INTRAOPERATIVE RADIOTHERAPY (IORT) – A NEW FRONTIER FOR PERSONALIZED MEDICINE AS ADJUVANT TREATMENT AND TREATMENT OF LOCALLY RECURRENT ADVANCED MALIGNANCY

EDITED BY : William Small, Jr. and Tarita O. Thomas PUBLISHED IN : Frontiers in Oncology

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# INTRAOPERATIVE RADIOTHERAPY (IORT) – A NEW FRONTIER FOR PERSONALIZED MEDICINE AS ADJUVANT TREATMENT AND TREATMENT OF LOCALLY RECURRENT ADVANCED MALIGNANCY

Topic Editors:

William Small, Jr., Stritch School of Medicine, Loyola University Chicago, United States Tarita O. Thomas, Stritch School of Medicine, Loyola University Chicago, United States

Image: © Carl Zeiss Meditec AG

Intraoperative radiotherapy (IORT) is a treatment delivery technique with reports starting in the early 20th century. There are numerous advantages of IORT in oncology including delivery of a tumoricidal radiation dose in a single treatment, direct visualization of the treatment area of interest, decreasing dose to surrounding tissues, among others. In this series we focus on the clinical application, radiobiology and physics of IORT with an emphasis on the Intrabeam system. As medicine and health care continue to evolve the new frontier of personalized medicine must continue to rigorously evaluate and implement technologies that limit costs and provide meaningful therapeutic benefit.

Citation: Small, W. Jr., Thomas, T. O., eds. (2019). Intraoperative Radiotherapy (IORT) – A New Frontier for Personalized Medicine as Adjuvant Treatment and Treatment of Locally Recurrent Advanced Malignancy. Lausanne: Frontiers Media. doi: 10.3389/978-2-88945-794-6

# Table of Contents

*04 Editorial: Intraoperative Radiotherapy (IORT)—A New Frontier for Personalized Medicine as Adjuvant Treatment and Treatment of Locally Recurrent Advanced Malignancy*

Tarita O. Thomas and William Small Jr

*06 Intraoperative Radiotherapy With INTRABEAM: Technical and Dosimetric Considerations*

Anil Sethi, Bahman Emami, William Small Jr. and Tarita O. Thomas


Stephanie A. Valente, Alicia Fanning, Robyn A. Stewart, Sharon Grundfest, Rahul D. Tendulkar, Sheen Cherian, Chirag Shah, Chao Tu, Courtney Yanda, Diane Radford, Zahraa Al-Hilli and Stephen R. Grobmyer

*33 Evaluation of Partial Breast Reirradiation With Intraoperative Radiotherapy After Prior Thoracic Radiation: A Single-Institution Report of Outcomes and Toxicity*

Christine Chin, Priya Jadeja, Bret Taback, David P. Horowitz, Sheldon M. Feldman, Richard Ha and Eileen P. Connolly

*39 Intraoperative Radiation "Boost" to the Surgical Resection Bed Following Pancreaticoduodenectomy for a Borderline Resectable Pancreatic Carcinoma: A Case Report*

Tarita O. Thomas, William Small Jr., Mark Fleming, Song Kang and Richard A. Hoefer

*43 Immunotherapy Combined With Large Fractions of Radiotherapy: Stereotactic Radiosurgery for Brain Metastases—Implications for Intraoperative Radiotherapy After Resection*

Carsten Herskind, Frederik Wenz and Frank A. Giordano

*62 Intraoperative Radiation Therapy: A Promising Treatment Modality in Head and Neck Cancer*

Lara Hilal, Karine A. Al Feghali, Paul Ramia, Ibrahim Abu Gheida, Jean-Pierre Obeid, Wassim Jalbout, Bassem Youssef, Fady Geara and Youssef H. Zeidan

*71 Future Directions of Intraoperative Radiation Therapy: A Brief Review* Tatjana Paunesku and Gayle E. Woloschak

# Editorial: intraoperative radiotherapy (iort)—a New Frontier for Personalized Medicine as adjuvant treatment and treatment of locally recurrent advanced Malignancy

*Tarita O. Thomas\* and William Small Jr*

*Loyola University Medical Center, Maywood, IL, United States*

Keywords: intraoperative radiotherapy, personalized medicine, Kv, breast cancer, physics

### **Editorial on the Research Topic**

**Intraoperative Radiotherapy (IORT)—A New Frontier for Personalized Medicine as Adjuvant Treatment and Treatment of Locally Recurrent Advanced Malignancy**

#### *Edited and Reviewed by:*

*Timothy James Kinsella, Warren Alpert Medical School of Brown University, United States*

> *\*Correspondence: Tarita O. Thomas tathomas@lumc.edu*

#### *Specialty section:*

*This article was submitted to Radiation Oncology, a section of the journal Frontiers in Oncology*

*Received: 04 June 2018 Accepted: 07 June 2018 Published: 25 June 2018*

#### *Citation:*

*Thomas TO and Small W Jr (2018) Editorial: Intraoperative Radiotherapy (IORT)—A New Frontier for Personalized Medicine as Adjuvant Treatment and Treatment of Locally Recurrent Advanced Malignancy. Front. Oncol. 8:234. doi: 10.3389/fonc.2018.00234*

Intraoperative radiotherapy (IORT) is a treatment delivery technique with reports starting in the early twentieth century with the use of orthovoltage energy with limited applicability due to the energy characteristics (1). This technology had a resurgence in approximately the 1960s with the use of electron energy in Japan (2) and subsequently the literature is replete with numerous other publications. Initially, the use of IORT was restricted by the cumbersome nature of treatment delivery as a shielded operating room was needed requiring large capital expenditure as well as expertise of staff with limited patient applicability. Over time the technology has evolved from requiring a shielded room to the development of a mobile device that can move into a standard operating room with minimal shielding requirements. Various current technologies exist that deliver electron or photon energies intraoperatively including Novac7 (Hitesys SPA, Aprillia, Italy; 7–10 MeV), Mobetron (IntraOp Medical Corporation, Sunnyvale, CA, USA; 4–12 MeV), Axxent system (Xoft, San Jose, CA, USA; 20–50 kV), and Intrabeam system (Carl Zeiss Meditec, Dublin, CA, USA; 30–50 kV).

There are numerous advantages of IORT in oncology. IORT has the benefit of delivering a tumoricidal radiation dose in a single treatment, while targeting the therapy to the region of highest risk of disease recurrence with direct visualization in the operating room. This provides a high relative biological effectiveness while limiting dose to normal tissue *via* tumor bed devascularization, elimination of inter-fraction tumor cell repopulation, and possibly providing a systemic immune effect (3). In addition, there are practical benefits to the patient by elimination or reduction in outpatient treatment visits that routinely last for 5–6 weeks for conventional postoperative radiotherapy, such as improved quality of life, decreased side effects and financial advantages.

In this series we focus on the use, radiobiology, and physics of IORT with an emphasis on the Intrabeam system. Sethi et al. describe the technical and dosimetric considerations for the various applicators now available to treat patients with disease intraoperatively in various locations including with flat, spherical, and even a needle applicator. Valente et al. discuss their experience with IORT from the surgical perspective and how their group decreased operative times in patients receiving breast IORT with increased utilization. Paunesku and Woloschak provide a review of the history of IORT as well as an engaging discussion of how IORT can be used in the future. Herskind et al.

**4**

extend this discussion into the theoretical usage of large radiation fraction size in brain metastasis and the potential combination with immunotherapy. The series then reviews the use of IORT in various malignancies, including head and neck cancer, pancreas cancer, and brain metastasis. Three articles finally review the use of IORT in breast cancer a highly prevalent cancer with numerous radiation treatment options available. Jacobson and Sochi provide a review of the various types of partial breast therapy and the toxicities associated. Chin et al. describe their experience using IORT for patients with prior thoracic radiation exposure. Harris and Small provide a comprehensive review of the data in support of the use of breast IORT as well as the toxicities, cosmesis, and quality of life with use of this treatment modality touching on both the use of electron and photon-based IORT.

The Organization for Economic Cooperation and Development evaluated the spending, supply, utilization, and price of health care across 13 high income countries and found that as a percent of GDP from 1980 to 2013 health care spending is approximately

### REFERENCES


17% in the United States versus 10% in the other countries evaluated (4). In the United States, health care spending consumes on average 1/5 of a households' income. It has been estimated that the use of breast IORT could provide at least \$1.2 billion in saving to the health care system in the United States over 5 years (5). Currently breast IORT using the IntraBeam system is being used in 35 countries in more than 300 major hospitals with more than 200,000 women treated. Here, we provide a series of articles that discuss the usage of IORT in various malignancies as well as the technical aspects of this technology. As medicine and health continue to evolve the new frontier of personalized medicine must continue to rigorously evaluate and implement technologies that limit costs to the health care system and provide meaningful therapeutic benefit.

## AUTHOR CONTRIBUTIONS

Both authors contributed to the editorial.

early-stage breast cancer. *Ann Surg Oncol* (2013) 20(9):2873–80. doi:10.1245/ s10434-013-2997-3

**Conflict of Interest Statement:** The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

*Copyright © 2018 Thomas and Small. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.*

*Anil Sethi\*, Bahman Emami, William Small Jr. and Tarita O. Thomas*

*Loyola University Medical Center, Maywood, IL, United States*

Purpose: We evaluate dose characteristics and clinical applications of treatment accessories used in intraoperative radiotherapy (IORT) and make site-specific recommendations for their optimal use.

Methods and materials: Dose measurements were performed for a low energy (50 kV) X-ray INTRABEAM source. For spherical, flat, surface, and needle applicators, the following dosimetric parameters were measured: depth-dose (DD) profiles, surface dose (Ds), output factors (OF), and target dose homogeneity (DH). Optical density versus exposure calibration films were employed to obtain 2-dimensional dose distributions in planes parallel and perpendicular to beam direction. Film results were verified *via* repeat dose measurements with a parallel-plate ionization chamber in a custom water tank. The impact of applicator design on dose distributions was evaluated.

### *Edited by:*

*Marco Durante, Trento Institute for Fundamental Physics and Applications, Italy*

### *Reviewed by:*

*Lidia Strigari, Regina Elena National Cancer Institute, Italy John Eley, University of Maryland Medical Center, United States*

> *\*Correspondence: Anil Sethi asethi@lumc.edu*

### *Specialty section:*

*This article was submitted to Radiation Oncology, a section of the journal Frontiers in Oncology*

*Received: 02 November 2017 Accepted: 05 March 2018 Published: 26 March 2018*

#### *Citation:*

*Sethi A, Emami B, Small W Jr. and Thomas TO (2018) Intraoperative Radiotherapy With INTRABEAM: Technical and Dosimetric Considerations. Front. Oncol. 8:74. doi: 10.3389/fonc.2018.00074*

results: Spherical applicators are commonly used for treating the inner-surface of breast lumpectomy cavity. Flat and surface applicators provide uniform planar dose for head and neck, abdomen, and pelvis targets. Needle applicators are designed for kypho-IORT of spinal metastasis. Typically, larger applicators produce a more homogeneous target dose region with lower surface dose, but require longer treatment times. For 4-cm diameter spherical, flat, and surface applicators, dose rates (DR) at their respective prescription points were found to be: 0.8, 0.3, and 2.2 Gy/min, respectively. The DR for a needle applicator was 7.04 Gy/min at 5 mm distance from the applicator surface. Overall, film results were in excellent agreement with ion-chamber data.

conclusion: IORT may be delivered with a variety of site-specific applicators. Appropriate applicator use is paramount for safe, effective, and efficient IORT delivery. Results of this study should help clinicians assure optimized target dose coverage and reduced normal tissue exposure.

Keywords: intrabeam, spherical, flat, surface applicators, dosimetry

## INTRODUCTION

Intraoperative radiotherapy (IORT) delivers a large tumoricidal dose to a well-defined target at the time of surgery while simultaneously minimizing exposure to nearby normal structures (1, 2). Compared to external beam radiotherapy, the advantages of IORT are: potential for dose escalation, reduced overall treatment time, and enhanced patient convenience. IORT may be delivered with

**6**

either an external beam of low-energy electrons (3), kV X-rays (4, 5), or *via* a miniaturized X-ray tube used as radiation source in electronic brachytherapy (6).

The IORT investigated in this report is based on an INTRABEAM X-ray source, PRS 500 (INTRABEAM, Carl Zeiss Surgical, Oberkochen, Germany) emitting low-energy (50 kV) photons at a high dose-rate (**Figure 1**) (4, 5, 7–12). The IORT's appeal lies in its ability to deliver a large dose (10–20 Gy) to the target volume with rapid dose fall-off and hence limited exposure to adjacent organs at risk. Furthermore, with appropriate precautions, the low energy X-rays result in minimal radiation risk to the operating room personnel. Recent IORT advances, such as the availability of novel treatment applicators to shape radiation dose to a desired target volume have resulted in tremendous gains in its clinical applications (13). Currently, some of the more common treatment sites for IORT include: breast, head and neck, brain, abdomen, pelvis, rectum, sarcoma, and spine.

At Loyola University Medical Center, we have commissioned and clinically used the INTRABEAM system with spherical, flat, surface, and needle applicators. Owing to their symmetric shape, spherical applicators are used to deliver a uniform dose at the inner-surface of the breast lumpectomy cavity. The surface and flat applicators are used when a constant dose is desired at a given tissue-depth: 0 and 5 mm, respectively. The needle applicator is designed for kypho-IORT (spine metastasis). These applicators are available in a range of sizes (diameter: 0.5–6 cm).

Although spherical applicators have been in use for a long time for breast IORT, the flat, surface, and needle applicators have recently become available and their clinical applications are becoming more popular. However, at present, there is a lack of available data for these applicators, which may limit their clinical use. Having a thorough understanding of IORT dose distribution is essential for safe, effective, and efficient treatment delivery. In this paper, we report on the dosimetric characteristics of each applicator, including, dose rate (DR), depth-dose (DD), dose homogeneity (DH), and treatment time. In addition, practical guidelines are provided for the optimal use of each applicator for various treatment sites. However, it is our recommendation that each institution intending to practice IORT must validate the dosimetric data of their equipment prior to clinical use.

## MATERIALS AND METHODS

All measurements were performed for an INTRABEAM 50-kV X-ray source fitted with either a spherical, flat, surface, or needle applicator to produce desired spherical or planar dose (**Figure 1**) (10). At our institution, the physician performing IORT may choose from any of the following treatment accessories: five spherical applicators (diameter: 3–5 cm in 0.5 cm increment), six flat applicators (diameter: 1–6 cm in 1 cm increment), and four surface applicators (diameter: 1–4 cm in 1 cm increment). For

each applicator, DD profiles, surface dose, output factors (OFs), and DH were measured:


Output factors in terms of absolute DR (Gy/min) and DD profiles were measured in a water tank (**Figure 2A**) with a suitable thin-window parallel plate ion chamber (PTW 34013A, Physikalisch Technische Werkstaetten, Freiburg, Germany) (**Figure 2B**) (14). The ion chamber was connected to a PTW UNIDOS electrometer T10010 to record measured charge. The water-phantom available from Zeiss (Carl Zeiss Surgical, Oberkochen, Germany) allows precise positioning of applicator tip relative to ion chamber for accurate dose output and dose distribution measurements.

The measured DR (*Gy/min*) at a specific depth *z* in water can be written as:

$$\text{DR}(z) = N\_{\text{k}} Q(z) C\_{\text{TP}} k\_{\text{Q}} k\_{\text{eke}} \tag{1}$$

where *N*k is the ion chamber calibration factor (*Gy/nC*), *Q(z)* is the ionization charge (C) collected in 60 s for the chamber located at depth *z* in water, *C*TP is the correction factor for room temperature (*T*) and pressure (*P*) at the time of dose measurement, *k*Q is the beam quality correction factor, and *k*elec is the electrometer calibration factor. The measured DR is plotted as a function of depth *z* to obtain depth dose profiles (DD).

DH data were obtained from the measured DD profiles. Surface-dose (Ds) data were acquired with a film dosimeter. Whenever possible, film data were confirmed with ion-chamber measurements.

The spherical applicator's isotropic dose distribution is easily mapped with an ion chamber. The dose characteristics and treatment times for a needle applicator are available from a look-up table available with Zeiss, Inc. (Carl Zeiss Surgical, Oberkochen, Germany). Dose distributions generated with flat and surface applicators, on the other hand, are more intricate, requiring the use of a 2-dimensional film dosimeter. Films are an efficient tool to measure planar dose distribution. We used Gafchromic EBT3 films (International Specialty Products - ISP, Wayne, NJ, USA) sandwiched between slabs of water equivalent phantom (Plastic water, CIRS, Norfolk, VA, USA) (15) (**Figure 3A**). The films were aligned in both parallel and perpendicular orientations relative to the radiation beam (only films in perpendicular orientation are shown in **Figure 3A**). For each measurement, the X-ray source was oriented vertically with the end of the applicator in contact with the phantom surface (**Figure 3A**).

First, we established a film characteristic or sensitometric curve (also known as Hurter and Driffield or H&D curve), which relates film optical density with film exposure or dose (**Figures 3B,C**). This was done by irradiating several films from the same batch to a known dose (range: 0–4 Gy) delivered at 5 mm depth from the phantom surface (**Figure 3B**). Absolute calibration of IORT source was validated *via* Eq. 1 given above. Since the EBT3 film response is known to be highly sensitive to environmental conditions, all necessary film precautions were

observed, for example, handling of films with tweezers and latex gloves (15–19). Furthermore, following each irradiation, films were allowed to self-develop for at least 24 h to stabilize dose response. An EPSON 11000XL PRO flatbed scanner was used with films placed at its center in the portrait orientation. The red color channel was used and film pixel values were converted to dose using the sensitometric (H–D) curve. Film scanning and analysis software, RIT ver. 6.4 (Radiological Imaging Technology, Colorado Springs, CO, USA) was used to generate 2-dimensional dose distributions. With each applicator, films were irradiated to deliver 1 Gy dose at the prescription depth. All film results (DD profiles, OF, etc.) were verified with parallel-plate ion chamber measurements in the water phantom.

## RESULTS

### Spherical Applicators

**Figure 4** shows typical dose distribution produced by a 4 cm diameter spherical applicator. A 20 Gy dose was prescribed at the applicator surface (or the lumpectomy cavity-inner surface). For effective skin sparing, the applicator surface must be at least 1 cm depth from the skin-surface. In the present case, the 20 Gy dose at the applicator surface would result in 5.7 Gy skin dose. Target DH, defined as the ratio of maximum and minimum doses (*D*max*/ D*min), was evaluated in the radial direction within a 1 cm thick spherical shell surrounding the applicator (as indicated in the figure). For the 4 cm spherical applicator, the measured DR at the applicator surface was 0.8 Gy/min, which resulted in a treatment time of 25 min. At 1 cm from the applicator surface, the DR was 0.23 Gy/min, corresponding to a DH = 3.5. In general, larger applicators require longer treatment time due to lower surface DR, but yield superior target dose homogeneity (DH values closer to unity) or a slower dose fall-off in the target region. Due to its unique design features, the 3-cm diameter spherical applicator has a lower surface dose and slower dose fall-off compared to the 3.5 cm applicator. This results in somewhat longer than expected treatment times with the 3 cm applicator (**Table 1**). **Figure 5** shows the surface DRs and dose fall-off with depth (DD

applicator. Doses are shown at the applicator surface and at 5, 10, and 20 mm distance from it. Dose homogeneity was evaluated in a 1 cm thick spherical shell surrounding the applicator. Applicator shown in inset.

X-ray source, etc. Table 1 | Dosimetric characteristic of various intraoperative radiotherapy applicators used with INTRABEAM.

#### Dosimetric comparison of applicators


profiles) for spherical applicators with diameters ranging from 3 to 5 cm (treatment times: 17–44 min). For each applicator, the DD curve starts at the applicator surface, for example, 15 mm from the X-ray source for a 3 cm applicator, 20 mm from the X-ray source for a 4 cm applicator, etc. Radial DH, in the 1 cm spherical shell surrounding each applicator (presumed target region) ranged between 3 and 4.

### Flat Applicators

**Figure 6** shows the dose distribution produced by a 4 cm diameter flat applicator. The dose uniformity (or flatness) perpendicular to the beam direction is greatest at the prescription depth of 5 mm with the dose being less uniform at other depths. At shallower depths (<5 mm from the skinsurface), "horns" in dose profiles corresponding to higher dose values are seen at points away from the central axis. For deeper depths (>5 mm), the opposite effect is observed: the measured dose is greatest along the central axis but tapers-off

away from it. For a prescription dose of 10 Gy at 5 mm depth, 19.4 Gy would be delivered to the skin-surface corresponding to a dose-homogeneity, DH of 1.94 along the beam direction within 5 mm thick surface layer. Typical treatment time is approximately 30 min. Larger applicators require longer treatment times, but result in a lower surface dose (Ds) and superior dose-homogeneity. **Figure 7** shows the dose fall-off with depth (DD) in water for various flat applicators ranging in diameter from 1 to 6 cm. Several interesting features of this figure are worth noting. First, small applicators are associated with (a) large surface (or skin) dose, (b) shorter treatment times, and (c) lower DH. Second, **Figure 7** shows a lack of measured data for depths <2 mm. This is caused by the design of the parallel-plate ion-chamber used for dose measurements. The effective point of measurement for this chamber is located at its entrance (inner) wall, which is ~2 mm below the chamber's outer surface. As shown in **Figure 2C**, the chamber wall thickness is 0.22 mm and the chamber is protected inside a 1 mm thick waterproof sleeve (**Figure 2A**) with an air gap of 0.5 mm between the sleeve and the chamber wall. Thus, a separation of 1.72 mm between the inner wall of the chamber and its outer surface represents the region where dose cannot be measured. To obtain missing data points in the superficial region, we repeated DD measurements with EBT3 films oriented parallel to the beam direction. **Figure 8** shows excellent agreement between film and ion chamber results for a 4 cm flat applicator.

### Surface Applicators

**Figure 9** shows dose distribution from a 4 cm diameter surface applicator. The prescription dose in this case is at the

evaluate dose homogeneity. Applicator shown in inset.

applicator surface and a rapid dose fall-off is observed with depth. The dose is highest along the central-axis and tapers off-axis. For a prescription dose of 10 Gy at the applicator (or skin) surface, a treatment time of 4.5 min is required. This corresponds to a surface dose-rate (Ds) of approximately

2.2 Gy/min. At a depth of 5 mm, the dose reduces to only 3 Gy for a DH = 3.33. Again, larger diameter surface applicators will require longer treatment times but produce superior DH and lower Ds. **Figure 10** shows the dose fall-off with depth for various surface applicators. To recover missing

Figure 9 | Dose distribution from a 4 cm surface applicator. Dose is prescribed at the surface of the phantom. Also indicated is 5 mm superficial layer used to evaluate dose homogeneity. Applicator shown in inset.

dose data points at the shallower depths (<2 mm), doses were re-measured with calibrated EBT3 films and the results are shown for the 4 cm-surface applicator. Good agreement between ion-chamber and film doses is seen (**Figure 11**). The discrepancy between film and chamber dose is probably due to X-ray spectral changes and rapid dose fall-off with surface applicators.

### Needle Applicators

**Figure 12** shows dose distribution produced with a needle applicator. Typically, 8 Gy prescription dose is given at 5 mm distance from the needle applicator surface to spine metastases for a treatment time of over a minute. Owing to the closeproximity of the prescription dose point to the XRS source, a rapid dose fall-off with depth is observed (**Figure 13**).

Figure 12 | Dose distribution from a needle applicator. Dose is prescribed at 5 mm from the applicator surface or 8 mm from the source. Needle applicator shown in inset.

A look-up table (available from Zeiss, Inc.) is used to determine treatment time for desired prescription dose.

**Table 1** summarizes main dosimetric results for various IORT applicators.

### DISCUSSION

from the source.

Intraoperative radiotherapy has shown clinical utility in a variety of treatment sites outside breast: intracranial (20), head and neck (21, 22), abdomen (23), pelvis (24, 25), spine (26, 27), and skin (28). Compared to 3-d conformal radiotherapy, the main advantages of IORT lie in its steep dose-fall off and the ability to give a large dose to target volume while limiting dose to nearby organsat-risk (OARs). Successful IORT requires a multi-disciplinary team of nurses, anesthesiologists, radiation oncologists, surgeons, and medical physicists. Also needed are quality assurance checks for safe and effective IORT delivery and a detailed understanding of each applicator's dose distribution. Since April 2014, our group has treated 73 IORT patients for a variety of treatment indications: breast (35 patients), H&N (23 patients), abdomen/pelvis (13 patients), and spine metastasis (2 patients).

The choice of IORT applicator depends on the treatment site and the extent of the disease. For example, breast IORT is commonly delivered using a spherical applicator with the prescription dose at the outer surface of the applicator (or inner-cavity surface). Flat and surface applicators provide a uniform planar dose as in the H&N and abdomen/pelvis regions. At our institution, H&N IORT has been delivered with flat applicators; however, both flat and surface applicators have been used in the treatment of abdomen/pelvis targets. The needle applicator has been designed for kypho-IORT of spine metastasis.

Each IORT applicator presents a unique dosimetric challenge and learning curve, due attention to which is essential for safe and effective treatments. Prior to their clinical use, dosimetric parameters: surface dose (Ds), depth dose profiles (DD), treatment times, etc., for each applicator must be measured and validated. These data, as a lookup table, can be helpful in performing quality assurance or independent checks of treatment times used for patient treatments. In addition, room survey measurements must be performed for each applicator to assess doses received by the OR personnel during IORT and to ensure they are within safe-limits.

The maximum dose with a spherical applicator is at the applicator surface. In general, radial dose fall-off with depth is sharper with smaller applicators. The use of small applicators may, therefore, result in greater skin sparing but a less uniform target dose. Small applicators are also associated with shorter treatment times. Using too small an applicator size, however, could cause air-gaps between the applicator and the surrounding cavity, thereby compromising treatments. Studies have shown significant attenuation of low energy photon spectrum in the presence of tissue inhomogeneities (29). These may produce unacceptably large variation in PTV dose. Based on film and ion chamber measurements, our group showed a 16% dose enhancement when a 2 mm layer of tissue is replaced by air and a 58% dose reduction when it is replaced by bone (29). This is further exacerbated by the above noted rapid variation in target dose with depth; for example, a 3.5 times reduction in dose within 1 cm thick shell surrounding a 4 cm diameter breast applicator. Therefore, Bouzid et al. have recommended CT-based treatment planning with Monte Carlo dose calculations for improved prescription and assessment of delivered IORT dose (30). With CT based planning, any concerns related to a lack of target coverage and/or OAR sparing can be addressed by the planner.

A flat applicator is used when a uniform dose at a given depth is desired in tissue (typically 5 mm depth from skin-surface). With an increase in applicator diameter, the dose-rate decreases, the treatment time increases, but the DH is improved in the

surface applicators. Notice the steep dose-fall of with depth for the surface applicator resulting in inferior dose homogeneity, larger output factor, and shorter treatment times.

shallow regions. These changes are most dramatic for smaller applicators (<3 cm diameter). In general, larger applicators produce the most homogeneous dose. Similar depth-dose variation is observed for surface applicators but the effect is more pronounced (**Figure 14**). A concern with the use of flat/surface applicators is the skin/surface dose, which could be a limiting factor in some treatments. It is important to note that very small

### REFERENCES


applicators (≤2 cm diameter) may yield relatively high surface dose; therefore, one must select the largest possible applicator size that is compatible with the treatment area.

Due to large prescription doses used with IORT, skintoxicity can be a big concern. We have investigated the use of SURGICEL® (Ethicon, Inc., Johnson and Johnson Health Care, Somerville, NJ, USA) to reduce surface dose. A 1 or 2-mm layer of SURGICEL® may be used to reduce skin toxicity (lower skin dose by up to 30%).

In summary, the dosimetric results presented here pertain to INTRABEAM IORT and associated applicators only. Furthermore, the results reported in this study are for guidance purposes only and must be validated by each institution with their own equipment prior to the clinical implementation of IORT.

### CONCLUSION

Intraoperative radiotherapy may be delivered with a variety of treatment applicators. Selection of appropriate applicator is important for safe, efficient, and effective delivery of IORT. The dosimetric results from this study should help design IORT treatments to assure optimized target coverage and reduced normal tissue exposure. These results may also be used in designing an effective IORT QA program including secondary check of treatment times.

### AUTHOR CONTRIBUTIONS

AS participated in study design, data measurement, and manuscript preparation. BE, WS, and TT contributed clinical data and recommended applications and reviewed manuscript.


**Conflict of Interest Statement:** The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

*Copyright © 2018 Sethi, Emami, Small and Thomas. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.*

# Low-Energy intraoperative Radiation Therapy and Competing Risks of Local Control and normal Tissue Toxicity

*Geraldine M. Jacobson\* and Ramon Alfredo Siochi*

*Department of Radiation Oncology, WVU Cancer Institute, West Virginia University, Morgantown, WV, United States*

Keywords: radiotherapy, breast cancer, breast conserving therapy, accelerated PBI, intraoperative radiation therapy, cardiac toxicity, lung toxicity

## INTRODUCTION

Radiotherapy (RT) following breast-conserving therapy has been demonstrated to reduce the risk of breast cancer local recurrence and death (1). The adverse normal tissue effects of radiation have been reduced, but not eliminated, with modern treatment planning techniques. Since the introduction of breast-conserving therapy in the 1980s, analyses of long-term results of randomized trials have identified patients that derive the most benefit in local control and disease-free survival. Partial breast irradiation (PBI) is an option for patients with defined, low-risk features that has the advantage of reduced radiation to the heart, lungs, and soft tissues. Low-energy intraoperative radiation therapy (IORT) is a subset of PBI that shortens the treatment course and reduces the

### *Edited by:*

*Tarita O. Thomas, Loyola University Chicago, United States*

### *Reviewed by:*

*Tamer Refaat Abdelrhman, Northwestern University, United States*

#### *\*Correspondence:*

*Geraldine M. Jacobson gmjacobson@hsc.wvu.edu*

#### *Specialty section:*

*This article was submitted to Radiation Oncology, a section of the journal Frontiers in Oncology*

*Received: 30 March 2017 Accepted: 28 August 2017 Published: 21 September 2017*

#### *Citation:*

*Jacobson GM and Siochi RA (2017) Low-Energy Intraoperative Radiation Therapy and Competing Risks of Local Control and Normal Tissue Toxicity. Front. Oncol. 7:212. doi: 10.3389/fonc.2017.00212*

dose to larger volumes of normal tissues compared to other techniques. Patients with a lower risk of local recurrence are likely to live decades beyond their treatment and are at continued risk of impaired cardiac and pulmonary function. For these patients, it seems

## WHOLE BREAST IRRADIATION (WBI) AND LOCAL CONTROL

prudent to select treatments that reduce the risk of late radiation effects.

While the proportional benefit of breast radiation in achieving local control is similar for all groups, the absolute benefit is related to patient and tumor characteristics. Data from the Early Breast Cancer Trialists' Collaborative Group showed that the absolute benefit of post-BCS conserving RT was greater in younger women; 10-year risk reduction was 24.6% for women younger than 40 years of age and 8.9% for women 70 years and older (2). An update of the European Organization for Research and Treatment of Cancer Boost or No Boost trial showed a cumulative incidence of breast tumor recurrence (IBTR) at 20 years of 34% in patients younger than 40 years of age and 11% in patients 50 years and older (3). Young age and presence of ductal carcinoma *in situ* were the predominant factors associated with IBTR. Patients with high invasive grade were at high risk for IBTR in the first 5 years posttreatment, but this effect declined with time. Liu et al. showed that breast cancer subtype was related to IBTR in node-negative patients older than 50 years but not associated with response to radiation (4).

In 1985, the reported 5-year ipsilateral breast recurrence rate following lumpectomy and radiation from the National Surgical Adjuvant Breast and Bowel Project B06 trial was 7.7% (5). This level of local control supported the introduction of breast conservation as an acceptable treatment for early breast cancer. Local recurrence rates following conservative surgery and radiation are lower in modern series, probably related to more effective systemic therapy and improved surgical and pathology techniques.

**16**

## ACCELERATED PBI

For localized breast cancer, the most common site of recurrence is at or near the index lesion. Recurrence in other quadrants is not impacted by WBI (6). Accelerated PBI (APBI) treats a limited volume surrounding the tumor cavity with larger fractions over a shorter period than WBI. Advantages of APBI include reduced treatment time, fewer patient visits, and reduced normal tissue effects. Several treatment delivery methods are utilized, including interstitial, intracavitary, intensity-modulated radiation therapy or 3-D conformal, proton therapy, and IORT.

A systematic review of eight randomized trials of APBI showed a small difference of local recurrence in favor of WBI but no difference in nodal recurrence, systemic recurrence, overall survival, or mortality (7). A randomized, non-inferiority trial of multicatheter brachytherapy versus WBI reported local recurrence of 1.44% with APBI versus 0.92% with WBI at 5 years, demonstrating non-inferiority with respect to local control and disease-free and overall survival (8). Several studies have identified age as a risk factor for local recurrence with APBI, with lower risk in patients older than 50 years (9, 10). This is reflected in the American Society for Radiation Oncology consensus statement regarding APBI, with age of 50 years or older in the suitable category (11). APBI has been widely used for decades and tested in clinical trials in over 1,000 patients. For appropriately selected patients, outcomes are comparable to WBI. In addition, there is a lower incidence of non-breast cancer mortality. A metaanalysis of randomized trials of PBI versus whole breast radiation reported lower 5-year non-breast cancer and overall mortality rates in patients treated with PBI, amounting to a 25% reduction in relative terms (12).

### LOW-ENERGY X-RAY IORT

Intraoperative radiation therapy is a subset of PBI that delivers a single fraction of radiation at the time of lumpectomy, saving patient and facility time and resources. There is a potential therapeutic advantage to delivering radiation to the operative tumor bed, with a steep dose fall-off and minimal dose delivered to noninvolved tissues. Several IORT delivery systems are available, most of which use kilovoltage (kV) photons or megavoltage electrons.

The intrabeam IORT device provides a point source of 50 kV X-rays at the center of a spherical applicator. Applicator diameters range from 1.5 to 5.0 cm. HVLs of the Intrabeam device correspond to effective energies of 20.7–36.3 keV. Depending on applicator diameter, the dose is reduced to 5–7 Gy at 1 cm from the applicator and 1 Gy in 2.3–6.4 cm of tissue. An *in vivo* dosimetry study showed a mean skin dose of 2.9 ± 1.6 Gy (13). A separate study demonstrated a skin dose of 0.29 ± 0.17 Gy at 5–10 cm from the applicator and a dose of 0.57 ± 0.23 Gy to the pectoral muscle in left breast patients (14). Based on these characteristics, dose to the heart and lungs from low-energy IORT is minimal. The absence of significant dose to organs at risk is a major difference between external beam radiation (**Figure 1A**) and low-energy, X-ray IORT (**Figure 1B**).

A prospective randomized trial of IORT versus whole breast RT using the Intrabeam device at the time of lumpectomy (TARGIT-A) reported a local recurrence rate at 4 years of 1.2% in the IORT group and 0.95% in the WBI group (15). Five-year results from the trial reported a 2.1% rate of breast recurrence in the IORT group versus 1.1% in the WBI group when IORT was delivered at the time of lumpectomy. When IORT was delayed until pathology was available, local recurrence was higher (5.4%) (16). Overall breast cancer mortality was the same in the TARGIT and WBI groups, but there were significantly fewer non-breast cancer deaths with TARGIT, attributable to fewer deaths from cardiovascular causes and other cancers (16).

### LUNG TOXICITY OF WBI

Lung toxicity is a known complication of thoracic radiation that is related to dose and volume. Postmastectomy radiation in the preimage guided 3-D conformal era was associated with a significant risk of radiation pneumonitis. The use of lung constraints in treatment planning has decreased clinical lung complications associated with RT. A Swedish group showed that use of 3-D planning with a lung constraint of V20 <30% reduced the rate of radiation pneumonitis and associated decrease in pulmonary function tests (PFTs) (17). A later report from the same group showed a significant reduction in PFTs compared to pre-RT values and observed that computed tomography (CT) changes observed 4 months after RT were still detectable

after 11 years (18). A prospective study of pulmonary sequelae of breast irradiation showed serial changes in lung imaging, with reduction in lung function following radiation. The study included patients receiving postmastectomy and intact breast radiation from 1996 to 1997 using 2-D planning. Lung functioning indices, including forced expiratory volume in 1 s, forced vital capacity, total lung capacity, and diffusing capacity of the lung for carbon monoxide, declined after radiation and were irreversible after 12 months (19).

Contemporary RT is CT-based and incorporates normal tissue constraints that reduce long-term toxicity. A study that coregistered quantitative single photon emission CT (SPECT)/ CT of the lung with treatment plans of patients receiving breast/ chest wall and regional node irradiation (V20 left lung limited to 33%) showed dose-related decreased perfusion to the lung (20). Although the use of these constraints prevented clinical symptoms of lung toxicity, patients experienced decreased lung perfusion, an indication of lung injury.

While modern radiation techniques can reduce pulmonary toxicity from external beam breast/chest wall irradiation, it occurs, is measurable, and persists long after treatment is completed.

### CARDIAC TOXICITY OF WHOLE BREAST RADIATION

Long-term follow-up of early randomized trials for breast cancer demonstrated an increased risk of ischemic heart disease in irradiated patients, with adverse effects on survival (21). Subsequent follow-up showed that the excess of early cardiac deaths was offset by a reduction of death due to breast cancer, particularly in the more recent trials. It was postulated that earlier trials had a higher rate of cardiac toxicity because of dose and radiation techniques (2, 22).

A population-based study of breast cancer patients treated between 1958 and 2001 in Scandinavia showed that radiation to the heart during breast cancer treatment was associated with an increased risk of coronary events. The increase was proportional to mean heart dose (MHD) with no apparent threshold, started within a few years after exposure, and continued for at least 20 years. The authors estimated that a 1 Gy increase in MHD equates to a 7.4% increase in significant coronary events (23). A study of radiation dose response relationship for risk of coronary heart disease (CHD) following radiation for Hodgkin lymphoma (treated 1965–1995) showed that risk of CHD increased linearly with increasing MHD, with a median interval between treatment and expression of CHD of 19 years (24).

Both cohorts of patients were treated in the 2-D era of radiation, before widespread use of CT-based planning and adoption of current cardiac and pulmonary dose constraints. These studies demonstrate that the clinical manifestation of cardiac injury is a late effect and may not be evident until decades after treatment.

Multiple techniques have been developed to decrease cardiac dose, including prone position, intensity-modulated radiation therapy, and deep inspiration breath hold, while dose constraints have been developed to reduce late toxicity. Still, current functional studies provide evidence of measurable cardiac damage following whole breast radiation.

A prospective study of cardiac injury using resting-gated SPECT cardiac perfusion pre- and post-breast/chest wall irradiation, using CT-based planning, showed that radiation therapy caused volume-dependent perfusion defects in about 40% of patients within 6 months to 2 years of treatment. The perfusion defects were more common in patients with a larger volume of the left ventricle within the treatment field and were associated with abnormalities in regional wall motion (25).

A review of published studies of cardiac toxicity demonstrated a decrease in cardiovascular events and cardiac death rate in more modern treatment eras (26). However, the previous example illustrates that measurable cardiac injury occurs with contemporary treatment methods, though symptoms may not be immediately evident. Cardiac injury is related to dose, does not have a threshold, and may become evident years after radiation. This is a relevant concern in the treatment of breast cancer patients who are likely to survive decades after their treatment.

### SUMMARY

Selection of radiation modality should consider treatment relatedmortality and long-term pulmonary and cardiac toxicity in addition to relative risks of local recurrence and disease-free survival. Multiple techniques have been developed to decrease cardiac and lung dose, and constraints have been developed to reduce late effects. Still, current functional studies provide evidence of measurable cardiac and pulmonary damage following whole breast radiation, which should be considered when selecting treatment.

Discussion of the merits of low-energy (50 kV) IORT has often focused on the issue of local control. Less attention has been directed to potential long-term normal tissue effects. Low-energy IORT has the distinct advantage of minimal dose to the heart and lungs as well as reduced dose to uninvolved soft tissue. In selected patients, local control is not inferior to WBI. In addition, PBI, including IORT, is associated with reduced non-breast cancer mortality.

### AUTHOR CONTRIBUTIONS

GJ: conception and design, research of supporting evidence, manuscript writing, and final approval. RS: critical revision of content, editing, creation of dosimetry model, and final approval.

### ACKNOWLEDGMENTS

We thank the West Virginia Clinical and Translational Science Institute for editorial assistance. The WVCTSI is supported by the National Institute of General Medical Sciences, U54GM104942. The content is solely the responsibility of the authors and does not necessarily represent the official views of the NIH.

## REFERENCES


**Conflict of Interest Statement:** The research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

*Copyright © 2017 Jacobson and Siochi. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) or licensor are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.*

#### *Eleanor E. R. Harris1 and William Small Jr.2 \**

*1Department of Radiation Oncology, Brody School of Medicine, East Carolina University, Greenville, NC, United States, 2Department of Radiation Oncology, Stritch School of Medicine, Loyola University, Chicago, IL, United States*

Intraoperative radiotherapy (IORT) for early stage breast cancer is a technique for partial breast irradiation. There are several technologies in clinical use to perform breast IORT. Regardless of technique, IORT generally refers to the delivery of a single dose of radiation to the periphery of the tumor bed in the immediate intraoperative time frame, although some protocols have performed IORT as a second procedure. There are two large prospective randomized trials establishing the safety and efficacy of breast IORT in early stage breast cancer patients with sufficient follow-up time on thousands of women. The advantages of IORT for partial breast irradiation include: direct visualization of the target tissue ensuring treatment of the high-risk tissue and eliminating the risk of marginal miss; the use of a single dose coordinated with the necessary surgical excision thereby reducing omission of radiation and the selection of mastectomy for women without access to a radiotherapy facility or unable to undergo several weeks of daily radiation; favorable toxicity profiles; patient convenience and cost savings; radiobiological and tumor microenvironment conditions which lead to enhanced tumor control. The main disadvantage of IORT is the lack of final pathologic information on the tumor size, histology, margins, and nodal status. When unexpected findings on final pathology such as positive margins or positive sentinel nodes predict a higher risk of local or regional recurrence, additional whole breast radiation may be indicated, thereby reducing some of the convenience and low-toxicity advantages of sole IORT. However, IORT as a tumor bed boost has also been studied and appears to be safe with acceptable toxicity. IORT has potential efficacy advantages related to overall survival related to reduced cardiopulmonary radiation doses. It may also be very useful in specific situations, such as prior to oncoplastic reconstruction to improve accuracy of adjuvant radiation delivery, or when used as a boost in higher risk patients to improve tumor control. Ongoing international clinical trials are studying these uses and follow-up data are accumulating on completed studies.

Keywords: breast cancer, intraoperative radiotherapy, breast conservation therapy, partial breast irradiation, radiation therapy

## INTRODUCTION

Partial breast irradiation has been established as a suitable treatment option for appropriately selected women with early stage breast cancer by numerous clinical trials dating back to the 1990s. There are several techniques which have been studied to accomplish irradiation of the periphery of the lumpectomy bed as sole therapy after lumpectomy, which is the target volume for any form of partial

#### *Edited by:*

*Sunil Krishnan, University of Texas MD Anderson Cancer Center, United States*

#### *Reviewed by:*

*Sonali Rudra, MedStar Georgetown University Hospital, United States John Austin Vargo, West Virginia University Hospitals, United States*

#### *\*Correspondence:*

*William Small Jr. wmsmall@lumc.edu*

#### *Specialty section:*

*This article was submitted to Radiation Oncology, a section of the journal Frontiers in Oncology*

*Received: 08 June 2017 Accepted: 06 December 2017 Published: 22 December 2017*

### *Citation:*

*Harris EER and Small W Jr. (2017) Intraoperative Radiotherapy for Breast Cancer. Front. Oncol. 7:317. doi: 10.3389/fonc.2017.00317*

**20**

breast treatment. Intraoperative radiotherapy (IORT) is one such technique. The major difference between IORT techniques and other forms of APBI is timing of the procedure. IORT is most often performed at the time of breast surgery as a single dose, while other APBI techniques are performed post-operatively, using target volumes are typically based on CT images and delivering multiple fractions. IORT requires specialized radiotherapy equipment, and there are several technologies available to provide IORT partial breast irradiation, which deliver treatment with either electrons or 50 kV X-rays. IORT has the advantage of completing the breast-conserving surgery and, in most cases, the partial breast irradiation as one combined procedure. All forms of APBI treat a smaller volume of normal tissue than whole breast radiation (WBRT), thereby reducing the potential lung and cardiac toxicities of radiation treatment, and reducing the overall treatment time compared with whole breast irradiation. IORT has the additional advantage of delivering a single dose at the time of surgery, potentially reduces non-compliance to post-operative radiation, and mastectomy rates among women without ready access to a radiotherapy center. There are two recently published large prospective randomized controlled trials comparing postlumpectomy standard whole breast irradiation to IORT, one using electrons and one using 50 kV photons, which have shown low-local recurrence rates for IORT with acceptable toxicity and excellent overall survival outcomes. These trials begin to inform our knowledge of selection criteria for optimal breast IORT candidates and provide the first evidence of outcomes and toxicity when using these techniques. Patient selection is important when recommending IORT, as the final pathology is not available at the time of treatment, so in order to avoid the potential use of subsequent whole breast irradiation, careful pre-operative, and intraoperative assessment can help ensure that high-risk features such as positive margins or positive sentinel nodes are minimized. As all techniques of partial breast irradiation leave some volume of the breast unirradiated, understanding of the selection criteria for each of the various techniques is critical information for clinicians when considering which patients may be appropriately treated with IORT or any other APBI technique. This review will discuss the clinical trial data, patient selection criteria, advantages and disadvantages of partial breast IORT, and published guidelines.

### RADIOBIOLOGY AND TUMOR MICROENVIRONMENT

A common calculation for assessing the efficacy of different dose fractionations in radiotherapy is the relative biological effectiveness (RBE = *Dx*/*D*). This parameter allows comparisons of radiation-induced cellular damage at a designated dose (*D*) relative to a reference dose (*Dx*) for the selected endpoint, such as percentage of cells surviving after 2 Gy (SF2). It is well known that the RBE of photons increases with decreasing energy, explained by a decrease in energy of secondary electrons with an increase in linear energy transfer (1, 2). Cell culture experiments utilizing cell survival methods such as SF2 confirm enhanced biological effects after exposure to lower energy X-rays. Brenner et al. modeled RBE at clinically relevant doses, at which the RBE for 40 kV photons was about 1.4 compared with 4 mV. Since effective RBE increases with depth this creates a less rapid fall-off of the biologically weighted dose at measured depth. A published review summarized RBEs for 10% cell survival established using different systems and tumor cell types for low energy X-rays (10–240 kV) to range from 1.1 and 1.7 (3). The RBE of 50 kV electronic brachytherapy sources has been estimated to exceed biological effectiveness by 40–50% over Co60 or Ir192 (4). Other investigators have employed a linear-quadratic formula to model the RBE of 50 kV X-rays modeled as an equivalent to a fractionated dose of 2 Gy (EQD2) as a function of depth, with the probability of local control estimated from clinical dose response data (5). This model resulted in a theoretical "sphere of equivalence" to explain the improved tumor control probability in the high-dose cloud near the applicator surface, where residual microscopic disease is most likely to be located, and compensating for a somewhat lower tumor control probability as the distance from the applicator increases. Overall local control patterns thus exhibited a different spatial pattern but were ultimately very similar to conventionally fractionated external beam irradiation. In the example of IORT for breast cancer, higher RBE for low-energy X-rays may result in higher tumor control rates in the breast tissue in closest proximity to the surgical excision bed and effectively eliminating the "marginal miss." In addition, cell culture data suggest that the RBE decreased at increasing distance, potentially reducing the effective dose to adjacent critical structures including heart and lung (6). The tumor bed intraoperatively is better oxygenated, which may also improve cell kill probability. There may also be some radiobiological advantages in using a higher dose per fraction in breast cancer, which has been estimated to have an alpha/beta ratio of around 4, therefore may demonstrate a higher radioresponsiveness to higher doses per fraction (7). The biologically equivalent dose (BED) for an alpha/beta of 4 in the linear-quadratic model for a prescribed single dose of 10 Gy is isoeffective to about 24 in 2 Gy fractions.

The microenvironment of the breast cancer cells likely plays a critical role as well in the risk of tumor recurrence, and this microenvironment is altered by the use of immediate radiation peri-operatively. Belletti et al. collected wound fluid from the lumpectomy cavity over 24 h after surgery, half of whom had IORT at the time of lumpectomy (8). The wound fluid was used to stimulate several breast cancer and control cell lines and analyzed for cell growth and motility. Normal wound fluid stimulated proliferation, migration, and invasion in breast cancer cell lines, while these effects were abrogated by wound fluid from immediately irradiated samples. The radiated wound fluid had altered expression cytokines, suggesting that the radiation had altered protein expression. Fabris et al. collected tissues from irradiated using IORT and non-irradiated tissue from breast cancer patients after surgery, and profiled the tissue for microRNA expression (9). IORT radiation changed the wound response by inducing expression of miRNA 223 in the peri-tumoral tissue, which downregulated expression of epidermal growth factor (EGF) and EGF receptor activation. This downregulation cascade prevented breast cancer cell growth and reduced local recurrence in mice models. A number of other studies have noted the stimulatory effect of wound fluid on breast cancer cells, suggesting a role of the fluid in cancer cell proliferation and possibly local recurrence, an effect that may be muted by immediate radiation, with its abrogating effect on protein expression. Clinical trial data are clinically consistent with these concepts and investigations of the biological effects of immediate high-dose radiation on the wound fluid and immunologic environment are ongoing.

### IO(E)RT TECHNOLOGIES

The Intrabeam® system (Carl Zeiss Meditec, Dublin, CA, USA) uses a 50 kV photon beam mobile X-ray unit that has been in clinical use since 1999 (10). The miniaturized accelerator produces an electron beam that is accelerated to the tip of a drift tube generating an isotropic point source of low-energy X-rays. The source is permanently integrated into the treatment unit, and is calibrated daily and externally yearly. This system has been designed for single fraction IORT and is calibrated at a single dose rate and output factor. IORT is delivered using multiuse solid state spherical applicators of size ranges 1.5–5 cm diameter. Treatment time typically runs 20–45 min depending upon the applicator size used.

The Axxent® System (Xoft Inc., Sunnyvale, CA, USA) is an electronic brachytherapy machine in clinical use since 2009 (11). The radiation source is a miniature, electronic, high-dose rate low-energy X-ray tube integrated into a flexible multi-lumen catheter producing 40–50 Kv X-rays at the catheter tip. The Axxent system was originally designed for fractionated balloon-based partial breast irradiation using variable currents and voltages to allow for changes in dose rates or depths, and can also be used for single fractions. The source is disposable and used for up to 10 fractions, with disposable balloons of spherical and ellipsoidal sizes (3–6 cm spherical and 5–6 × 7 cm elliptical), over treatment times of 10–20 min. Both technologies can be used in a standard operating room with portable shielding only.

Mobile electron accelerators use electrons energies ranging from 3 to 12 MeV. The lumpectomy cavity of the breast is treated with a cone inserted intraoperatively (12). Electrons are more penetrating than low-energy X-rays, requiring breast tissue to be mobilized and for shields to be inserted into the posterior lumpectomy cavity in order to shield tissues inside the thorax. Doses of 20–21 Gy usually delivered at a low-electron energy for the measured depth. This technology is often notated as intraoperative electron radiation therapy (IOERT).

### IO(E)RT AS SOLE PARTIAL BREAST RADIATION

To date, there are two large prospective randomized trials published using breast IO(E)RT, the TARGIT-A trial and the ELIOT trial. TARGIT-A compared conventional WBRT (EBRT) to single dose IORT (TARGIT) and enrolled 3,451 patients from 33 centers in 10 countries between the years 2000 and 2012. This study used a non-inferiority statistical design which anticipated a 15% probability of adverse pathologic features on final pathology leading to additional WBRT after initial IORT (13). There were pre-specified strata described as IORT at the time of lumpectomy (pre-pathology), or IORT performed during a subsequent procedure at a different time (post-pathology).

Women enrolled to TARGIT-A were age 45 years or older with operable unifocal invasive ductal carcinoma. Per eligibility criteria pathologic findings requiring subsequent WBRT after IORT included positive excision margins, extensive intraductal component, or the presence of invasive lobular carcinoma. Participating centers could prospectively specify additional other factors. Overall, 22% of women enrolled in the pre-pathology and 3.6% of those in the post-pathology strata received additional WBRT after randomization to IORT. The majority enrolled had lower risk pathologic features, including ≤2 cm (87%), low to intermediate grade (85%), negative nodes (84%), estrogen receptor positive (93%), and mammographic detection (63%). With median follow-up of 2.5 years of the whole cohort and over 1,200 patients with 5-year median follow-up, for the primary endpoint of in-breast recurrence (IBR), the investigators reported 5-year IBR in the EBRT arm of 1.3%, and in the TARGIT arm of 3.3%, a 2% difference which was within the pre-specified 2.5% non-inferiority margin (*p* = 0.042). The impact of the timing of IORT was analyzed between the two strata. For pre-pathology stratum, IBR was 1% in the EBRT arm and 2.1% in the TARGIT arm (*p* = 0.31). For post-pathology stratum, IBR was 1.7% in the EBRT arm and 5.4% in the TARGIT arm (*p* = 0.069). These findings prompted the trialists to conclude that pre-pathology timing was more optimal. Overall survival was higher in the TARGIT arm (3.9%) compared with the EBRT arm (5.3%; *p* = 0.099), mainly due to higher rates of cardiopulmonary deaths. Toxicity comparisons including hematoma requiring treatment, post-operative infection, delayed wound healing, and all major toxicities were similar between the two arms.

The largest series outside of the TARGIT-A trial using 50 kV IORT has been reported by the TARGIT-R North American multi-institutional IORT retrospective registry trial (14). Nineteen institutions participated in this registry and reported outcomes on 822 women treated from 2007 through 2013 with minimum 6 months follow-up and a median follow-up of 2 years. Registrants were treated with IORT without (*n* = 537) or with WBRT (*n* = 110) or IORT as intended boost (*n* = 115). As is typical of APBI studies and registries, patients were mainly lower risk, or meeting American Society for Radiation Oncology (ASTRO) 2009 "suitable" criteria, with a median age of 67, <2 cm (90%), estrogen receptor positive (91%), Her2 non-amplified (89%), grade 1–2 (83%), without lymphovascular invasion (91%), and sentinel node negative (89%). Interestingly, 52% of registrants had a pre-operative breast MRI performed. Post-operative WBRT was recommended in 17% of IORT patients due to unfavorable pathologic findings, and 14% of registrants received IORT as a planned boost. A small number (*n* = 60) had delayed IORT as a second procedure rather than at the time of lumpectomy. Local IBRs were seen in 2.3% (*n* = 19 of 822), and axillary nodal recurrences in 0.2% (*n* = 2), at a median time to recurrence of 19 months. One death was attributed to breast cancer. Local recurrence by type of IORT was reported as follows: IORT alone, 2.4%; secondary IORT, 6.6%; IORT + WBRT, 1.7%; IORT boost, 1.8%. Thirteen of 19 local recurrences occurred >1 cm from the lumpectomy site. Several of these patients had higher risk features including ER negative tumors or positive sentinel nodes but elected not to undergo WBRT. Complications were low, including post-operative seroma in 9%, hematoma in 1.5%, and infection requiring antibiotics in 2.8%. The early results of this large retrospective registry are similar to those seen in the TARGIT-A randomized trial. There is also an ongoing prospective United States registry study, TARGIT-US (15), which should complete accrual this year.

The ELIOT study had a similar design to the TARGIT-A, but used mobile electron technology to deliver IORT. It was a single institution study completed by the Institute Milan. This trial randomized 1,305 women between the years 2000 and 2007 between external conventional whole breast irradiation (EBRT) (50 + 10 Gy boost) and single dose electron IOERT (21 Gy) with no additional WBRT (16). The study statistical design was an equivalence endpoint, with a pre-specified margin for local recurrence of 7.5% after IOERT. Women eligible for the study had stage I–II invasive breast cancer up to 2.5 cm in size between ages 48 and 75 years old. Lower risk tumors were pre-dominant, with patient characteristics including estrogen receptor positive in 90%, Her2 negative in 97%, and negative nodes in 74%. With median follow-up of 5.8 years, 5 years IBR was recorded in an unexpectedly low percentage in the EBRT arm (0.4%) as well as in the IOERT arm (4.4%), which was within the pre-specified equivalence margin of 7.5% (*p* < 0.0001). There was no difference in overall survival (96.8 and 96.9%, respectively; *p* = 0.59). Cutaneous toxicities were significantly better for all recorded endpoints in the IOERT arm, although a higher incidence of fat necrosis was seen after IOERT, with no overall differences in other side effects including breast fibrosis, retraction, pain, or burning. The ELIOT trialists concluded that the unselected population helped to define stricter selection criteria which could result in a lower IBR rate. They identified risk factors associated with local recurrence after IOERT as tumor size >2 cm, grade 3, 4, or more positive nodes, and triple negative histology.

The published randomized trials and large multi-institutional studies, provide guidance in selecting appropriate patients for breast IORT, and provide the basis for guidelines and consensus statements of selection criteria. The ASTRO consensus guideline was updated in 2017 to include a key question on breast IO(E) RT. The statement notes that IO(E)RT use should be restricted to women who otherwise meet "suitable" criteria for partial breast irradiation, and Coverage with Evidence Development on a registry or trial applies to low-energy X-ray IORT while awaiting longer follow-up on accrued clinical trials (17). The limited follow-up time was a major concern of this panel. There have been published commentary which disagree with some of the conclusions of the ASTRO consensus statement on IORT, including by a group representing other professional societies and IORT users, which highlighted inconsistencies in interpretation of the TARGIT-A data (18). TARGIT investigators advocate use of the study criteria and offering patients WBRT as was done on the study when pathologic features indicative of more diffuse disease are present on final pathology. Similarly, the ELIOT investigators discussed the potential for use of stricter selection criteria for IOERT than used in the trial, with initial use or inclusion of WBRT when those features are present on final pathology. With any type of IO(E)RT, radiation most commonly delivered at the time of surgery when final pathologic features are not yet available, therefore selection criteria should be based on the information available prior to as well as during surgery. Many surgeons and radiation oncologists prefer to have rigorous intraoperative assessment of sentinel nodes and margins to assist in decision making and patient selection.

### Toxicity and Cosmesis

In studies using 50 kV X-rays, several have reported acute and late toxicity profiles after IORT ± WBRT. In the original TARGIT-A publication, all clinically significant complications occurred in 3.3% or fewer patients, including hematoma or seroma requiring intervention, infection, wound healing, or any grade 3 toxicity, and were similar between IORT and WBRT arms. In the study update published in 2013, it was noted that complications at 6 months showed no difference between arms for any wound-related complications, with fewer grade 3–4 skin toxicities after IORT. Keshtgar reported cosmesis up to 4 years after IORT on a TARGIT-A subprotocol as assessed by photograph-analyzing software. IORT patients were about twice as likely to have excellent or good cosmetic scores as WBRT treated patients (19). One German institution examined their 48 TARGIT-A enrolled patients in a subgroup analysis of post-treatment mammogram findings (20). They noted a higher rate of radiographic fat necrosis after IORT (56%) than after conventional WBRT (24%), and more scar calcifications as well. Sperk et al. noted no differences between IORT ± WBRT versus WBRT with respect to fibrosis, breast edema, lymphedema, pain, or hyperpigmentation (21). Fibrosis was higher after IORT + WBRT (37.5%) than after IORT alone (6%) or WBRT only (18%) at 3 years. Telangiectasias were not seen in any IORT only patients, compared with 17% of women after WBRT ± IORT. The Copenhagen group conducted a subgroup analysis of post-treatment pain in their enrolled TARGIT-A cohort (*n* = 244) conducted using patient reported outcomes data, and found that persistent pain was reported in 34% of WBRT patients compared with 25% of IORT patients (22).

In studies using electron IOERT, the ELIOT trial reported on the subset of patients in whom toxicity data were available, noting that side effect profiles significantly favored IOERT compared with WBRT, especially as related to skin toxicity, with less erythema, hyperpigmentation, dryness, and pruritis (all *p* < 0.04). No differences were recorded for fibrosis, pain, or burning sensation. Only radiologic presence of necrosis was higher in the IOERT group. The ELIOT group randomly selected 119 patients treated with sole IOERT for further late toxicity assessment using standard rating scales (23). At a median of 6 years, grade 2 fibrosis was noted in 32% and grade 3 fibrosis in 6%. Excellent or good cosmesis was scored by patients in 77% and by physicians in 84%. The Netherlands group compared institutional data for women treated with IOERT (*n* = 26) or conventional WBRT (*n* = 45) based on seven asymmetry features (24). Features favoring IOERT with smaller differences between treated and untreated breast included breast contour, relative breast area, and breast overlap. Excellent and good cosmesis after IOERT was scored as 88% by patients and 96% by physicians.

### Quality of Life

Increasingly, study of quality of life accompanies more traditional outcomes endpoints and influences patient preferences and informed consent discussions for patients and their providers facing breast cancer treatment. Investigators in Australia conducted a survey of Western Australia breast cancer health professionals and reported that among those surveyed, 3–7.5% considered breast IORT unacceptable treatment at any risk of local recurrence, 18–21% considered IORT acceptable at risks equivalent to that of WBRT, while 56–59% considered IORT acceptable if associated with a 1–3% increased local recurrence risk (25). In a survey of patient preference considering treatment options of breast IORT or fractionated multi-week whole breast irradiation which described to participants alternative increases in rates of local recurrence risk over 10 years, this survey found that patients accepted a median increase in local recurrence risk for IORT of 2.3% (26). In addition, 91% surveyed would accept IORT if the treatment were equivalent or associated with a slightly higher risk of local recurrence compared with WBRT. Another quality of life study among a subgroup of German patients enrolled in the TARGIT-A trial administered the EORTC QLQ-C30 and BR23 survey instruments (27). Patients in the IORT alone arm indicated significantly less general pain, breast, and arm symptoms and better overall functioning than patients in the WBRT arm.

## BREAST IO(E)RT IN OTHER CLINICAL CONTEXTS

Intraoperative radiotherapy, as with all types of APBI, is less well studied in conjunction with breast-conserving surgery for ductal carcinoma *in situ* (DCIS). The recent update of the ASTRO APBI consensus statement now classifies APBI for pure DCIS as "suitable" when meeting certain specific criteria. A California group has published the only series to date using 50 kV IORT for patients with DCIS. In this series, selection criteria included tumor size <4 cm on pre-operative imaging with pure DCIS on biopsy and deemed resectable with breast conservation (28). Thirty-five patients had IORT, with a mean tumor size of 1.5 cm. Five patients had close or positive margins, two of whom had mastectomy due to extent of DCIS in the specimen, and three had re-excision followed by WBRT, for a 14% rate of additional treatment after surgery plus IORT. The 3-year local recurrence rate was 5.7%.

A study from the Milan group used IOERT in conjunction with nipple-sparing mastectomy, comparing 800 patients receiving IOERT to the retroareolar region of the nipple to 201 patients with nipple-sparing mastectomy followed by delayed one-dose radiation later (29). At median follow-up of 20 months, they noted nipple-areolar necrosis in 3.5%, and nipple removal in 5%. Of the 1.4% local recurrences, none were seen in the nipple, but at the site of the primary tumor. No difference in outcomes was noted between the two techniques. This report does not discuss any comparison with patients who have not received any radiation to the nipple-areolar complex. A second series using 50 kV IORT single dose of 16 Gy after nipple-sparing mastectomy describes only seven patients with 7 months follow-up, with no acute toxicity attributable to radiation, no necrosis of the nipple complex or local recurrences (30).

In these special clinical scenarios, further data are needed to define the appropriate role of IORT. The direct visualization and elimination of the possibility of marginal miss makes IORT an attractive technique in situations where accuracy of dose targeting is particularly critical, such as women with higher risk cancers, as being investigated in the TARGIT-B trial of IORT versus conventional boost. Areas for further study of the efficacy and toxicity of IORT may include as sole therapy for lower risk DCIS, as a boost prior to planned oncoplastic reconstruction, and as part of re-treatment after prior whole breast irradiation for limited local recurrence.

### IO(E)RT BOOST

A boost dose to the periphery of the surgical lumpectomy bed has been shown to further lower the risk of local recurrence, especially for younger women, women with higher grade, triple negative or larger tumors, and those with positive margins or extensive lymphovascular invasion (31). The tissue in closest proximity to the primary cancer has the highest density of residual microscopic cells and is therefore at highest risk for local recurrence. Several institutional studies have reported using IORT as a boost with planned WBRT to follow. The theoretical advantages of using IORT as a boost include the ability to directly visualize the tumor bed and thereby avoid marginal misses when boosting a CT-based volume. The same BED, oxygenation, and biological advantages theoretically present for single dose IORT may be relevant for IORT boost as well, and are under investigation. Several studies using either IOERT (intraoperative electrons) or 50 kV IORT as a boost have been reported. Ongoing studies going IORT boost include the TARGIT-B(oost) (32) and HIOP trials (33).

Intraoperative electron radiation therapy boost has been reported as a pooled analysis by the International Society of Intraoperative Radiotherapy (34). In this analysis, 1,109 unselected patients from seven European centers, 60% of whom had at least one high risk factor, were treated similarly with IOERT boost at a median dose of 10 Gy and a subsequent whole breast dose of 50–54 Gy. After a median follow-up of 5 years, the local recurrence risk was 0.8%, half seen in the index quadrant. Risk factors for recurrence included high grade, age under 40, and ER negative. Upon examining the impact of delays from IOERT boost to WBRT, no impact on local recurrence of delays up to 140 days was seen. The Salzburg IOERT group conducted a matched-pair analysis of IOERT boost and external electron boost patients, who had IBR rates at 10 years of 1.6 and 7.2%, respectively (35).

Low-energy X-rays IORT as a boost has been reported in two cohort series. One multicenter pilot study treated with 20 Gy to cavity surface intraoperatively followed by 45–50 Gy whole breast in 299 women undergoing lumpectomy. After a median followup of 5 years, the observed local recurrence rate was 2.7% (36). A single institution series of 197 patients received an IORT boost of 18–20 Gy then 46–50 Gy whole breast, reporting a 5-year local relapse free survival of 97% (37).

TABLE 1 | Prospective randomized controlled trials of partial breast irradiation compared to whole breast radiation.


*PBI, partial breast irradiation; MC, multicatheter; Brachy, brachytherapy; WBRT, whole breast radiation; IORT, intraoperative radiotherapy; IOERT, intraoperative electron radiotherapy; IMRT, intensity modulated radiotherapy; F/U, follow-up; ER, estrogen receptor; pTis, pathologic ductal carcinoma in situ; NR, not reported.*

Regardless of technique the toxicity of IORT boost appears to be acceptable. The virtual complete skin sparing associated with use of IORT is likely to have a positive impact on any cutaneous toxicity profiles. Acutely there are no reports of increased post-operative infections or delayed wound healing. Late fibrosis has been reported to range from about 0 to 15% for grade 3 toxicity, depending upon technique and dose. Cosmesis does not seem to be compromised in the studies described when compared with conventional boost techniques, although assessment tools have varied. Lemanski has reported the longest term experience in using IOERT boost, reporting 9-year outcomes on 50 women receiving 10 Gy IORT then 50 Gy whole breast, with no grade 3 fibrosis and 14% grade 2 fibrosis (38). Mayo Arizona conducted a prospective study of 10 Gy IOERT then 48 Gy to the whole breast, and reported a 3.8% 6-year local recurrence rate, with excellent or good cosmesis in 87% of patients (39). An Australian group reported that 55 patients treated with 5 Gy at 1 cm IORT boost then 50 Gy whole breast had no local recurrences at 3 years, but grade 3 fibrosis was 15% (40). There is one report of IOERT boost (12 Gy) followed by hypofractionated whole breast (37.05 Gy in 13 daily fractions of 2.85 Gy) in 204 pre-menopausal women. Reporting only acute toxicity, grade 3 skin toxicity was 4%, grade 2 skin toxicity was 29%; late skin toxicity at 12 months was grade 3 and 4 in one patient each (41).

For context in comparison, the EORTC boost trial, the 5- and 10-year rates of moderate to severe fibrosis was reported in 11 and 28% of boost patients compared with 10 and 13% on the no boost arm. In this study, at 3 years, excellent and good cosmesis was somewhat worse in the boost arm compared with no boost, 71 versus 86%, respectively (42, 43).

There are several reports of using IORT boost in conjunction with breast-conserving surgery and WBRT in special patient cohorts. One retrospective series reported on IORT boost after neoadjuvant chemotherapy in 61 patients and a contemporaneous consecutive cohort of 55 patients who received a conventional external beam boost, all receiving WBRT (44). At 4 years follow-up, there was no difference in 5-year local recurrence (9.8% with IORT and 8.3% with external boost), and there was a better overall survival in the IORT arm (97 versus 82%) related to fewer non-breast cancer deaths. The Salzburg group has also reported IORT boost in a retrospective series of triple negative patients undergoing breast conservation, with an 8-year actuarial local recurrence rate of 11%, all of those recurrences occurring in high-grade cancers (45).

A German group has reported on IORT boost in the setting of oncoplastic reconstruction. This is a particularly appealing clinical scenario for the use of an IORT boost, as the oncoplastic reconstruction which immediately follows the definitive oncologic surgery to remove the cancerous cells can eradicate any clear delineation of the tumor bed and preclude the use of external beam boost, causing some patients to be underdosed. Performing IORT after the lumpectomy but prior to the oncoplastic rearrangement eliminates the risk of target volume miss, the possibility of dissemination of microscopic disease during the reconstruction, or inability to identify the boost target on post-operative treatment planning image sets. However, minimal data exist to support the efficacy of this approach. The Cologne group has used IORT boost in 149 patients who also underwent an oncoplastic reconstruction (glandular rotation or mammoplasty), and have reported only post-operative toxicity, with a 2% seroma formation rate. Additional outcome and efficacy data from this and other series will be welcome (46).

### CONCLUSION

Breast IO(E)RT is currently primarily a technique for partial breast irradiation which has been well established as an option for patients who are otherwise appropriate candidates for APBI. Two large randomized trials, the TARGIT A trial establishing 50 kV IORT and ELIOT establishing electron based IOERT, both have published excellent results regarding local control and acceptable toxicity. While local recurrence was slightly higher after IORT in both studies, it was within the clinically relevant range seen in numerous other clinical trials of various radiation techniques, including APBI and WBRT approaches (see **Table 1**). Patient preference analyses have shown that some women will accept the risk of a small increase in local recurrence in order to preserve their breast, to mitigate toxicity or to reduce the burden of lengthier radiation treatment courses. Overall survival in TARGIT-A was significantly better in the IORT group, an intriguing observation that warrants additional investigation. It is appropriate for these considerations to be discussed with patients as part of the clinical decision making and informed consent process for radiation treatment.

When considering use of IO(E)RT techniques for partial breast treatment after lumpectomy, it is recommended to select patients who fall into the low-risk categories among published guidelines, using the "suitable" or "good risk" criteria for patients who are general candidates for APBI. IO(E)RT has potential advantages over external or brachytherapy-based techniques given the direct visualization of and contact with the target tissue and the immediacy of treatment, but has the disadvantage of lacking final pathologic assessment of the margins and sentinel nodes, placing a percentage of women at risk of being recommended to undergo additional external beam irradiation. Therefore, when intended to be used for partial breast treatment, patient selection should focus on clinicopathologic factors predictive of negative nodes and negative margins. Careful assessment of pre-operative mammographic and other imaging studies for features, such as extent

### REFERENCES


of calcifications, may be helpful. Intraoperative techniques can be useful as well, including assessment of margins and sentinel nodes intraoperatively, and careful excision technique to maximize clear margins, such as taking additional shave margins as needed.

Potential emerging indications for breast IO(E)RT under investigation include as a boost in higher risk patients, as a boost in patients undergoing oncoplastic reconstruction, as sole therapy for lower risk DCIS, and as re-treatment for IBR after prior breast irradiation. Longer term follow-up on the completed trials is anticipated and ongoing studies and registries will help define these new modalities.

### AUTHOR CONTRIBUTIONS

EH wrote manuscript, created table, completed literature review. WS wrote sections of manuscript and edited entire manuscript.

### ACKNOWLEDGMENTS

The authors would like to acknowledge Ms. Pamela Simons for her assistance in editing this manuscript.

radiation using low-kilovoltage X-rays for breast cancer. *Ann Surg Oncol* (2016) 23:2809–15. doi:10.1245/s10434-016-5240-1


results in breast-conserving therapy. *Strahlenther Onkol* (2016) 192:705–13. doi:10.1007/s00066-016-1029-1


**Conflict of Interest Statement:** EH and WS are both members of the TARGIT-US Scientific Steering Committee, which is not associated with any financial remuneration. EH received travel funds from the TARGIT Collaborative Group to give an invited lecture in 2017. WS has received travel reimbursement from and is on a speaker's bureau for Carl Zeiss Meditec.

*Copyright © 2017 Harris and Small. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) or licensor are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.*

# Intraoperative Radiation for Breast Cancer with Intrabeam**™**: Factors Associated with Decreased Operative Times in Patients Having IORT for Breast Cancer

*Stephanie A. Valente1 , Alicia Fanning1 , Robyn A. Stewart1 , Sharon Grundfest1 , Rahul D. Tendulkar2 , Sheen Cherian2 , Chirag Shah2 , Chao Tu3 , Courtney Yanda1 , Diane Radford1 , Zahraa Al-Hilli1 and Stephen R. Grobmyer1 \**

#### *Edited by:*

*William Small, Jr., Stritch School of Medicine, United States*

#### *Reviewed by:*

*John C. Roeske, Loyola University Medical Center, United States Eric D. Donnelly, Northwestern Memorial Hospital, United States*

> *\*Correspondence: Stephen R. Grobmyer grobmys@ccf.org*

#### *Specialty section:*

*This article was submitted to Radiation Oncology, a section of the journal Frontiers in Oncology*

*Received: 13 April 2017 Accepted: 14 September 2017 Published: 09 October 2017*

#### *Citation:*

*Valente SA, Fanning A, Stewart RA, Grundfest S, Tendulkar RD, Cherian S, Shah C, Tu C, Yanda C, Radford D, Al-Hilli Z and Grobmyer SR (2017) Intraoperative Radiation for Breast Cancer with Intrabeam™: Factors Associated with Decreased Operative Times in Patients Having IORT for Breast Cancer. Front. Oncol. 7:237. doi: 10.3389/fonc.2017.00237*

*1Division of Breast Services, Department of General Surgery, Cleveland Clinic, Cleveland, OH, United States, 2Department of Radiation Oncology, Cleveland Clinic, Cleveland, OH, United States, 3Quantitative Health Sciences, Cleveland Clinic, Cleveland, OH, United States*

Introduction: Intraoperative radiation with Intrabeam™ (IORT) for breast cancer is a newer technology recently implemented into the operating room (OR). This procedure requires time and coordination between the surgeon and radiation oncologist, who both perform their treatments in a single operative setting. We evaluated the surgeons at our center, who perform IORT and their OR times to examine changes in OR times following implementation of this new surgical procedure. We hypothesized that IORT is a technique for which timing could be improved with the increasing number of cases performed.

Methods: A prospectively maintained IRB approved database was queried for OR times (incision and close) in patients who underwent breast conserving surgery (BCS), sentinel lymph node biopsy with and without IORT using the Intrabeam™ system at our institution from 2011 to 2015. The total OR times were compared for each surgeon individually and over time. Next, the OR times of each surgeon were compared to each other. Continuous variables were summarized and then a prediction model was created using IORT time, OR time, surgeon, and number of cases performed.

Results: There were five surgeons performing IORT at our institution during this time period with a total of 96 cases performed. There was a significant difference observed in baseline surgeon-specific OR time for BSC (*p* = 0.03) as well as for BCS with IORT (*p* < 0.05), attributable to surgeon experience. The average BCS times were faster than the BCS plus IORT procedure times for all surgeons. The overall mean OR time for the entire combined surgical and radiation procedure was 135.5 min. The most common applicator sizes used were the 3.5 and 4 cm, yielding an average 21 min IORT time. Applicator choice did not differ over time (*p* = 0.189). After adjusting for IORT time and surgeon, the prediction model estimated that surgeons decreased the total BCS plus IORT OR time at a rate of −4.5 min per each additional 10 cases performed.

Conclusion: Surgeon experience and applicator size are related to OR times for performing IORT for breast cancer. OR time for IORT in breast cancer treatment can be improved over time, even among experienced surgeons.

Keywords: intraoperative radiation therapy, surgery, operation, time, applicator, experience, radiation oncology, breast surgery

### INTRODUCTION

Adjuvant intraoperative radiation with the Intrabeam™ system (IORT) is an excellent therapeutic option for selected patients with breast cancer. Evidence to support the use of IORT comes from the TARGIT-A randomized trial showing that for early stage breast cancer, risk of local recurrence with IORT performed at the time of lumpectomy surgery is not statistically different than whole breast radiation (WBRT) (2.1% with IORT compared to 1.1% with WBRT, *p* = 0.31) (1, 2). This was confirmed with the large North American TARGIT-Retrospective Study supporting similar low local recurrence rates and showing utilization of IORT for breast cancer treatment is growing in North America and globally (3).

In the treatment of breast cancer, IORT has been shown to have many advantages for patients including convenience, improved quality of life (4), and lower cost compared to more traditional treatments (5). However, the use of IORT as adjunct for breast cancer treatment does prolong operating room (OR) times for patients having breast-conserving surgery. OR time is a costly and precious resource within health-care systems (6). There is increasing pressure on value-based health-care delivery and efficient care (7). Little is known about the impact of performing IORT with Intrabeam™ on OR times and factors associated with decreased operative times (8).

We hypothesized that the time to perform IORT in combination with breast conserving surgery (BCS) can be decreased with the increasing number of cases performed. In this study, we sought to document OR times associated with performance of BCS and IORT and analyze factors associated with operative time. This information is critical for assigning appropriate resources, OR allocations, and for optimizing efficient use of ORs within centers offering IORT for the treatment of breast cancer.

### MATERIALS AND METHODS

A prospectively maintained database was queried for OR times (incision and close) in patients who underwent lumpectomy, sentinel lymph node biopsy with and without IORT using the Intrabeam™ system (Carl Zeiss AG: Oberkochen, Germany) at our institution from 2011 to 2015. Only IORT cases performed as a unilateral procedure at the time of initial lumpectomy were included. OR time was defined from incision to closure in this study. OR times included combined performance of lumpectomy, sentinel node biopsy, and IORT. In our practice, frozen section analysis was performed on axillary sentinel nodes in patients having IORT. The size applicator used for IORT can vary at our institution from 2.5 to 5 cm. Surgeons choose an applicator size based on the size of the lumpectomy cavity, which is a result of the size tumor removed. IORT was performed by 1 of 2 radiation oncologists who were present in the OR for these procedures. IORT delivery time was calculated based on the size applicator used.

Surgeons who performed conserving surgery (BCS) operation with IORT at our institution during this time period were identified. The IORT applicator sizes used and total OR times for all the identified surgeons were then analyzed. As a baseline control for OR time, surgeons individual OR times for BCS alone during the study period were averaged and compared. Then, their individual OR times for BCS with IORT were analyzed and compared. Next, the OR times for each surgeon who had performed greater than 10 IORT cases during the study period were evaluated. Statistical analysis was performed using the Kruskal–Wallis test and repeated measures ANOVA. Continuous variables were summarized and then a prediction model was created using IORT time, OR time, surgeon, and number of cases performed. Since IORT time is a standard prescribed time based on the size applicator used, this was controlled for in the prediction model. A *p*-value < 0.05 was considered statistically significant. This study was carried out in accordance with the recommendations of the Cleveland Clinic Foundation Institutional Review Board, under which it was reviewed and approved. A waiver of informed consent was granted, as all patient data were de-identified.

Table 1 | Applicator sizes used for IORT cases with Intrabeam™ in this series.


Table 2 | Operating room times for five different surgeons performing BCS IORT with Intrabeam™ in this series.


*The surgeon with prior significant experience performing IORT.*

*a*

associated with increasing surgeon number of cases with IORT using Intrabeam™ system. Every surgeon decreased OR time by −4.5 min for each 10 cases performed.

### RESULTS

There were five surgeons identified performing IORT at our institution during this time period with a total of 96 cases performed. The most common applicator sizes used were the 3.5 cm (33%) and the 4 cm (36%), yielding an average 21-min IORT time (**Table 1**). Applicator size did not differ over time (*p* = 0.189). Longer OR times were significantly associated with use of larger applicator sizes, as a longer time is required to deliver the prescribed dose of radiation (*p* < 0.0001).

The overall mean OR time for all surgeons for the entire combined BCS and IORT procedure was 135.5 min (102–173 min). There was a significant difference observed in surgeon specific mean OR time for BCS with IORT (131 vs 185 min, *p* = 0.03) (**Table 2**). Only one of the five surgeons had significant prior experience with performing IORT (Surgeon E). This surgeon with the greatest IORT experience had the lowest mean operating time (131 min), which was significantly lower than the surgeon with least IORT experience (185 min, *p* = 0.02).

For the purpose of prediction modeling, only surgeons with at least 10 operations performed were used, giving a total of three surgeons evaluated in this analysis (**Figure 1**). First, a control group was created using the BCS only OR times for the three surgeons during 2011–2015. **Figure 1A** shows that at baseline, the three surgeons did differ significantly in their average overall BCS only OR times [(Surgeon C 86.8 min, D 95.2 min, E 101.8 min), *p* < 0.001] and this difference was maintained over the study period without significant improvement in OR time over time. Next, BCS with IORT times for the three surgeons were analyzed (**Figure 1B**). This demonstrates that there were baseline differences among surgeons in total OR time for BCT with IORT (131 vs 176 min, *p* < 0.001). Surgeon E, with the prior IORT experience, had the fastest average BCS with IORT time (131 min). Similar to the BCS only procedure, the surgeon's individual OR time differences for BCS with IORT were maintained over time. Interestingly, for BCS with IORT, despite differences in individual OR times, all surgeons (C, D, and E) regardless of prior IORT OR experience were found to have significantly decreasing operating time with an increasing number of cases performed over the study period (*p* = 0.02). After adjusting for IORT time and surgeon, the prediction model estimated that each surgeon decreased the total OR time for BCS with IORT at a rate of −4.5 min per each additional 10 cases performed.

### DISCUSSION

This study documents that total OR time with IORT Intrabeam™ does decrease with increasing experience with the technique. The decreased time is likely related to surgeons and operating teams becoming more familiar with the procedure and improved

### REFERENCES


coordination of the required steps to complete the breast conserving surgery, sentinel node biopsy, and IORT. It is noteworthy that even a highly experienced IORT surgeon (**Table 2**, Surgeon E) can improve OR times as experience with an operating team and radiation oncology team grows. The reported times in this series compare favorably to operating times reported utilizing an alternative form of IORT, which was reported to be 140 min (9). We hypothesize that while the delivery of radiation is longer with Intrabeam™ compared to other systems, the reproducible simplicity of setup and cavity preparation of Intrabeam™ accounts for the lower overall observed OR times with Intrabeam™.

We did not investigate the specific time of each step as it relates to the observed decrease in operative times. However, there are several specific areas in the procedure where efficiencies can be realized to decrease total operation time such as: surgeon technique including performance of intraoperative radiation and purse-string suture placement; equipment setup and having the proper equipment in the OR by OR staff (shielding drapes, ultrasound, etc.); and coordination of the arrival of the radiation oncologist and physicist to coincide with the start of the IORT portion of the case.

In this series, all patients had frozen section performed as part of the procedure as our programmatic approach during the time of the study was to not perform IORT in patients with axillary metastasis. There is a potential to reduce OR times further if this step was eliminated and patients were treated with IORT regardless of nodal status. IORT has been shown to be an effective boost replacement for patients requiring WBRT therapy (10). Other opportunities to improve OR times include: having standard operating teams and nursing teams who are familiar with the procedure, use of anticipatory paging of radiation oncology team members to avoid delays waiting for their arrival, and performing procedures at standard time and in a standard location.

The data in this report are important, as this is the first documentation of time associated with the performance of BCS and IORT using the Intrabeam™ system. These data can be used for planning operating time and resource allocation and serve as a benchmark for planning operating days for new teams adopting the Intrabeam™ system. Importantly, this series shows us that surgeons and treatment teams can become more efficient over time with IORT delivery as experience with the technique grows.

### AUTHOR CONTRIBUTIONS

Each author has collaborated together to make contributions to conception and design, with the acquisition of data, and the analysis and interpretation of data. Each author has offered revisions and input on the manuscript. Each author gave their final approval of the submitted manuscript.

radiotherapy for breast cancer: 5-year results for local control and overall survival from the TARGIT-A randomised trial. *Lancet* (2014) 383:603–13. doi:10.1016/S0140-6736(13)61950-9

3. Valente SA, Tendulkar RD, Cherian S, O'Rourke C, Greif JM, Bailey L, et al. TARGIT-R (retrospective): north American experience with intraoperative radiation using low-kilovoltage X-rays for breast cancer. *Ann Surg Oncol* (2016) 23:2809–15. doi:10.1245/s10434-016-5240-1


to deliver intraoperative radiation therapy in the treatment of early-stage breast cancer. *Int J Radiat Oncol Biol Phys* (2015) 93:E24–5. doi:10.1016/ j.ijrobp.2015.07.607

10. Vaidya JS, Baum M, Tobias JS, Wenz F, Massarut S, Keshtgar M, et al. Long-term results of targeted intraoperative radiotherapy (Targit) boost during breast-conserving surgery. *Int J Radiat Oncol Biol Phys* (2011) 81:1091–7. doi:10.1016/j.ijrobp.2010.07.1996

**Conflict of Interest Statement:** ZEISS Meditech provides travel and education funding for SG. All other authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

*Copyright © 2017 Valente, Fanning, Stewart, Grundfest, Tendulkar, Cherian, Shah, Tu, Yanda, Radford, Al-Hilli and Grobmyer. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) or licensor are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.*

*Christine Chin1 , Priya Jadeja2 , Bret Taback2 , David P. Horowitz1 , Sheldon M. Feldman3 , Richard Ha4 and Eileen P. Connolly1 \**

*1Radiation Oncology, NYP-Columbia University Medical Center, New York, NY, United States, 2Breast Surgery, NYP-Columbia University Medical Center, Herbert Irving Pavilion, New York, NY, United States, 3Breast Surgery and Breast Surgical Oncology, Montefiore M-E Center for Cancer Care, Bronx, NY, United States, 4Radiology, NYP-Columbia University Medical Center, Herbert Irving Pavilion, New York, NY, United States*

#### *Edited by:*

*William Small Jr., Stritch School of Medicine, United States*

### *Reviewed by:*

*Andrew O. Wahl, University of Nebraska Medical Center, United States John Austin Vargo, West Virginia University Hospitals, United States*

#### *\*Correspondence:*

*Eileen P. Connolly epc2116@cumc.columbia.edu*

#### *Specialty section:*

*This article was submitted to Radiation Oncology, a section of the journal Frontiers in Oncology*

*Received: 28 March 2017 Accepted: 31 July 2017 Published: 28 August 2017*

#### *Citation:*

*Chin C, Jadeja P, Taback B, Horowitz DP, Feldman SM, Ha R and Connolly EP (2017) Evaluation of Partial Breast Reirradiation with Intraoperative Radiotherapy after Prior Thoracic Radiation: A Single-Institution Report of Outcomes and Toxicity. Front. Oncol. 7:175. doi: 10.3389/fonc.2017.00175*

**33**

Keywords: breast, accelerated partial breast irradiation, intraoperative radiation therapy, recurrence, reirradiation, toxicity

## INTRODUCTION

Based on a number of large randomized trials, the estimated 10-year rate of an isolated ipsilateral breast tumor recurrence (IBTR) after breast-conserving therapy is approximately 10% (1, 2). At the time of local recurrence the current standard of care is salvage mastectomy given the unacceptable toxicity to normal tissues with repeat whole breast irradiation (WBI). Local excision alone of an IBTR results in subsequent local recurrence in approximately 35% of patients based on retrospective series (3–8). Interestingly, in women who develop late local recurrences more than 5 years after treatment of their primary disease, disease-free, and overall survival is not significantly different compared to women who do not experience an IBTR (4, 9, 10). The prolonged interval until recurrence reflects a favorable tumor biology, and retrospective studies report no difference in survival between patients who undergo salvage mastectomy and breast-conserving surgery (BCS) for small, localized recurrences (10, 11). Given that many IBTRs are detected early on surveillance-imaging, many patients desire a breast-conserving option at the time of recurrence. Breast-conserving therapies have therefore become increasingly popular in treating these patients.

Accelerated partial breast irradiation (APBI) is a novel technique that offers the opportunity to limit radiation dose to previously irradiated breast tissue while improving rates of local control after BCS (12–16). There have been a limited number of small retrospective and prospective studies examining the use of APBI after local excision of an IBTR in the setting of prior radiation using various dose fractionations and delivery techniques (17–23). The use of intraoperative radiation therapy (IORT) to deliver a single, high-dose radiation to the lumpectomy surface at the time of surgery has been compared to adjuvant whole breast radiotherapy in the treatment of unifocal, early-stage breast cancers with non-inferior results (13). Its use in the setting of reirradiation has been reported with acceptable toxicity and cosmesis in small retrospective studies (13, 24, 25). Here, we report our single-institution experience with partial breast reirradiation (PBrI) with IORT after BCS in patients who decline salvage mastectomy.

## MATERIALS AND METHODS

### Patient Eligibility

We performed a retrospective review approved by the Columbia University Institutional Review Board of 228 patients treated with breast IORT between September 1, 2013, and November 31, 2016. Written informed consent was obtained from research participants. Patients were included in this study if they had developed a unifocal IBTR or new primary breast cancer (PBC) in the setting of prior WBI for early-stage breast cancer or a PBC after definitive thoracic radiation for another primary malignancy and declined salvage mastectomy. IBTR was defined as a breast tumor recurrence with the same histology and location as the initial PBC.

### Radiation Treatment

Intraoperative radiotherapy was delivered using the Intrabeam™ device (Carl Zeiss, Oberkochen, Germany) at the time of lumpectomy. A spherical applicator was chosen at the radiation oncologist and operating surgeon's discretion to most appropriately fit the lumpectomy cavity. Ultrasound was used to confirm a minimum skin to applicator margin of at least 10 mm. A medical physicist was present to confirm delivery of 20 Gy to the surface of the lumpectomy cavity using 50 kV X-rays.

### End Point Analysis

Patients were encouraged to follow-up at 2 weeks after surgery and at least every 6 months for the first year after treatment. End points of local control, mastectomy-free survival, distant metastasis, and overall survival were determined from the time of IORT. Acute and long-term side effects including breast pain, dermatitis, fibrosis, seroma, and infection were graded according to the National Cancer Institute Common Terminology Criteria for Adverse Events (CTCAE) version 4.0.

## RESULTS

### Patient and Primary Tumor Characteristics

The median age of our patients at the time of recurrence was 65 years old (range: 52–85 years). The median time from prior external beam radiation to biopsy-proven recurrence was 18 years (range: 2 months to 46 years). Nine patients developed an IBTR or new PBC after prior WBI for early-stage breast cancer, 2 patients with a new PBC after mantle-field radiation for Hodgkin's lymphoma, and 1 patient with a synchronous diagnosis of stage III non-small cell lung cancer (NSCLC) treated with definitive radiation to the ipsilateral lung and mediastinum. Only one patient with the synchronous diagnosis of stage III NSCLC and breast cancer underwent breast IORT at an interval of less than 1 year from the time of prior definitive radiation. Further available details of patient primary tumor and treatment characteristics are presented in **Table 1**.

## Pathologic Findings

Based on final pathology, the median tumor size was 0.55 cm (range: 0–3.9 cm). All patients with invasive primaries were hormone receptor positive, defined as ≥1% staining of the estrogen

#### Table 1 | Primary tumor and treatment characteristics.


*IDC, invasive ductal carcinoma; ER, estrogen receptor; PR, progesterone receptor; NSCLC, non-small cell lung cancer; IMRT, intensity modulated radiation therapy; CMF,* 

*cyclophosphamide, methotrexate, fluorouracil; ddAC, dose dense doxorubicin, cyclophosphamide; AT, doxorubicin, paclitaxel.*

*a Patient was treated with neoadjuvant chemotherapy and definitive radiation therapy for her initial primary. bTherapy given in the neoadjuvant setting. bRight breast mean dose 10.9 Gy, max point dose 59.6 Gy.*

Table 2 | Patient treatment characteristics.


*IBTR, ipsilateral breast tumor recurrence; SBC, second breast cancer defined as an ipsilateral breast cancer of a different histology from the initial primary breast cancer; PBC, primary breast cancer in the setting of another prior malignancy; AI, aromatase inhibitor; TC, docetaxel and cyclophosphamide.*

*a Patient 3 had no residual disease at the time of lumpectomy.*

*bInterval time from prior diagnosis to biopsy-proven breast recurrence.*

and/or progesterone receptor by immunohistochemistry (26). Two patients were HER2/neu positive per American Society of Clinical Oncology/College of American Pathologists (ASCO/CAP) guideline recommendations (27). One of two patients with ductal carcinoma in situ (DCIS) was hormone receptor negative. Five patients underwent axillary sampling at the time of surgery for their IBTR. One patient was found to have involved sentinel lymph nodes and underwent completion of an axillary lymph node dissection. All patients with invasive primaries were found to have negative margins with no tumor on the final inked margin. One patient with DCIS was found to have a final close margin of 1 mm with no further re-excision. A summary of these findings is shown in **Table 2**.

### IORT Details

The median applicator size used in our cohort of patients was 3.0 cm (range: 3.0–3.5 cm). All patients were prescribed 20 Gy to the lumpectomy surface using 50 kV X-rays. The median treatment

time was 24.2 min (range: 17.1–24.7 min). The median applicator to skin distance was superiorly 15.3 mm (range: 5.1–23.0 mm), inferiorly 13.9 mm (range: 8.3–21.9 mm), medially 19.4 mm (range: 11.0–29.7 mm), and laterally 13.9 mm (range: 6.3–22.1 mm).

### Adjuvant Systemic Therapy

All patients with hormone receptor-positive disease were recommended adjuvant hormonal therapy. All patients were compliant with therapy except three patients due to intolerance. One patient with disease metastasis to the axilla was treated with adjuvant docetaxel and cyclophosphamide chemotherapy. Two patients with HER2-positive invasive disease received adjuvant trastuzumab therapy.

### Outcomes and Toxicity

At a median follow-up of 14 months (range: 4–25 months) there were no events of local or distant recurrence, and all women were able to preserve their breast. There was a single non-breast cancer-related death due to heart failure. In the acute setting, we observed grade 1 dermatitis in 25% (*n* = 3), grade 1 breast pain in 8% (*n* = 1), grade 1 seroma in 25% (*n* = 3), grade 2 seromas requiring drainage in 17% (*n* = 2), and grade 2 infection in 8% (*n* = 1). No grade 3 or higher acute toxicity was observed. In the late setting, defined as 3 months after treatment with IORT, we observed grade 1 hyperpigmentation and/or fibrosis in 50% (*n* = 6), grade 1 seromas in 8% (*n* = 1), persistent grade 2 seromas in 33% (*n* = 4), and grade 2 infection in 17% (*n* = 2). There was a single patient who developed a grade 3 abscess requiring hospitalization and intravenous antibiotic therapy. There were no grade 2 or higher toxicity for the patient who underwent breast IORT within a year of prior definitive RT. Details of patient toxicity regarding breast seroma formation and infection are presented in **Table 3**.

### DISCUSSION

In our single-institution experience, all 12 of our patients were able to successfully undergo local excision of their breast cancer followed by IORT with acceptable toxicity and preserve their breast. After a median follow-up of 14 months, there were no events of local or distant recurrence. In the late setting after treatment, we observed a significant incidence of persistent grade 2 seromas requiring drainage and antibiotic therapy. In the current literature, there are conflicting accounts of post-APBI seroma formation using various brachytherapy techniques. Evans et al. reported persistent seromas in about 75% of patients (greater than 6 months after treatment) treated with APBI using MammoSite®. Evaluating various dosimetric, clinical, and treatment-related variables, higher body weight was the only significant variable that correlated positively with seroma formation (28). On the contrary, Kraus-Tiefenbacher reported no difference in the rate of palpable seromas between patients who underwent BCS with or without IORT at the time of lumpectomy. While radiographically detected seromas were higher in the IORT group (81 vs. 52%, *p* < 0.001), the rate of palpable clinically significant seromas was not different. In their analysis, the addition of adjuvant chemotherapy correlated with higher rates of seromas detected on follow-up CT imaging (contingency coefficient 0.22, *p* = 0.003) (29). The higher incidence of seromas we observed in our study may reflect the lower tolerance of normal tissue in the reirradiation setting.

There has been one other published report of PBrI with IORT in a cohort of 17 patients who developed localized breast recurrences after previous external beam radiation (25). Overall, with a median follow-up of 26 months there were no reported local recurrences. Acute toxicity consisted mainly of mild induration of the tumor bed, and there were no instances of grade 3 or 4 toxicity. Interestingly, in comparison to our study, a larger median size applicator was used to treat the lumpectomy cavity (median 4.0 cm, range: 2.5–5.0 cm) and 3 of the 17 patients treated with ≥4.5 cm applicator were prescribed a lower dose of 14.7 Gy to the lumpectomy surface. While both of our cohorts are small, the difference in our reported experiences highlights the importance of applicator selection. Larger applicator selection may help to optimize apposition of the applicator against surrounding breast tissue and improve dose homogeneity at the tissue–applicator interface.

There are a number of single-institution studies that have reported on the toxicity and outcomes of PBrI. Deutsch et al. reported on their experience with PBrI using external beam electrons to cover the involved breast quadrant to 50 Gy in 2 Gy per daily fraction. They reported a recurrence free rate of 77% at 52-month follow-up, and overall good cosmesis with mainly skin pigmentation changes (14). Interstitial brachytherapy has been used by several institutions, reporting late toxicity of grade 3 fibrosis in up to 10–16% of patients (15, 17, 19, 20). Freedom from a second local recurrence at a median follow-up of 5 years was 89–93%. Existing studies support the efficacy and safety of PBrI to treat IBTR. To date, however, there is no clear optimal delivery technique or dose fractionation. RTOG 1014 is the most recent prospective trial examining the safety and efficacy of PBrI for IBTR after prior WBI using 3D-conformal external beam radiation. Their preliminary outcomes were reported at the recent American Society for Radiation Oncology conference, describing a 3-year subsequent IBTR of 3.7%, DMFS and


*All toxicity graded per the NCI CTCAE version 4.02; acute defined as within 3 months from intraoperative radiotherapy treatment.*

OS of 94.8% in a cohort of 58 patients. Four patients underwent subsequent mastectomy, two for a subsequent IBTR, one for a non-healing wound and another patient who underwent bilateral mastectomy after discovery of contralateral disease (30). They describe grade 1 late toxicity (greater than 1 year from treatment) in 24.1% of patients mainly consisting of breast pain and fibrosis, grade 2 late toxicity in 22.4%, and grade 3 toxicity in 6.9% with one instance of grade 3 infection. Comparable to the 3-year toxicity data from RTOG 1014, we observed a significant number of grade 2 and 3 toxicities in the late setting.

### Limitations

The major limitations of this study include the retrospective nature of our data with limited follow-up to observe further delayed toxicities and recurrences. Given the small sample population of our study, it is difficult to draw conclusions regarding the differences in toxicity between our PBrI experience and previously published studies. Finally, we did not report on patient satisfaction and quality of life in our study. The pursuit of breast-conservation was driven mainly by patient

### REFERENCES


preference, and ultimately the low-grade toxicities we observed with breast conservation may be outweighed by the success of breast preservation.

## CONCLUSION

Breast-conserving surgery with IORT is a feasible salvage option for patients desiring breast conservation after prior thoracic radiation; however, continued follow-up of these patients is warranted given the incidence of delayed treatment toxicity. Further studies are needed to determine optimal treatment strategy, dose, and dose fractionation for PBrI.

## AUTHOR CONTRIBUTIONS

Each of the listed authors contributed to the completion of this submission. Both CC and PJ worked on data collection and drafting of the final manuscript. BT, SF, RH, DH, and EC all contributed to the critical revision of the article with final approval for publication.

mastectomy vs. salvage breast-conserving surgery and prognostic factors for salvage breast preservation. *Int J Radiat Oncol Biol Phys* (2005) 63:845–51. doi:10.1016/j.ijrobp.2005.02.035


breast previously treated with lumpectomy and radiation therapy for breast cancer. *Brachytherapy* (2008) 7:22–8. doi:10.1016/j.brachy.2007.10.006


in breast cancer: American Society of Clinical Oncology/College of American Pathologists clinical practice guideline update. *Arch Pathol Lab Med* (2014) 138:241–56. doi:10.5858/arpa.2013-0953-SA


**Conflict of Interest Statement:** The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

*Copyright © 2017 Chin, Jadeja, Taback, Horowitz, Feldman, Ha and Connolly. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) or licensor are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.*

# Intraoperative Radiation "Boost" to the surgical Resection Bed following pancreaticoduodenectomy for a Borderline Resectable pancreatic Carcinoma: a Case Report

*Tarita O. Thomas1,2\*, William Small Jr.1,2, Mark Fleming3,4, Song Kang3,4,5 and Richard A. Hoefer 6,7*

*1Department of Radiation Oncology, Stritch School of Medicine, Loyola University, Chicago, IL, United States, 2Cardinal Bernardin Cancer Center, Maguire Center, Loyola University Medical Center, Maywood, IL, United States, 3Virginia Oncology Associates, Hampton, VA, United States, 4Division Chief Hematology and Oncology, Eastern Virginia Medical School, Norfolk, VA, United States, 5Department of Radiation Oncology, Sentara CarePlex Hospital, Sentara Healthcare, Hampton, VA, United States, 6Department of Surgical Oncology, Sentara CarePlex Hospital, Sentara Healthcare, Hampton, VA, United States, 7Eastern Virginia Medical School, Norfolk, VA, United States*

#### *Edited by:*

*Brian Timothy Collins, Georgetown University, United States*

#### *Reviewed by:*

*Joseph M. Herman, Johns Hopkins University, United States Thomas Kole, Valley Hospital, United States*

#### *\*Correspondence:*

*Tarita O. Thomas tathomas@lumc.edu*

#### *Specialty section:*

*This article was submitted to Radiation Oncology, a section of the journal Frontiers in Oncology*

*Received: 07 April 2017 Accepted: 16 January 2018 Published: 23 March 2018*

#### *Citation:*

*Thomas TO, Small W Jr., Fleming M, Kang S and Hoefer RA (2018) Intraoperative Radiation "Boost" to the Surgical Resection Bed following Pancreaticoduodenectomy for a Borderline Resectable Pancreatic Carcinoma: A Case Report. Front. Oncol. 8:12. doi: 10.3389/fonc.2018.00012*

Neoadjuvant therapy including chemotherapy alone or concurrent chemotherapy with external bream radiation is a standard treatment strategy for borderline resectable pancreatic adenocarcinoma and is also used routinely for primary operable cancers at some institutions (1). The use of intraoperative radiation therapy (IORT) has been limited largely because of the logistical issues in delivery of radiation during surgery (2). This is the first reported case of a borderline resectable pancreas cancer patient who underwent neoadjuvant chemo-radiation therapy followed by resection with the use of IORT using the mobile IntraBeam device to boost the resection bed and improve local control by dose escalation.

Keywords: intraoperative radiation therapy, IORT, pancreas cancer, borderline resectable, neoadjuvant therapy

### INTRODUCTION

Surgical resection remains the mainstay therapy for pancreas cancer; however, more than 80% of patients have disease that is not resectable at the time of diagnosis (3). There is a subgroup of patients who are considered potentially resectable following neoadjuvant therapy. This group has been termed borderline resectable pancreas cancer (BRPC) based on the assessment of the association of tumor with the major regional vessels. Generally, neoadjuvant therapy includes chemotherapy followed by concurrent chemo-radiation therapy or in some cases up front chemo-radiation therapy alone (4). Neoadjuvant chemo-radiation has been increasingly used for patients with localized pancreatic carcinoma including resectable or BRPC with improved outcomes (5, 6).

Retrospective studies have shown that adding local radiation to a BRPC leads to improvement in survival at 1 year due to improved local control (7). Phase II data and single institution data support the use of neoadjuvant therapy (8–10). In addition, a meta-analysis has shown that in about one-third of initially unresectable patients can be converted to resectable with the use of neoadjuvant therapy (7). Intraoperative radiation therapy (IORT) had been used in pancreas cancer in the past primarily with the goal of pain reduction and control of locoregional tumor progression (11–14). Formerly IORT was given with an electron energy source that produced promising results but was difficult to implement. Additionally, further radiation following neoadjuvant radiation after surgery for a close or positive margin is typically prohibited by normal tissue tolerances. Intraoperative radiation has been used as a method to deliver additional radiation boost to areas at risk for residual disease. Here, we describe the use of low-kilovoltage (lowkV) IORT to boost an area at risk for residual disease recognized during the surgical resection.

### CASE REPORT

A 50-year-old African-American male presented with a 14-day history of abdominal pain and anorexia in the spring of 2008. Abdominal CT scan revealed a 3.4 cm × 3.7 cm × 4.7 cm hypoattenuated mass in the region of the uncinate process, contacting directly 50° of the superior mesenteric vein and approximately 25° of the superior mesenteric artery as well as contacting the left renal vein (**Figure 1**). The pancreatic duct was dilated measuring 0.7 cm. An endoscopic ultrasound-guided FNA biopsy was non-diagnostic, and percutaneous CT-guided biopsy confirmed a pancreatic carcinoma. CEA was 14.5 and Ca19-9 was 76. The case was discussed in prospective tumor board and the tumor was deemed to meet NCCN criteria for borderline resectable disease of less than or equal to 180° of contact of major vessels. The patient underwent neoadjuvant chemo-radiation with protracted infusion 5FU and involved field radiation to a dose of 5,040 cGy. Repeat staging showed a good response with the tumor measuring 3.3 cm × 3.1 cm, and the association of the tumor to the SMV and SMA being improved (**Figure 2**). Repeat Ca19-9 was 3. The patient was taken to surgery and underwent pylorus-preserving pancreaticoduodenectomy (**Figure 3**). A boost dose of 1,800 cGy

Figure 1 | CT abdomen at initial work-up that shows a hypo-attenuated mass in the region of the uncinate process contacting 50% of the superior mesenteric vein, and approximately 25% of the superior mesenteric artery (see arrow) as well as contacting the left renal vein.

was prescribed to the surface of a 3.5 cm spherical applicator and was administered intraoperatively to the uncinate margin at the superior mesenteric vein, superior mesenteric artery, and inferior vena cava. This area was felt to be at the highest risk for a close or positive margin and formed a concave region making it amendable for treatment with a spherical applicator. Normal tissue was retracted including the left kidney, neck of pancreas, small bowel, and colon which were protected with lead shields and several centimeters of tissue equivalent material in all directions other than the target. Once the radiation dose was delivered, the patient was reconstructed in the standard fashion. Pathology confirmed a 0.5 cm area of tumor over the inferior vena cava. Biopsies of the soft tissues along the right lateral border of the superior mesenteric vein were benign. The patient's final pathology stage was ypT3, N0, M0 with 12 regional nodes negative for metastatic disease, and margins of resection were negative (R0). The patient had a satisfactory postoperative course and was discharged on the 11th postoperative day with the evidence of delayed gastric emptying requiring home TPN. The patient was, over time, able to return to a regular diet, and the TPN was discontinued and then received 6 months of adjuvant chemotherapy with gemcitabine. In our surgical series, we have found ~40% rate of some element of delayed gastric emptying. We have routinely placed patients with clinical manifestation of delayed gastric emptying in the first postoperative week on TPN. This has decreased readmission rates. In this case report, there was no need for vessel resection or reconstruction. This patient did undergo a pylorus-preserving procedure which may have a higher rate of delayed gastric emptying. In addition, it is possible that nerves in the retroperitoneal space could have had radiation exposure. The cause of this patient's delayed gastric emptying is likely multi-factorial including the effects of the surgical technique and radiation exposure.

Figure 2 | CT abdomen at restaging following neoadjuvant chemoradiation therapy showed a 3.3 cm × 3.1 cm mass, decreased from initial size with involvement of the SMA (see arrow) and SMV improved.

pancreaticoduodenectomy in region of the uncinate margin at the superior mesenteric vein (thin arrow), superior mesenteric artery, and inferior vena cava were intraoperative low-kV radiation therapy was administered in retroperitoneal space (thick arrow).

The patient has been followed with repeat three-dimensional imaging at routine intervals (**Figure 4**). Since surgery, the patient has been in surveillance follow-up for over 87 months and is considered free of disease.

### DISCUSSION

Intraoperative radiation therapy refers to the delivery of a single dose of radiation therapy directly in the tumor bed at the time of surgery. IORT can be delivered with both electron and low-energy kV X-rays. IOERT requires a specially shielded and approved operating room with a linear accelerator or prior to closing the incision the patient is physically transported to a radiation oncology suite for treatment. In most institutions, this is prohibitive. In addition, electron therapy has more energy scatter and thus is more difficult to spare adjacent normal tissue compared to lowenergy kV X-rays (15).

The Intrabeam system offers many potential advantages over IOERT including accessibility, easier patient positioning, rapid dose gradient, and beam characteristics. The system is portable allowing for use in multiple operating rooms. Therefore, no designated operating room is necessary for delivery of treatment. The system has six degrees of freedom permitting the machine to conform to multiple treatment positions. The low-kV X-rays have a steep dose gradient (15). The rapid dose gradient allows for delivery of high dose to the target tissue while relatively sparing the surrounding normal tissues (16). Any tissue at risk can be shielded with tungsten-filled silicone or even wet gauze. The system has minimal radiation protection requirements allowing

Figure 4 | (A) 1 year post-operative scan showing SMA (see arrow) is clear of disease. (B) 7 year post-operative scan showing renal vein at IVC (see arrow) widely patient and without disease.

essential staff to stay in the room with the anesthetized patient during delivery of the radiation.

In addition to treatment parameters that make the Intrabeam technology easier to deliver, there are also potential radiobiological benefits of IORT. Delivery of a single high-dose treatment to a confined area rather than a fractionated postoperative course of therapy does not allow for tumor cell proliferation between surgery and start of radiotherapy as well as sublethal damage repair between fractions in a standard course of radiation therapy (17, 18).

In summary, potential advantages of an intraoperative radiation include direct visualization of the tumor bed, no temporal delay between surgery and radiation not allowing for tumor cell repopulation, and improved displacement and shielding of normal tissues. Loyola University is accruing to a Phase I, 3 × 3 trial of 10, 15, and 20 Gy IORT prescribed to the surface of a flat applicator for patients with resectable pancreas cancer (19). This trial will further establish the dose of kV IORT to increase treatment to the tumor bed and retroperitoneal margin; the areas of highest risk of local recurrence. Currently, flat applicators are available in six sizes ranging from 10 to 60 mm to accommodate patient size and area of concern.

In BRPC, vascular involvement remains an important factor as to whether or not a patient can become resectable. Once respectable, the margin negative rate is a key determinant of local control. This case demonstrates the feasibility of using IORT for target dose escalation while minimizing the normal tissue exposure.

### REFERENCES


This is the first known successful use of intraoperative radiation using the IntraBeam device for a BRPC patient.

### ETHICS STATEMENT

This study was carried out in accordance with the recommendations of Sentara CarePlex Hospital regulations with written informed consent from all subjects. All subjects gave written informed consent in accordance with the Declaration of Helsinki. The protocol was approved by the Sentara CarePlex Hospital committee on events related to human subjects.

### AUTHOR CONTRIBUTIONS

TT and WS provided intellectual background in analyzing the data for this report and writing the report. MF, SK, and RH were directly involved in the treatment of the patient reported in this case report.


**Conflict of Interest Statement:** WS is a member of US Targit trial Steering Committee and has received travel and speaker honorarium funds from Zeiss. RH and WS are board members of the TARGIT Collaborative Group. All other authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

*Copyright © 2018 Thomas, Small, Fleming, Kang and Hoefer. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.*

#### *Carsten Herskind1,2\*, Frederik Wenz1 and Frank A. Giordano1,3*

*1Medical Faculty Mannheim, Department of Radiation Oncology, Universitätsmedizin Mannheim, Heidelberg University, Mannheim, Germany, 2Cellular and Molecular Radiation Oncology Laboratory, Medical Faculty Mannheim, Department of Radiation Oncology, Universitätsmedizin Mannheim, Heidelberg University, Mannheim, Germany, 3 Translational Radiation Oncology, Department of Radiation Oncology, Universitätsmedizin Mannheim, Heidelberg University, Mannheim, Germany*

### *Edited by:*

*Tarita O. Thomas, Loyola University Chicago, United States*

#### *Reviewed by:*

*Joshua Silverman, New York University, United States Kiri Sandler, University of California, Los Angeles, United States*

#### *\*Correspondence:*

*Carsten Herskind carsten.herskind@medma. uni-heidelberg.de*

#### *Specialty section:*

*This article was submitted to Radiation Oncology, a section of the journal Frontiers in Oncology*

*Received: 03 April 2017 Accepted: 22 June 2017 Published: 24 July 2017*

#### *Citation:*

*Herskind C, Wenz F and Giordano FA (2017) Immunotherapy Combined with Large Fractions of Radiotherapy: Stereotactic Radiosurgery for Brain Metastases—Implications for Intraoperative Radiotherapy after Resection. Front. Oncol. 7:147. doi: 10.3389/fonc.2017.00147*

Brain metastases (BM) affect approximately a third of all cancer patients with systemic disease. Treatment options include surgery, whole-brain radiotherapy, or stereotactic radiosurgery (SRS) while chemotherapy has only limited activity. In cases where patients undergo resection before irradiation, intraoperative radiotherapy (IORT) to the tumor bed may be an alternative modality, which would eliminate the repopulation of residual tumor cells between surgery and postoperative radiotherapy. Accumulating evidence has shown that high single doses of ionizing radiation can be highly efficient in eliciting a broad spectrum of local, regional, and systemic tumor-directed immune reactions. Furthermore, immune checkpoint blockade (ICB) has proven effective in treating antigenic BM and, thus, combining IORT with ICB might be a promising approach. However, it is not known if a low number of residual tumor cells in the tumor bed after resection is sufficient to act as an immunizing event opening the gate for ICB therapies in the brain. Because immunological data on tumor bed irradiation after resection are lacking, a rationale for combining IORT with ICB must be based on mechanistic insight from experimental models and clinical studies on unresected tumors. The purpose of the present review is to examine the mechanisms by which large radiation doses as applied in SRS and IORT enhance antitumor immune activity. Clinical studies on IORT for brain tumors, and on combined treatment of SRS and ICB for unresected BM, are used to assess the safety, efficacy, and immunogenicity of IORT plus ICB and to suggest an optimal treatment sequence.

Keywords: brain metastases, immune therapy, radiotherapy, stereotactic radiosurgery, intraoperative radiotherapy

## INTRODUCTION

Brain metastases (BM) are an advanced-stage manifestation of cancer that affect up to a third of patients with systemic disease. BM predominantly originate from primary lung, breast, or gastrointestinal cancers and melanoma. Given the change in demographics in industrialized countries with increasing cancer frequencies, combined with the increase in numbers of long-term survivors owing

**43**

to improved diagnostics and therapy, the incidence is believed to rise further. Depending on the clinical condition, treatment options for BM include surgery, whole-brain radiotherapy (WBRT), stereotactic radiosurgery (SRS), or a combination of these, while chemotherapy has only limited activity owing to low penetration of the blood–brain barrier (BBB). A considerable proportion of patients undergo upfront surgery for debulking the tumor mass or for the determination of histology and/or mutational status. In such cases, local relapse occurs in roughly 60% of patients 1–6 months after surgery alone (1), indicating that tumor stem cells capable of forming recurrent tumors have microscopically invaded the borders of the surgical cavity. While some degree of local control may be achieved by adding WBRT, this is associated with high morbidity and intracranial recurrences are common. Randomized phase III trials did not show improved overall survival by adding adjuvant WBRT (2, 3) and most patients now undergo SRS directed to the tumor bed, a procedure that was proposed and developed even before these trials were done (4, 5). Although level I evidence for this treatment is lacking, initial data suggest a low toxicity profile (6–8). However, even with the best treatment available, the median survival is rarely much longer than 1 year and, thus, there is a strong need for improved treatment beyond the BM and the tumor bed around the excised cavity.

Similar to SRS, intraoperative radiotherapy (IORT) to the cavity and margins treats the tumor site while minimizing dose to the surrounding normal tissue. Early clinical studies on IORT after the resection of glioma were conducted especially in Japan and in Germany, typically applying 15–25 Gy of high-energy electrons in a single fraction. Results were encouraging, with comparable or better local control and overall survival, and less radionecrosis than after fractionated WBRT with external X-ray beams (9–12). A large retrospective study of IORT as a boost combined with external beam WBRT versus WBRT alone did not show any significant improvement by IORT (13). However, failures were found to be associated with insufficient dose coverage (14) and a case of long-term (9 years) survival was indeed observed (15). Because of technical limitations, few centers were able to pursue this treatment at the time, but in the last decade, dedicated mobile machines for delivering IORT by high-energy electrons or lowenergy X-rays (LEX) in the operating room have become more widely available.

Compared to SRS for resected metastases, IORT eliminates the healing time between surgery and the beginning of RT during which tumor cells may proliferate and possibly spread beyond the tumor bed. In contrast, IORT requires the total dose to be applied in a single fraction, whereas hypofractionated treatment is optional with SRS (e.g., for larger tumors or cavities). Recently, the potential use of IORT for brain tumors may be supported by encouraging results from a phase I/II trial on IORT with 50 kV X-rays for glioblastoma (16), which was found to be safe in these patients (Giordano et al., submitted) and prompted the initiation of a randomized phase III trial (NCT02685605). The rationale for IORT in glioblastoma has been reviewed by Giordano et al. (17). Notably, the treatment of solitary BM with excision and IORT alone using 50 kV X-rays has been shown to be feasible with disease-specific outcome comparable to other modalities (18).

It has been suggested that in addition to targeted cell killing induced by conventional fraction sizes, vascular, cohort (bystander), and immune effects may contribute to the biological effect of very large doses per fraction (19–22). In contrast, it has been disputed whether additional effects other than the 5R's of radiotherapy (reassortment, repair, reoxygenation, repopulation, and radiosensitivity) need to be invoked to explain the successes of SRS, SBRT/SABR, and IORT (23). Nevertheless, there is a strong case that large radiation doses may act as an adjuvant for immunogenic cell death by releasing tumor antigens and danger signals (24). At the same time, the identification and characterization of immune checkpoints has led to a surge in clinical studies on immune therapy using immune checkpoint blockade (ICB) antibodies (frequently termed "checkpoint inhibitors" although to date no small-molecule inhibitors are available). For example, an early phase II study showed dramatic effects in melanomas, which generally are immunogenic tumors (25). Thus, combining RT and ICB is considered to offer potential synergies, in particular since antitumor immune effects outside the irradiated target volume, so-called abscopal effects (26), might help control microscopic systemic disease.

Although the brain has, for decades, been regarded as a "privileged site" that provided limited scope for antitumor immunity, activated T cells are known to be able to cross the BBB (27). While conventional radiotherapy mildly increases BBB permeability (28), SRS disrupts the BBB within hours after application, allowing cells and substances to easily cross into the CNS for a period of roughly a month (29). In the case of BM, early studies suggested improved overall survival rates when ICB was combined with SRS for unresected metastases (30, 31), reaching levels similar to patients without BM (32). In contrast, a study applying ICB in patients previously treated with SRS found no significant difference to SRS alone (33) and ICB combined with conventionally fractionated WBRT after resection also failed to show an effect (34), suggesting that timing and fractionation may be important.

Whereas a potential interaction between SRS and ICB is readily understandable in the case of non-resected metastases where radiation can release tumor antigens, it is not obvious if the irradiation of residual tumor cells in the tumor bed after resection of the tumor is sufficient to elicit a tumor-directed immune response. Since no systematic studies on resected tumors have been published, a rationale for combining IORT with ICB must be based on an understanding of the mechanisms involved. Therefore, the purpose of the present review is to examine the immunological interaction between radiation and ICB to elucidate whether high single doses to the resection cavity and the residual cancer cells within its margins might act as an immunizing event opening the gate for ICB therapies in the brain. Because of the complexity and dynamic nature of this topic, we first give a brief overview of the antitumor immune response and immune checkpoints for the non-expert. We then present the experimental evidence for the interactions between radiation and the immune system. Finally, we review the clinical studies on SRS combined with ICB and discuss the implications and potential for combining IORT with ICB for BM.

### ACTIVATION OF THE IMMUNE RESPONSE

The innate immune system acts as a non-specific first-line defense against infection and foreign antigens but also activates the adaptive immune system to provide an antigen-specific response. Upon infection, trauma (including irradiation), or during tumor initiation and progression (35), an inflammatory cascade is induced. In case of an infection, this is initiated by pathogenassociated molecular pattern (PAMP) molecules such as bacterial liposaccharides. Similarly, trauma release damage-associated molecular pattern (DAMP) molecules including proteins such as nuclear high-mobility group box 1 (HMGB1) and endoplasmatic calreticulin (CRT) as well as non-protein molecules adenosine triphosphate (ATP) and mitochondrial peptides and DNA (in the case of necrotic cell death) (36–38).

Soon after the appearance of PAMP or DAMP molecules, neutrophils enter the tissue secreting a large variety of chemokines and cytokines, including pro-inflammatory interleukin (IL)-12 (39), which in turn recruit monocytes and lymphocytes into the inflamed tissue. Depending on the cytokines, monocytes can differentiate into inflammatory or anti-inflammatory macrophages, and dendritic cells (DC). Phagocytes (neutrophils and macrophages) have pre-formed *pattern recognition receptors* (PRRs), mainly toll-like receptors (TLRs) and *receptors for advanced glycation end-products* (RAGE) that bind to PAMPs on microbial surfaces or to DAMPs from damaged cells. DAMPs are found on cell membranes, released into the extracellular space, or detected in the cytoplasm by intracellular PRR sensors such as TLR-9, which activates the STING [stimulator of interferon (IFN) genes] pathway (40) inducing the expression of type 1 IFN, e.g., IFNβ.

Natural killer (NK) cells are an important component of immune surveillance that remove cells with low expression of major histocompatibility complex (MHC) class I surface molecules. NK cells are CD3<sup>−</sup> CD8<sup>+</sup> lymphocytes lacking the T-cell receptor (TCR), which CD3<sup>+</sup> lymphocytes use for the detection of antigens on MHC. Instead, they express activating receptors belonging to the family of killer-cell immunoglobulin-like receptors (KIRs). The body's own cells are protected by inhibitory KIRs that recognize MHC class I presenting "self " antigens. Combinations of IL-12 or IL-15 with IL-18 stimulate NK cells activated by target cell recognition to secrete chemotactic cytokines, e.g., macrophage inflammatory protein followed by inflammatory cytokines IFNγ and tumor necrosis factor (TNF)-α in different subpopulations (41).

The adaptive immune system reacts to specific antigens and is made up largely of T and B lymphocytes, which are responsible for the cell-mediated and humoral adaptive immune responses, respectively. This part of the system carries a memory of previous antigens with lymphocytes being distributed between lymph nodes and the body tissues. Antigens need to be presented to lymphocytes by antigen-presenting cells (APCs). Most cell types present a small fraction of degraded proteins as peptide antigens on MHC molecules on their surface. Non-professional APCs (essentially all cell types) present 3–18 amino acid (a.a.) peptides from degraded cellular protein on 105 –106 MHC I molecules found on each cell (42), while so-called professional APCs (DC found mainly in superficial tissue, macrophages, and B cells) also present peptides on MHC class II molecules. The peptides presented on MHC class II are generated from antigens taken up by endocytosis and can be longer than 18 a.a. but are often degraded by peptidases to approximately 12 a.a. (42). Tumor cells and dying normal cells translocate CRT to the cell surface acting as an "eat me" signal. If CRT is able to overcome the inhibitory "do not eat me" signal from CD47, it will activate TLRs on phagocytes (43, 44). Together with the release of other DAMP molecules, this stimulates phagocytosis by DC or macrophages which process the antigens and present them on MHC class II leading to activation of these APCs (45). Activated professional APCs migrate to the nearest lymph nodes (or *via* the blood vessels to the spleen) where the MHC:peptide complexes are presented to lymphocytes that recognize specific antigens by their T- or B-cell receptors (BCR). B cells recognize native antigens by their BCR and can internalize, process, and present antigen peptides on their MHC class II molecules to T cells (46). According to the clonal selection theory, the highly variable TCR and BCR give rise to an extremely large number of mature, so-called naive, lymphocytes that each recognize different epitopes made up of antigen peptides presented on MHC molecules. While an adaptive antitumor immune response requires CD8<sup>+</sup> and CD4<sup>+</sup> T cells, the role of B cells and the humoral adaptive immune response is unclear.

The two major classes of T cells, cytotoxic ("killer") T cells (Tc) and helper T cells (Th), express different co-receptors, CD8 and CD4, respectively. CD8 on Tc bind to MHC class I (on all cells), while CD4 on Th cells bind to MHC class II (on professional APC). The binding between the Th and professional APCs is reinforced by induced expression of co-stimulatory molecules, mainly CD28, which binds to B7.1 (CD80) and B7.2 (CD86) on APCs, and CD40 ligand (CD40L), which binds to the CD40 receptor. Once a specific MHC II antigen-peptide combination binds the TCR and CD4 co-receptor of a naive Th, co-stimulatory binding results in its activation with clonal expansion and differentiation to a secretory effector Th cell releasing different cytokines.

Subsets of differentiated Th cells mediate either a cytotoxic immune response (mainly Th1 cells characterized by secretion of IFNγ) or a humoral immune response (follicular helper, TFH) (47). Th1 cytokine IFNγ stimulates the function of macrophages and the activation of CD8<sup>+</sup> T cells, binding to MHC I:peptide complexes. TFH are thought to activate B cells, while Th1, Th2 (characterized by IL-4, IL-5, and IL-13), and Th17 (characterized by IL-17a, IL17b, and IL-22) direct immunoglobulin class switching according to different types of pathogens. Since B cells function as professional APCs they may activate Th cells recognizing the antigen peptides presented on the MHC II molecules of the B cell and the secreted cytokines in turn activate the B cell causing it to proliferate and produce specific antibodies. An overview of the immune activation is shown schematically in **Figure 1** (48, 49).

### IMMUNE TOLERANCE AND CHECKPOINTS

Various mechanisms prevent the immune system from attacking its own body cells (autoimmune reactions) and from excessive

FIGURE 1 | Schematic overview of the interaction between the innate and adaptive immune systems. The innate system initiates the immune response by reacting to pathogens or trauma. Pathogens release pathogen-associated molecular pattern molecules (e.g., liposaccharides) while trauma release damage-associated molecular pattern molecules [mainly calreticulin (CRT); high mobility group box (HMGB)-1; ATP; DNA]. These molecules bind to pattern recognition receptors (PRR) on phagocytes (neutrophils, macrophages). Neutrophils entering the tissue secrete a large variety of chemokines and cytokines which recruit monocytes and lymphocytes. Natural killer (NK) cells remove cells with low expression of major histocompatibility complex (MHC) class I surface molecules *via* a set of activating and inhibiting receptors (AR and IR, respectively). In the adaptive system, antigens are presented to lymphocytes by MHC molecules on antigen-presenting cells (APCs). All cells express MHC class I molecules but only professional APC (mainly dendritic cells, macrophages, and B lymphocytes) express MHC class II molecules. Professional APCs migrate to the secondary lymphoid tissue (lymph nodes and the spleen) where they activate naïve CD4+ and CD8+ T-lymphocytes. Depending on the cytokine expression of CD4+ T helper (Th) cells, these activated cells regulate class switching of naïve B lymphocytes to mediate the humoral immune response. Th1 also stimulate activation of CD8 cells to become cytotoxic ("killer") T cells (Tc) that infiltrate the peripheral inflamed tissue and target specific antigens expressed on MHC class I molecules, e.g., on tumor cells. Interactions between MHC–antigen complexes and T cells are mediated by the T-cell receptor (TCR) and are reinforced by binding between pairs of complementary costimulatory molecules (e.g., B7 and CD28, CD40 and CD40 ligand, 4-1BB ligand and 4-1BB). Please also see text. For more detailed mechanisms, the reader is referred to comprehensive text books, e.g., Ref.(48, 49).

normal immune reactions. Basically, tolerance to "self "-antigens is induced by the deletion of naive Tc recognizing MHC:peptide complexes that present fragments of the individuals own proteins. In addition, a number of other mechanisms help limit the physiological immune response. A special type of CD4+ regulatory T cells (Tregs, characterized by CD25high and the canonical transcription factor FoxP3) limit or modulate the adaptive immune reaction by a variety of mechanisms [reviewed in Ref. (50)]. Tregs secrete inhibitory cytokines IL-10 and TGF-β1 and express CTLA-4 (cytotoxic T-lymphocyte-associated antigen 4) which is a negative regulator competing with CD28 for costimulatory binding to the B7 molecule on APCs [reviewed in Ref. (51)]. Tregs constitutively express CTLA-4 (52), but CTLA-4 is also induced during Tc activation, thus providing a feedback mechanism for downregulating APC-mediated Tc activation to prevent an excessive inflammatory reaction (53). Another member of the CD28 family, programmed death-1 (PD-1), is expressed on lymphocytes and inhibits the function of activated T cells, by binding to the B7 family ligand PD-L1. PD-L1 is not expressed in most normal cells but can be induced in tumor cells by IFNγ in the tumor microenvironment (54). PD-L1 can bind to B7.1, and PD-L1 signaling *via* PD-1 mediates immune suppression by stimulating apoptosis of T cells, inducing IL-10 and inducible Tregs, which contributes to a dysfunctional state

FIGURE 2 | Schematic model of immunosuppressive mechanisms during T-cell activation in the secondary lymphoid tissue (lymph nodes or spleen) and during the anti-tumor immune response in the tumor. Naïve CD8+ lymphocytes express TCR which bind to a specific antigen presented by major histocompatibility complex (MHC) I on professional antigen-presenting cell (APC). Binding is reinforced by binding of CD8 to MHC, and secretion of IFNγ by Th1 cells leads to expression of the costimulatory molecules CD28 which binds to B7. Together, these signals activate the CD8+ lymphocyte to become a cytotoxic Tc lymphocyte. However, CTLA-4 on regulatory T cells (Treg) competes for B7 in the APC thus dampening T-cell activation. Furthermore, induced CTLA-4 (i-CTLA-4) may contribute to inhibiting the activity of Tc. Cytotoxic Tc lymphocytes infiltrate the tumor and engage tumor cells by binding of TCR to the MHC I antigen complex, which is reinforced by binding of costimulatory molecules CD28 to B7. However, myeloid-derived suppressor cells (MDSC) and tumor-associated macrophages (TAM) secrete IL-10 and TGF-β1 which stimulate Treg to express CTLA-4 competing for B7, and also directly inhibit Tc cells. Furthermore, tumor cells may upregulate expression of the programmed death (PD) ligand (L)1 which binds PD-1 on Tc thus inhibiting the activity of Tc against the tumor cells. In addition, TAM express PD-L1 binding to PD-1 on Tc and natural killer (NK) cells, and also B7 binding to CTLA-4 on NK cells. Tumor cells can upregulate these immune checkpoints to escape attack by the immune system. Use of immune checkpoint blockade (ICB) antibodies directed against CTLA-4 in the secondary lymphoid tissue or PD-1/PD-L1 in tumors can help override these immune checkpoints thereby stimulating immune activation (anti-CTLA-4) or inhibition of cytotoxic T-cells (anti-PD-1, anti-PD-L1).

termed T-cell "exhaustion" (55). Thus, according to the current model of immune checkpoints, CTLA-4 exerts its action mainly during antigen presentation and Tc activation, i.e., in the afferent arm of the adaptive immune response (leading to the secondary lymphoid tissue). By contrast, PD-L1/PD-1 is considered to act mainly in the efferent arm (leading from the lymph nodes back to the affected tissue) by modulating the cytotoxic action of CD8<sup>+</sup> T cells in the tumor, although PD-1 is also expressed on Tregs, NK, and B cells, while PD-L1 is expressed on myeloid cells in tumors (56, 57). In addition to Tregs, myeloid-derived suppressor cells (MDSC; of monocytic and granulocytic lineages) contribute to immune suppression *via* secretion of immunosuppressive cytokines IL-10, and TGF-β1, and other mechanisms (58). The major immune checkpoints currently exploited in cancer therapy are shown schematically in **Figure 2**.

Because tumor cells arise from the body's own cells they might be expected to escape immune surveillance. In spite of this inherent tolerance, an immune response may be elicited by overexpressing naturally occurring "self " proteins (tumor-associated antigens), mutated "self " proteins, or foreign proteins such as viral proteins (tumor-specific antigens, TSA) (59). However, genetic and epigenetic changes during tumor progression may select for mechanisms that avoid detection or suppress the immune response. Thus, an inflammatory response in tumors may upregulate PD-L1 and cause tumor-associated macrophages and MDSC to express IL-10 and TGF-β1 (60, 61).

Targeting the immune checkpoints by antibodies against CTLA-4 and PD-1/PD-L1 has recently shown to result in clinically relevant responses in some cancer patients (62–66). Antibodies against CTLA-4 broadly stimulate the adaptive immune response but may be associated with severe side effects, while anti PD-1/ PD-L1 therapy may be more specific toward tumors and appears to be better tolerated (51). However, ICB antibodies given alone are effective only if the tumor is immunogenic *per se*.

### RADIATION-INDUCED ENHANCEMENT OF IMMUNE ACTIVITY

Although low doses of radiation are immunosuppressive, it has become clear in the last 10–15 years that higher doses may stimulate the antitumor immune response (45, 67, 68). Indeed, some evidence suggests that immunogenic cell death contributes to the efficacy of hypofractionated or single-dose radiotherapy (37, 69, 70). However, data regarding the influence of dose and fractionation are conflicting, thus warranting a critical review of the dose dependence of immune activation.

The first evidence that irradiation releases DAMP molecules was found in murine thymoma cells that released HMGB1 after a dose of 10 Gy in an apoptosis-dependent fashion since the release was suppressed by the caspase inhibitor Z-VAD-fmk (71). Golden et al. found that CRT translocation and the release of DAMP molecules ATP and HMGB1 in a murine breast adenocarcinoma cell line were increased by single doses in the range of 2–20 Gy (72). The data indicated a quasi-linear increase up to 10 Gy, whereas 20 Gy produced a moderate further increase for CRT and ATP but only little further increase of HMGB1. Radiation-induced release of DNA into the cytosol (e.g., from the mitochondria) activates the STING pathway leading to the induction of type I IFN, the first step in the inflammatory cytokine cascade (73). Thus, IFNβ was induced after a single dose of 20 Gy to B16 melanoma tumors (74). NK cells and lymphocytes are very radiosensitive and undergo apoptosis after doses <2Gy. Furthermore, translocation of nuclear HMGB1 into the cytosol was recently reported after irradiation of human skin fibroblasts with doses in the range 4–12 Gy (75). Therefore, it seems a distinct possibility that high-dose irradiation of the normal tissue in the tumor bed may contribute to producing an inflammatory microenvironment conducive of an antitumor immune reaction.

Irradiation induces cytokines in various cell types, most importantly *via* nuclear factor (NF)-κB [reviewed in Ref. (76)], which can be activated by DNA damage-induced kinases, ATM, and DNA-PKcs (77, 78). Furthermore, HMGB1 is a ligand for RAGE and TLR4 signaling to NFκB, which may contribute to cytokine induction after higher doses (79). NFκB regulates transcription of a large number of cytokine genes, including pro-inflammatory cytokines such as IL-1β, IL-6, IL-8, IL-33, and TNF-α. Thus, expression of IL-1β and TNF-α was induced within a few hours of irradiating macrophages with doses of 3–20 Gy *in vitro* (80–82). Early upregulation of IL-1β was also observed after *in vivo* irradiation with 18.5 Gy (83), whereas a lower dose of 3 Gy caused upregulation approximately 5–7 days later, during macrophage differentiation preceding regeneration of the spleen (80). Early transcriptional upregulation of a number of cytokines including IL-1β and TNF-α occurred in brain or lung tissue after irradiation with doses of 7–25 Gy (84, 85). Thus, robust expression seems to require high single doses although daily fractions of 4 Gy also produced sustained expression in lung macrophages (85). Strong, dose-dependent secretion of IL-6 regulated by NFκB and activator-protein-1 was found in HeLa cells 24 h after irradiation with 3–20 Gy, while no significant increase was observed after 1 Gy (86, 87). In another study, secretion of IL1-α, IL-6, and IL-8 over 24 h was induced 1.7-, 1.6-, and 2.1-fold, respectively, by a low dose of 1.5 Gy in a monocytic cell line but not in A549 adenocarcinoma cells (88). However, irradiation of murine lymphoma with a single high-dose of 30 Gy initially decreased IFNγ and TNF-α in splenocytes but expression recovered 7–10 days after irradiation (70). A comprehensive review of the inflammatory response to ionizing radiation was given recently by Di Maggio et al. (89).

While it is not surprising that leukocytes express cytokines, it may be important for other cell types that p53 and NF-κB show a reciprocal relationship (90, 91). Veldwijk et al. (22) tested the secretion of 36 cytokines by p53 wild-type MCF7 breast cancer cells over 24 h after irradiation with 15 Gy. Only six cytokines (CD40L, IFNγ, IL-6, IL-8, IL-23, and Serpine E1) were detectable, and none showed significant upregulation after irradiation. Thus, it is conceivable that radiation-induced p53 may limit induction of the ATM/DNA-PKcs/NFκB pathway in p53 wild-type normal and tumor cells (A549 and MCF7) and that stronger induction in HeLa cells is due to the suppression of p53 by expression of the HPV18 E6 protein. Whereas *in vitro* induction may require high doses, there is ample evidence for radiation-induced expression of pro-inflammatory cytokines after moderate doses given *in vivo* (76). For example, dose-dependent upregulation was demonstrated in peritoneal mouse macrophages isolated 16 h after whole-body irradiation with 0.075–6 Gy with maximum upregulation at 4 Gy showing twofold increase for IL-12 and fivefold increase for IL-18 (92). The apparently higher sensitivity *in vivo* may be explained by additional activation due to lymphocyte apoptosis that may release DAMP molecules *in situ* including HMGB1 which activates the NF-κB pathway (79, 93).

Tumor cells frequently downregulate MHC surface molecules, thus reducing the opportunity of antigen presentation. However, radiation doses of 10–20 Gy upregulated MHC class I expression by ≥10% in 8/23 human colon, lung, and prostate, tumor cell lines tested (94). In a human melanoma cell line, MHC class I was increased in a dose- and time-dependent fashion with maximum expression at 48–72 h yielding a twofold increase for 10–25 Gy compared to 1.3-fold after 4 Gy (95). This study also showed that intracellular peptides for antigen presentation were initially generated by the degradation of existing proteins, but at later time points, novel peptides from new protein synthesis were presented on MHC class I. In a similar system, upregulation of MHC class I appeared to depend partly on radiation-induced IFNγ (96). Further aspects of different radiotherapy schemes on immune stimulation *in vitro* and *in vivo* have been reviewed in Ref. (97, 98).

Experiments using a tumor antigen-specific adenoviral vaccine showed that a single, moderate dose of 8 Gy given before vaccination produced a synergetic antitumor effect against a murine colorectal tumor, which was also observed when irradiation was given in three fractions of 3.5 Gy each (99). Since irradiation after vaccination had no effect, this seems to suggest a role of irradiation as an adjuvant creating a local microenvironment that supports immunization rather than a role in antigen presentation in this setting. Such a model is supported by the strong immunogenic effect of a TLR-7 agonist on a colorectal tumor, which was potentiated by fractionated radiotherapy with 5 × 2 Gy beginning simultaneously with the first application of the agonist but without any further immune therapy (100). However, the complexity and multiple components of the dynamic immune reaction may explain why combining radiotherapy with systemic type I or II IFNs was mostly unsuccessful, while the combination with IL-2 or IL-12 showed only limited effects in early clinical studies [reviewed in Ref. (101, 102)].

### TUMOR-DIRECTED, RADIATION-INDUCED IMMUNE EFFECTS *IN VIVO*

Few studies have investigated antitumor immunogenic effects of radiation *in vivo* without applying immune stimulation or checkpoint inhibitors. Lugade et al. found that a single radiation dose of 15 Gy increased the number of APC capable of activating IFNγ-secreting cells in lymph nodes in an experimental mouse model B16 of melanoma genetically modified to express ovalbumin (OVA) as a non-self antigen (67). A fractionated schedule of 5 × 3 Gy showed smaller increases of such APC in the lymph nodes. A similar difference between single and fractionated irradiation was seen for infiltration of the tumor by CD45<sup>+</sup>, CD4<sup>+</sup>, CD8<sup>+</sup>, CD11c, and CD11b immune cells 7 days after irradiation, indicating the recruitment of T cells, DC, macrophages, and possibly NK cells (CD8<sup>+</sup> but CD3<sup>−</sup>). Interestingly, the difference between single and fractionated doses was observed for specific T cells, activated by tumor-derived peptide presentation on MHC I but not on MHC II in both lymph nodes and tumors. Infiltration into the tumors was due to lymphocyte trafficking and was dependent on the upregulation of vascular cell adhesion molecule-1 on endothelial cells (96). A study by Lee et al. using unmodified B16 melanoma confirmed a growth inhibitory effect after a single dose of 20 Gy when tumors were grown from 2 × 106 injected cells, and local control was observed after 15 Gy when the number of injected cells was reduced to 1 × 105 (69). In the same study, local tumor control was also observed when an MHC class I-binding peptide ("SIY") was introduced as antigen and tumors grown from 2 × 105 injected cells were irradiated with 25 Gy. Growth delay for 5 × 105 injected cells and irradiation with 20 Gy was dependent on CD8<sup>+</sup> and was not seen for fractionated irradiation with 4 × 5 Gy. The effect of dose and fraction size was studied by Schaue et al. who irradiated B16-OVA tumors with single doses of 5–15 and 15 Gy applied in 1, 2, 3, or 5, fractions (103). Inhibition of tumor growth was seen at 7.5–15 Gy, whereas no significant effect was seen after 5 Gy. Applying a dose of 15 Gy in 1, 2, 3, or 5, fractions reduced tumor size and increased antigen-specific IFNγ expressing cells in the spleen for all schedules with a trend for 2 × 7.5 Gy being more effective. Notably, doses of 1 × 7.5 Gy or 2 × 7.5 Gy, but not other doses, also seemed to reduce the number of Tregs in the spleen. Taken together, single doses of 15-25 Gy, or hypofractionated irradiation with large dose fractions (7.5 Gy), seem capable of eliciting an immunogenic antitumor response against the primary tumor in the B16 murine melanoma system even without including ICB in the treatment.

The combination of radiotherapy with ICB has shown considerable synergies on local tumor control in experimental systems. Demaria et al. found that a single dose of 12 Gy followed by CTLA-4 blockade showed a synergistic growth delay of mammary tumors and two fractions of 12 Gy separated by 48 h combined with CTLA-4 blockade produced local control in a small number of animals (104). In a study by Dewan et al., a single dose of 20 Gy, or daily fractionated irradiation with 3 × 8 Gy or 5 × 6 Gy, caused similar growth delay but adding anti-CTLA-4 antibody 2, 5, and 8 days after the first irradiation inhibited growth for all schemes with an apparent, small advantage of 3 × 8 Gy (105). Incidentally, 5 × 3 Gy fractionated irradiation of B16 melanoma produced slightly more tumor infiltration than a single dose of 15 Gy for CD8<sup>+</sup> T cells activated by tumor-derived peptide presented on MHC class II, whereas 1 × 15 Gy produced higher numbers of cells activated by peptide presentation on MHC class I (67). This would seem consistent with a model in which hypofractionated irradiation combined with CTLA-4 blockade increases MHC class II-mediated antigen presentation by APC, while high single doses may be more efficient in promoting antigen presentation *via* MHC class I. In a radioresistant triple-negative mammary tumor studied by Verbrugge et al., a single dose of 12 Gy combined with anti-CD137 and anti-PD-L1 antibody treatment produced regression with some control of subcutaneous tumors while 4 × 5 Gy daily fractionated irradiation in combination with the same antibodies was effective in orthotopic tumors (106). Sharabi et al. showed regression of murine melanoma and mammary tumors irradiated with a single dose of 12 Gy combined with anti-PD-1 treatment (107). Irradiation with five fractions of 2 Gy upregulated expression of PD-L1 in colorectal cancer cells isolated from murine tumors but did not control tumors in a study by Dovedi et al. (108). However, concomitant administration of anti-PD-1 or anti-PD-L1 during and after irradiation resulted in 66–80% local control, and significant effects were confirmed in two other tumor lines. Irradiation combined with both anti-PD-1 and anti-PD-L1 showed no further enhancement. While local control was influenced by NK cells, survival was dependent on CD8<sup>+</sup> T cells that also induced PD-L1 *via* IFNγ. Azad et al. irradiated syngenic pancreatic ductal adenocarcinoma (PDAC) tumors combined with anti-PD-L1 antibody therapy (109). In the KPC line, combined treatment produced non-significant growth delays after 1 × 6 Gy or 5 × 2 Gy, while a single dose of 20 Gy produced growth inhibition but excessive dermatitis required termination of the experiment. By contrast, combined treatment with a single dose of 12 Gy, or 5 × 3 Gy fractionated irradiation, caused significant growth delay in KPC and regression in the Pan02 line. This was associated with an increase in T-cell infiltration and a reduction in myeloid cell numbers and was only seen for simultaneous treatment and not when anti-PD-L1 was started 1 week after irradiation. In another study, Twyman-Saint Victor TABLE 1 | Preclinical results on the effect of immune reactions on the growth of the irradiated tumor.


(*Continued*)

#### TABLE 1 | Continued


α*, anti; APC, antigen-presenting cells; TIL, tumor-infiltrating lymphocytes; MHC, major histocompatibility complex; Treg, regulatory T cells; VCAM-1, vascular cell adhesion molecule-1; PD-1, programmed death-1; PD-L1, PD-ligand 1.*

et al. showed that resistance in patients against hypofractionated SBRT combined with anti-CTLA4 was caused by the upregulation of PD-L1. Mimicking this in a mouse model, the resistance could be overcome by combining CTLA-4 and PD-L1 blockade with radiation, thus confirming and exploiting that the two immune checkpoints are non-redundant (110).

An overview of preclinical studies on immune effects in irradiated tumors is given in **Table 1**. Overall, dose fractions larger than 7–8 Gy seem to be more efficient in eliciting an inflammatory response and immune effects in irradiated tumors (67, 69, 103, 109). In many systems, tumor-infiltrating lymphocytes are increased after irradiation and an increase in the CD8<sup>+</sup>/Treg ratio seems to be associated with a successful immune reaction in some systems (103, 109, 110), although this is not universally found and MDSC reduction in tumors also seems to play a role (57, 110, 111). The question whether high single doses or hypofractionated irradiation with large fraction sizes is more efficient may depend on the tumor system, the role of antigen presentation by MHC class II, and the immune checkpoint being targeted.

### RADIATION-INDUCED ABSCOPAL EFFECTS

Sporadic cases of abscopal effects of radiotherapy were first described in clinical case reports (112–114), but meanwhile, this rare phenomenon is well documented although in some cases it may be associated (or to some extent overlap) with spontaneous regression [reviewed in Ref. (115)]. Experimentally, a nonspecific abscopal effect on unirradiated distant tumors (Lewis lung carcinoma or T241 fibrosarcoma) was found by irradiating a non-tumor-bearing leg of mice with five fractions of 10 Gy each, whereas a lower dose of 12 × 2 Gy normo-fractionated irradiation was less effective (116). Interestingly, this effect was dependent on wild-type p53 function in the host animal cells. Irradiation and a special form of immunotherapy prevented distant metastases in the lung when primary tumors of a melanoma B16 line overexpressing CC chemokine receptor-7, or the breast cancer cell line 4T1, were irradiated with 2 × 12 Gy followed by adenoviral transduction with LIGHT, a TNF superfamily member, which enhances host immune responses (69). However, the systemic potential of radiation was much clearer when DC were stimulated by a growth factor or an ICB antibody was added (26, 105, 117). An early study achieved 60% long-term survival in a metastatic Lewis lung tumor model by irradiating the primary tumor with a single, very high dose of 60 Gy combined with the DC growth factor Flt-3 ligand (Ftl3-L) given for 10 days beginning 1 day after irradiation (117). Significant growth retardation was also obtained in a mammary tumor model after irradiation of one of the two tumors with a moderate dose of only 2 Gy combined with Flt3-L (26). In metastatic mammary tumors, the number of lung metastases was reduced in a CD8<sup>+</sup>-dependent fashion after 12 Gy followed by CTLA-4 blockade (104). Another study compared different fractionation schemes in combination with CTLA-4 blockade in irradiated primary and unirradiated secondary tumors (105). The growth delay in secondary tumors was larger for 3 × 8 Gy, intermediate for 5 × 6 Gy, and smallest for 1 × 20 Gy. For 3 × 8 Gy, delaying the CTLA-4 antibody until 4 days after the first fraction (2 days after the last fraction) reduced the abscopal effect. The alternative approach of combining radiation with a PD-L1 checkpoint inhibitor was tested using two mouse mammary tumors irradiated with single doses of 12 or 20 Gy combined with anti-PD-L1 every third day on days 0–9 (57). After regression of the primary tumor, rechallenge did not result in tumor growth, and furthermore, an abscopal effect on growth delay was seen in unirradiated secondary tumors. Similarly, blocking PD-1 at the time of irradiation showed abscopal effects on the growth of unirradiated secondary tumors (melanoma and renal cell carcinoma) when the primary tumors were irradiated with single fractions of 15 Gy (118). A recent study reported an anti-metastatic effect of radiation and anti-PD-L1 after *ex vivo* irradiation of tumor cells with 12 Gy but because no primary tumor was irradiated,


α*, anti; PD-1, programmed death-1; PD-L1, PD-ligand 1*

this experimental design detected tumor take and not an abscopal effect (109). An overview of preclinical studies on abscopal effects of irradiation is given in **Table 2**.

Most studies found that immune effects of RT alone or in combination with ICB were dependent on CD8<sup>+</sup> T cells (57, 69, 70, 94, 104, 106, 108). However, there is also evidence on an influence of NK cell (106, 108), though this has been less often tested and was not found in an earlier study (69). The role of CD4<sup>+</sup> T cells is more ambiguous with little or even a negative influence in most studies (104, 106, 108), while an important role was reported in a glioma model (119). This variation may be explained by the fact that CD4<sup>+</sup> represents not only tumor-reactive Th cells but also Treg cells. Since the latter constitutes a significant but variable fraction, the stimulating effect of Th and the inhibitory effect of Treg may frequently cancel each other. Although PD-L1 may enhance Treg, their number was not affected in the mammary tumors (57). Instead irradiation combined with anti-PD-L1 treatment was found to confer a delayed decrease in immunosuppressive MDSC mediated by TNF secreted by infiltrating Tc cells (57). Similarly, no change in the CD8<sup>+</sup>/Treg ratio but a late decrease in myeloid cell numbers was observed in PDAC tumors after irradiation with a single dose of 12 Gy combined with PD-L1 blockade (109).

In accordance with the stimulating effect of Flt3-L on antigen presentation and the effect of CTLA-4 inhibition on Tc activation and Treg downregulation, these agents were effective when applied concurrently with and immediately after irradiation though full abscopal effects were only manifested several weeks later. Since blocking the PD-1/PD-L1 checkpoint is considered to prevent the exhaustion of cytotoxic Tc lymphocytes infiltrating the tumor in the efferent phase, one might expect a synergistic effect by applying radiation and anti-PD-1/PD-L1 antibody sequentially. However, delaying the beginning of PD-L1 blockade until 6 days after irradiation abrogated the synergistic immune effect on irradiated tumors (109). Since four anti-PD-L1 treatments were given in 10 days, this seems to imply that irradiation acts on the tumor microenvironment *before* modulation by ICB, while ICB acts on the inflammatory microenvironment *induced* by irradiation. This suggests that although the PD-L1/PD-1 checkpoint is considered to be effective mainly in the efferent pathway of the adaptive immune response (120), it may be more important in the afferent pathway (activation and antigen presentation) after irradiation than previously thought. If this finding is confirmed in other systems, it would provide a strong argument for starting ICB immediately after irradiation (which is supported by initial clinical data, see below).

The success of ICB antibodies in preclinical and early clinical trials has prompted a large number of clinical trials applying different ICB antibodies with radiotherapy in different schedules and tumor sites [reviewed in Ref. (121)].

### COMBINING SRS WITH IMMUNE THERAPY FOR BM

With the discovery of a lymphatic vessel system in the CNS (122), and the knowledge that antigen presentation to T cells occurs in the (deep) cervical lymph nodes (123), it is becoming clear that TABLE 3 | Outcomes of combined application of stereotactic radiosurgery (SRS), and ipilimumab (IPI) in melanoma brain metastases (BM), whole-brain radiotherapy (WBRT).


the immune system of the brain communicates with its systemic counterpart (124). In fact the traditional concept of CNS immune privilege no longer seems appropriate (124, 125). Microglial cells representing CNS innate immune cells perform many functions similar to macrophages, including recognition of DAMP, while DC appear to be important for antigen presentation in the cervical lymph nodes (125). Thus, the general model of immune response and immunosuppression also applies to tumors located in the brain (126).

A series of articles by Lim and colleagues examined the interaction between stereotactic irradiation with a single dose of 10 Gy and different ICB antibodies in an intracranial glioma model using a small-animal irradiator. Anti-PD-1 antibody given three times in 4 days beginning the day of irradiation produced significant survival at 3 months in approximately 28% of the animals (127). Challenging the survivors with glioma cells in the flank demonstrated adaptive immune memory. Triple treatment with a CD137 agonist, an anti-CTLA-4 antibody, and radiation resulted in 50% long-term survival (119). Omitting the CD137 agonist yielded approximately 20% survival for concurrent treatment starting before or on the day of irradiation but only 10% when CTLA-4 inhibition was started 2 days after irradiation. Survivors after triple treatment also produced a memory response. A different triple treatment combining anti-TIM-3 and anti-PD-1 ICB antibodies with irradiation achieved 60% survival (128).

These preclinical data are in line with a number of clinical studies that suggested considerably improved overall survival rates by adding the antibody ipilimumab (IPI, anti-CTLA-4) to SRS (30–33, 129–131) (**Table 3**). In two of the studies, a median number of two BM was present (32, 131), but generally the number and size of metastases varied over a wide range. In some of the studies, information on prescription dose and fractionation was missing or incomplete but the treatment of individual BM with a single fraction of 20–21 Gy (median dose) appeared to be common (129, 131). However, doses and the number of fractions to individual BM varied: 14–24 Gy and 1–5 fractions (31), 15–20 Gy (129), 15–24 Gy in a single fraction (131), or 15–21 Gy with 16/20 patients receiving a single fraction and 3–5 fractions given to the last four (33). These early studies used retrospective or prospective series of patients, the sequence of IPI and SRS varied greatly, which may have contribute to the variable outcome, and frequently little detail was given regarding timing. Thus, clearly prospective studies with defined protocols are needed. Nevertheless, some of the studies seem to support the preclinical results that this ICB antibody shows better efficacy when given concurrently or immediately after SRS compared to delayed treatment although differences may exist between the irradiated metastases and abscopal effects on out-of-field disease (31, 33, 129, 131). However, although one trial included four patients who underwent prior resection of metastases before SRS to the cavity plus IPI (131), none have *a priori* addressed therapy of a purely resected population. Combining SRS with an anti-PD-1 antibody (nivolumab) has only been described in a single study on 73 lesions in 26 patients with median 9.4 months follow-up (132), including patients with resected lesions. Overall, local control (82% at 12 months) was comparable to conventional treatments, while distant control (53%) was higher than for other treatments. Interestingly, seven patients with resected BM appeared to have superior overall survival with five patients surviving after 24 months.

## BIOLOGICAL EFFECTS OF IORT

Although the application of radiotherapy during surgery to inactivate any malignant cells remaining after tumor excision is not a new concept, IORT has only become a practical option during the last decade owing to the development of novel, dedicated machines. Thus, mobile linear accelerators producing high-energy electrons, or miniature X-ray machines emitting LEX allow irradiation of the tumor bed in the operating room with minimal radiation protection issues directly after the tumor has been removed (133–135). Different dose distributions can be achieved using special applicators in combination with the type and energy of the beam (136–138). However, IORT differs from conventional adjuvant RT in several aspects that may potentially influence the biological effect [reviewed in Ref. (20, 21)].

Intraoperative radiotherapy is given as a single fraction during surgery, whereas fractionated RT has been the established procedure for decades, applying daily fractions of typically 1.8–2.0 Gy. Thus, IORT eliminates the time of some weeks required for wound healing between surgery and the beginning of RT during which residual cancer stem cells may proliferate and increase the number of recurrence-forming cells that need to be inactivated, or possibly spread by migration out of the tumor bed and thus escape focused SRS (139). SRS represents an intermediate between the two since it is usually applied as a single, large-dose fraction a few weeks after surgery. When comparing the biological effects of IORT and conventionally fractionated RT, the radiation quality, distribution of dose, and dose rate must be considered. High-energy electrons show a relative biologic effectiveness (RBE) similar to that of high-energy X-rays (20) and produce a relatively uniform dose distribution at dose rates of 1–5 Gy/min. IORT with LEX involves increased RBE values, a non-uniform dose distribution with a steep radial dose gradient, and protracted irradiation with reduced dose rates allowing the repair of sublethal damage during irradiation. The biological implications of these characteristics have been studied by radiobiological modeling and experimental measurements (140–142). Adverse reactions of the normal, healthy tissue are limited to a small volume around the applicator, while the risk of recurrence is predicted to be similar to that of conventional external beam radiotherapy within a spherical shell, the "sphere of equivalence," thus defining a new target volume for tumor bed irradiation with LEX (140–145).

## POTENTIAL OF COMBINING IORT WITH IMMUNE THERAPY FOR BM

The treatment of solitary BM by excision and IORT in 23 patients using 50 kV X-rays at a dose of 14 Gy in 2 mm depth yielded a disease-specific outcome at 5-year follow-up that was comparable to other modalities (18). In a large retrospective study from the same institution, localized RT versus WBRT alone or in combination was compared in 212 patients including 37 patients treated with SRS only and 19 patients treated with IORT only (146). The results indicated a slightly higher local recurrence rate for SRS/ IORT, though this was not significant (*P* = 0.27). Rates of distant intracranial recurrences were higher than for local recurrences in both groups (WBRT and SRS/IORT) and were significantly higher after SRS/IORT compared with WBRT (*P* < 0.001). In spite of this, overall survival was comparable in the two groups and perhaps even marginally higher for SRS/IORT (*P* = 0.27). These results emphasize that distant recurrence is an issue when treating single lesions, especially with adjuvant localized RT although it may not directly affect overall survival.

At present, no studies combining IORT with ICB have been published. However, IORT differs from single fraction SRS by eliminating the delay between tumor excision and postoperative SRS. Thus, residual tumor cells are irradiated before they can be stimulated by factors released during the wound-healing process. Another important aspect is that the primary tumor is not irradiated but only the tumor bed, consisting mainly of normal brain tissue with an unknown, presumably low number of residual tumor cells. This poses the question whether the radiation-induced immune activity will suffice to elicit a tumordirected immune response on which an interaction with ICB may be based. In the following, key points relevant to the potential use of ICB in combination with IORT for BM are discussed.


FIGURE 3 | Hypothetical immune activation by IORT to the tumor bed after tumor excision of the metastasis. Irradiation of the normal tissue induces inflammatory "danger" signals, damage-associated molecular pattern (DAMP), leading to expression of chemokines and cytokines which recruit immune cells to the tumor bed (see also Figure 1), and may thus act as an adjuvant for the tumor-directed immune response. Cytotoxic Tc cells may target tumor cells as a result of being activated by antigen-presenting cell (APC) presenting tumor-specific or tumor-associated antigens before surgical excision. Immunogenic cell death of residual tumor cells in the tumor bed may contribute to antigen presentation and further inflammatory signals, creating a positive feedback loop. This would provide opportunities for synergy with immune checkpoint blockade (ICB) in the tumor bed or the secondary lymphoid tissue (see also Figure 2).

(132). This strongly supports that irradiation of the resected cavity is indeed capable of eliciting an antitumor immune response and furthermore suggests that the tumor cell load may be an important factor in controlling residual disease. Further support that irradiation of normal tissue may play a role as an adjuvant in this response comes from the abscopal anti-tumor effect seen after irradiation of an unaffected leg with large fraction sizes (116). As discussed in the previous sections, preclinical studies indicate that the inflammatory microenvironment induced by high-dose irradiation may play an important role in enabling a tumor-directed immune response. Thus, while most irradiated lymphocytes in the tumor bed will undergo apoptosis after IORT, DAMP signals produced by irradiated immune cells and stromal cells, and cells damaged by the surgical procedure, may start a cascade of chemokines and cytokines that will attract and activate cells of the innate and adaptive immune systems. This will renew the lost lymphocyte population, which in turn may attack remaining tumor cells, thereby releasing more antigens and DAMP molecules.


during SRS (0% 1-year local recurrence) than in those receiving SRS before (13%) or after (11%) the administration of IPI (131). Similar data were shown in a retrospective analysis of 75 patients receiving SRS and anti-CTLA-4 or anti-PDtherapy: the study found that lesion responses were greater and more rapid with concurrent administration of immunotherapy and SRS (149). Translated into the setting of IORT, this would require administration of immunotherapy at the same day of surgery, provided that surgery is not complicated by the administration of the substances.

(5) *Safety of concurrent immunotherapy and surgery*: as concurrent application of immunotherapy and surgery appears to be required to achieve maximum therapeutic efficacy, safety is a major concern. Although not prospectively assessed, we believe that at least for the anti-CTLA4 antibody IPI and the anti-PD-1 antibody nivolumab, these concerns can be dispelled. Gyorki et al. analyzed 34 operations on 23 patients treated with IPI (150). Beside some grade 1 or 2 wound complications (22%), no grade 3–5 complications were seen. In line with this, a systematic review from Baker et al. also detected no IPI-related surgical complications so far (151). Similarly, neoadjuvant administration of nivolumab 2 or 4 weeks prior to surgery was seen to be safe and feasible in operable NSCLC (Forde et al. ESMO 2016, NCT02259621).

### CONCLUSION

Brain metastases have a high likelihood of local recurrence after resection, but at present, there is no standard radiotherapy technique to boost the surgical cavity. Thus, SRS to a narrow high-dose volume (e.g., by focusing different beam angles and/ or by modulating the beam intensities) with Gammaknife or Cyberknife, or a linear accelerator are being used. An intraoperative boost of IORT appears a promising alternative, which does not require irradiating large volumes of healthy tissues or organs and which would eliminate the time required for wound healing (typically 2–4 weeks) before SRS is initiated. For both modalities, high single doses may elicit immunological effects that can reach beyond the tumor bed. A review of the mechanisms of radiationinduced immune reactions supports a model in which doses >~8 Gy may act as an adjuvant for antitumor immune reactions present before irradiation or enhanced by the release of tumor antigens from irradiated residual cancer cells in the tumor bed and possibly by immunogenic cancer cell death elsewhere. The efficacy of an immune response is supported by retrospective studies on SRS for (mainly) unresected BM combined with ICB antibodies (mostly IPI), suggesting that the antibody must be present at the time of and immediately after irradiation. Recent data on a small number of patients with resected BM indicate that SRS in combination with ICB antibodies, and in particular anti-PD-1, might increase overall survival in these patients, thus supporting the rationale for combining IORT with ICB for resected BM. Since IORT limits the dose to a small volume of normal brain tissue, one might even hypothesize that this approach would not preclude adding SRS in the case of oligometastases. Although these effects need to be more comprehensively understood, a combination therapy of very large dose fractions with ICB antibodies appears to be specifically synergistic, warranting further prospective clinical evaluation.

### AUTHOR CONTRIBUTIONS

CH performed the literature search, wrote the manuscript, and drafted the figures. FW included clinical aspects and suggested literature. FG performed the clinical literature search and wrote

### REFERENCES


the manuscript. All authors conceived of the aim of the review and read the final manuscript.

### ACKNOWLEDGMENTS

The authors acknowledge the financial support of the Deutsche Forschungsgemeinschaft and Ruprecht-Karls-Universität Heidelberg within the funding programme Open Access Publishing. We thank Mr. A. Yazgan for expert graphical assistance.

radiotherapy (IORT) in 77 brain tumor patients: adequate target volume coverage improve results. *Rev Cancer (Madrid)* (2008) 22(extra):1–58.


radiotherapy. *EMBO Mol Med* (2017) 9(2):167–80. doi:10.15252/emmm. 201606674


recurring glioblastoma]. *Zh Vopr Neirokhir Im N N Burdenko* (2007) (3):39–40.


**Conflict of Interest Statement:** Carl Zeiss Meditec AG, Jena, Germany, and Elekta, Crawley, UK, support training and radiobiological research at Universitaetsmedizin Mannheim. FG serves as a consultant and speaker for Carl Zeiss Meditec AG, NOXXON Pharma AG, Merck Serono GmbH, Roche Pharma AG, and Siemens Healthcare Diagnostics GmbH and holds patents related with Carl Zeiss Meditec AG. All other authors declare no conflict of interest.

*Copyright © 2017 Herskind, Wenz and Giordano. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) or licensor are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.*

*Lara Hilal1 , Karine A. Al Feghali1 , Paul Ramia1 , Ibrahim Abu Gheida1 , Jean-Pierre Obeid2 , Wassim Jalbout1 , Bassem Youssef1 , Fady Geara1 and Youssef H. Zeidan1 \**

*1Department of Radiation Oncology, American University of Beirut Medical Center, Beirut, Lebanon, 2Miller School of Medicine, University of Miami, Miami, FL, United States*

Every year, almost 62,000 are diagnosed with a head and neck cancer (HNC) and 13,000 will succumb to their disease. In the primary setting, intraoperative radiation therapy (IORT) can be used as a boost in select patients in order to optimize local control. Addition of external beam radiation to limited volumes results in improved disease control over surgery and IORT alone. In the recurrent setting, IORT can improve outcomes from salvage surgery especially in patients previously treated with external beam radiation. The use of IORT remains limited to select institutions with various modalities being currently employed including orthovoltage, electrons, and high-dose rate brachytherapy. Practically, execution of IORT requires a coordinated effort and careful planning by a multidisciplinary team involving the head and neck surgeon, radiation oncologist, and physicist. The current review summarizes common uses, outcomes, toxicities, and technical aspects of IORT in HNC patients.

Keywords: recurrent cancer, locally advanced, head and neck tumors, salivary gland tumors, intraoperative

## INTRODUCTION

radiation therapy

Head and neck cancers (HNCs) continue to take a high toll with an estimated incidence of around 62,000 new cases in the United States in 2016 (1). Radiation therapy (RT) is commonly used as adjuvant treatment or, as definitive modality when surgical resection is not possible. Delivering radiation at the time of resection of HNCs is particularly helpful in cases at high risk for recurrence, particularly where there is gross or microscopic residual disease or for recurrent disease (2). The safety and effectiveness of intraoperative radiation therapy (IORT) for HNCs have been established in studies from several institutions (3–5). Although IORT is mainly used in recurrent patients, few studies reported on its use in the primary setting. Two forms of IORT have been studied for HNCs: high-dose rate (HDR) brachytherapy (6) and external beam that includes electrons and orthovoltage photons IORT (3, 4). The purpose of this manuscript is to review experience over the past four decades with the use of IORT in patients with primary or recurrent cancer of head and neck.

## ROLE OF IORT IN HEAD AND NECK TUMORS

### Recurrent HNC

The treatment of recurrent HNC is challenging, especially in the setting of previous irradiation. As per the NCCN 2017 guidelines, surgery is the mainstay of treatment for resectable locoregional recurrences with or without the addition of postoperative reirradiation (7). However, the NCCN

### *Edited by:*

*Tarita O. Thomas, Loyola University Chicago, United States*

#### *Reviewed by:*

*Sunyoung Jang, Princeton Radiation Oncology, United States Sonali Rudra, MedStar Georgetown University Hospital, United States*

> *\*Correspondence: Youssef H. Zeidan yz09@aub.edu.lb*

#### *Specialty section:*

*This article was submitted to Radiation Oncology, a section of the journal Frontiers in Oncology*

*Received: 07 March 2017 Accepted: 22 June 2017 Published: 07 July 2017*

#### *Citation:*

*Hilal L, Al Feghali KA, Ramia P, Abu Gheida I, Obeid J-P, Jalbout W, Youssef B, Geara F and Zeidan YH (2017) Intraoperative Radiation Therapy: A Promising Treatment Modality in Head and Neck Cancer. Front. Oncol. 7:148. doi: 10.3389/fonc.2017.00148*

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adds a word of caution that reirradiation should be limited to a highly selected group of patients (8). The main challenge in reirradiation is the dose limiting tolerance of surrounding normal tissue. Hence, many studies have reported on the use of IORT since it provides the advantage of decreasing the treatment volume to the site that is directly observed in the operating room (OR), in addition to the possibility of operative mobilization of organs at risk, and normal tissue shielding. Encouraging results have been observed with the use of IORT for recurrent head and neck tumors at the primary site, neck, or salivary glands (**Table 1**).

### Neck Recurrences

Intraoperative radiation therapy for neck recurrences is one of the most common IORT uses in head and neck tumors probably due to the difficulty of completely resecting recurrent tumors close to critical structures such as the carotid artery or due to fixation to deep tissues especially after fibrosis induced by previous irradiation. We previously reported one of the largest retrospective series on neck IORT (12); it included 231 patients with advanced cervical metastasis, 88% (198 patients) had recurrent tumors. All included patients had either microscopic or gross residual disease. Intraoperative electron radiation therapy (IOERT) with a median dose of 15–20 Gy was used. Postoperative EBRT was offered to 50 patients (21.6%). With a median follow-up of around a year, 5-year recurrence-free survival (RFS) and overall survival (OS) were 49 and 26%, respectively, for all included patients (12). Another study on neck IORT included 52 patients with recurrent tumors who received a median dose of 20 Gy of IOERT. With a median follow up of 2 years, 2-year local control (LC) and OS were 68 and 45%, respectively, for all 75 patients (18). Most of the other series on IORT for neck recurrences, listed in **Table 1**, included between 17 and 84 patients (10, 14, 15, 19). The majority used IOERT while two of the studies used high dose radiation brachytherapy known as HDR IORT. The median dose ranged from 12 to 20 Gy. Most used adjuvant EBRT in addition to IORT with a median dose range of 41–50 Gy. The 2-year in-field LC reached up to 62%. LC was higher in patients with gross total resection (GTR), where 2-year LC was 50–100% after GTR vs. 0–24% with gross residual disease. It was also higher for primary recurrences as compared to neck recurrences (LC: 100 vs. 75%, respectively). As for survival outcomes, in a study, by Teckie et al, on 57 patients who received 15 Gy of HDR IORT, 3-year OS reached 50% in patients who had in-field control vs. 32% in those not achieving in-field control (10). Survival was also superior for patients with clear margins; where 2-year OS reached 70% in patients with clear margins in a study by Toita et al. which included 17 patients with 22 treated sites (19). In summary, IORT, with a median dose of 15–20 Gy, results in very good LC in patients with neck recurrences and can improve survival outcomes, especially in those who attain negative surgical margins.

### Primary Site Recurrences

Good outcomes have been also reported for recurrences at the primary site treated with IORT. Perry et al. presented a study on 34 patients that included salivary gland tumor (SGT) recurrences (21%) in addition to tumor recurrence at other head and neck the primary sites. Most of the patients had a history of irradiation with EBRT to a median dose of 63 Gy. Adjuvant EBRT at recurrence was given to 15% of the patients with a median dose of 50 Gy. With 10–20 Gy of HDR IORT, 2-year LC and 2-year OS were 56 and 55%, respectively (6). An earlier series by Nag et al. reported less favorable outcomes as compared to other listed studies. It included 38 patients, 29% of which were treated with IOERT for primary site recurrence. The dose was 15 Gy for close or microscopically positive margins and 20 Gy for gross disease. It is worth noting however that patients did not receive adjuvant EBRT in addition to IORT resulting in a 2-year LC and OS of 13 and 21%, respectively (15).

### SGT Recurrences

Recurrent SGTs were addressed in two main studies, which included patients, treated with IOERT, with the parotid gland being the most common site. Patients in both studies had a history of irradiation (EBRT) with a median dose of 60 Gy. We reported on 46 patients with recurrent parotid tumors treated by 15–20 Gy IOERT in addition to EBRT in 54% (dose of 45 Gy) and chemotherapy in 19%. Favorable outcomes were observed with a relatively long follow-up of 5.6 years. For those with recurrent tumors, 5-year RFS and 5-year OS were both 48% (11). The second study included 37 patients with recurrent SGT who received a median dose of 15 Gy IOERT in addition to EBRT (54 Gy) in 15%. LC at 5 years was better with IORT compared to no IORT (82 vs. 60%) and 5-year OS was 34% for the entire sample (2).

### Prognostic Factors

Most of the studies on IORT in the recurrent setting showed a significant correlation between in field control and margin status. Scala et al. reported that 1-year in-field control for patients with negative margins was 82% compared to 56% in those with a positive margin (9). At least five other studies also showed that positive margins (more so for gross residual than microscopic residual) at the time of IORT significantly predicted for in-field failure when compared to close or clear margins (2, 11, 13, 18, 19). In addition, doses of IORT of more than 15 Gy were shown to be associated with better LC (10, 12). Other prognostic factors for LC and RFS include pre-reirradiation recurrence-free interval of more than 12 months (10), use of adjuvant EBRT (9), absence of nodal extra-capsular extension (10), and tumor size (11). Furthermore, patients with neck metastasis who had no PNI, no LVSI, and no involvement of the carotid artery were reported to have better OS after IORT (12). Taken together, these results underscore the prognostic importance of surgical pathology details in addition to treatment dose in this patient cohort.

## Locally Advanced (LA) Primary HNC

One of the potential benefits of intraoperative RT in LA HNC is minimization of the time interval between surgery and RT as studies have shown that delayed radiotherapy compromises LC outcomes (20, 21). The importance of IORT in LA HNC is in boosting microscopic or gross residual disease in close proximity to or extending to critical structures, in a setting where negative surgical margins cannot be achieved without significant morbidity. A larger volume is usually irradiated postoperatively using external beam radiation therapy (EBRT). IORT has the advantage



IORT in HNC

Hilal et al.

TABLE 1 | Continued


(*Continued*) IORT in HNC

Hilal et al.



*LRC, locoregional control. aN: patients with recurrent tumors who received IORT.*

*b12 Gy: negative margins, 15–17.5 Gy for microscopically positive margins.*

*c1% LR in the IORT field, 20% regional, 13% distant in all.*

 *dIt included primary and recurrent tumors.*

*eMedian field size, when available, is given when median tumor size is not reported.*

*fLC: 78% primary and 43% for neck.*

of reducing the volume of the radiation boost field, allowing dose escalation to target tissue with selective shielding of sensitive structures.

### IORT As a Boost to Primary HNC

The Methodist Hospital of Indiana introduced IORT for HNC in 1982 to improve on LC rates and select patients' survival outcomes (3, 22). Few studies have examined the role of IORT in the primary HN setting exclusively. Most studies included patients with recurrent HNC, and most included a heterogeneous patient population with a wide variety of HNC disease sites. One of the first published studies by Garrett et al. reported on 28 patients with LA or recurrent HNC treated with surgery followed by IORT with 1-year OS rates of 67%. Indications for IORT in this study were as follows: (1) gross residual disease, (2) microscopic residual disease, or (3) close margins. In this series, all patients with residual gross disease recurred locally, whereas LC rates were 87 and 75% for close surgical margins and microscopic residual disease, respectively. The 43% of the patients received EBRT, in addition to IORT, with a median dose of 50 Gy. Carotid blowout was found to be a major treatment complication (in two patients). Results were not stratified by primary versus recurrent treatment (22). The same group at the Methodist Hospital of Indiana also reported on their experience with IORT in 104 patients with LA and recurrent HNCs (40 patients previously untreated, and the rest with recurrent disease). Patients were treated with surgery followed by IORT at a dose of 15–20 Gy. Some of the indications for IORT use were close surgical margins, fixation to the carotid sheath, deep muscles of the tongue, pre-vertebral fascia, extension to skull base or dura, or preservation of critical structures function, such as facial nerve. The percentage of patients who also received EBRT in addition to IORT was not reported. Results were promising, with 2-year LC rates of 54%, with better LC rates for parotid cancers (*n* = 19, 2-year LC = 69%) and tongue cancers (*n* = 16, 2-year LC = 57%) (3). Another small study by Freeman et al. included 25 patients with primary (*n* = 11) or recurrent (*n* = 14) tumors close to the skull base, who were treated with IORT for close surgical margins, or residual gross or microscopic disease, with LC rates of 64% at 1 year. The 36% of those patients also received postoperative EBRT (23). A third paper by Freeman et al., mentioned in the above section on neck recurrences, that studied 75 patients with advanced cervical lymph node metastases with 2-year LC and OS of 68 and 45%, respectively, included 22 patients with primary advanced untreated disease. This study did not report on the percent of patients also receiving adjuvant EBRT (18). These three studies with promising results paved the way for more single-institution studies.

In a study by Pinheiro et al., 44 patients with recurrent (*n* = 31) and LA (*n* = 13) HNCs (56% with skull base cancers) were treated with IORT at doses between 12.5 and 22.5 Gy, with around 50% tumor control rates overall. All patients with primary LA tumors received adjuvant EBRT (dose not reported) after IORT (13). Similar control rates were also documented in another study including 25 patients with mainly primary (*n* = 17) LA HNC treated with surgery and 12 Gy of IORT, with 2-year locoregional recurrence-free survival and disease-free survival rates of 58.5 and 50.6%, respectively (21). Nag et al. also studied

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53 patients with primary HNC (out of 65 included patients) who were treated with 7.5–20 Gy intraoperative HDR brachytherapy to sites inaccessible to intraoperative electron beam radiotherapy, with 5-year LC rates of 59% and 5-year OS rates of 42%, and no major intraoperative or postoperative complications (24). Although the abovementioned studies have limitations inherent to their retrospective design, they all report LC rates of 50–68% at 2–5 years. It is worth noting that the majority of those studies do not report on adjuvant EBRT use.

### Primary SGTs

Locally advanced SGT may involve or be in close contact with vital nerves or blood vessels within the head and neck. Adequate surgical margins might be difficult to attain in such a context. IORT might therefore be a good option in patients with salivary gland cancers at high risk of recurrence. The largest report of IORT in the multimodal management of patients with parotid cancers is a single-practice experience, which included 96 patients with primary (50 patients) or recurrent (46 patients) parotid cancers treated between 1982 and 2007 (11). In this study, 5-year recurrence-free survival (RFS) rates were 77.8% and OS rates were 65.7% for patients treated in the primary setting with IORT boost dose of 15–20 Gy using 4–6 MeV electrons. Larger tumor size was predictive of recurrence after IORT, and patient age was predictive of survival on multivariate analysis. These results have to be put into perspective, as other RT modalities were tried in the management of LA parotid cancers with promising results. The use of fast neutron RT yielded 5-year locoregional control (LRC) rates of 92 and 63% in patients treated with RT alone (without surgery), and with postoperative RT for gross residual disease, respectively (25). Garden et al. also reported LRC of 85% with the use of postoperative EBRT in patients with malignant tumors of the parotid gland (26). Taken together, these studies demonstrate that IORT could be considered as an option in multimodal management of patients with primary HNC, to address gross or microscopic residual disease in a setting where complete resection would be too morbid due to proximity to a major vessel or critical nerve, or as a boost for traditionally radioresistant tumors, such as SGT.

## TOXICITY OF IORT

Intraoperative radiation therapy-related complications rate reported in the literature (**Table 1**) ranges between 22 and 52% in both the primary and recurrent setting (11–13, 18, 19, 27). Early studies by Toita et al. reported a significant increase in the rate of toxicities with doses exceeding 20 Gy (19), whereas other studies failed to show significant changes in toxicity rates with different doses (10, 12). Of the several reported complications, carotid artery rupture incidence ranged between 2 and 5% (3, 13, 18) and up to 10% in older series (19). Carotid blow out is a treatment complication associated with the highest mortality rates. Fistula/abscess rate ranged between 4 and 15% (3, 4, 10–12, 14, 18, 27). Wound related toxicity from cellulitis to flap necrosis ranged from 0 to 12% (2–4, 6, 10–12, 14, 27). Osteoradionecrosis rates are reported to range from 0 to 13% (3, 4, 6, 11, 12, 14, 19). Furthermore, some studies report on treatment related neuropathy ranging from 1 to 3% and mostly treated by symptomatic pain management (2, 4, 6, 11–13). Of note, higher neuropathy rates were noted in a study from MSKCC reporting the outcomes of 57 patients with recurrent tumors, where neuropathy rates reached 26%, trismus rate of 24%, and fibrosis rate of 29% (10). Similar rates of trismus and fibrosis, 28 and 23% respectively, were reported from MSKCC in another retrospective study of 34 patients with recurrent disease (6). It is worth mentioning, however, that the above studies used different toxicity scales and had variable median follow up. Taken together, IORT in experienced centers has reasonable toxicity profile, and does not increase perioperative mortality (2, 4, 5, 11, 12) or hospital length-of-stay (5).

### RADIOBIOLOGY OF IORT

Using IORT in different clinical settings, including HNCs, is well grounded in several well-known radio-biologic principles. IORT provides a significant dose–response relationship advantage with the high dose given in a single IORT fraction having 1.5–2.5 times the biological effectiveness of the same dose given at standard fractionation (28). While it is debatable to rely solely on the linear quadratic model at very high doses, there is little doubt that tumor cell survival is significantly reduced when using a higher dose per fraction as compared to conventional fractionation. Moreover, there remains a need to better evaluate the effects radio-sensitizing compounds may have on normal tissue tolerances as well as tumor cell survival in the IORT setting (29).

Despite the efforts made intraoperatively to decrease the treatment volume, provide adequate retraction, and adequate shielding, the high fraction dose that is used during IORT does not give ample time for normal tissue repair. This may contribute to vessel injury, neuropathies, fibrosis, and other late effects in surrounding healthy tissues (30, 31). However, the quick drop off in dose with IORT HDR may help prevent treating critical structures surrounding the post-op bed to high dose. Regardless of treatment modality (photons/electrons or brachytherapy), treating residual tumor cells with radiation during surgery as opposed to days or weeks postoperatively reduces the disadvantageous role of tumor cell re-population. Also, it is well established that ischemic tumor areas may be more resistant to radiation treatment due to the paucity of oxygen fixing DNA damage caused by free radicals, and thus, a single high IORT dose does not provide ample time for tumor re-oxygenation. However, it is quite likely that the effect of this unfavorable ischemic milieu is counterbalanced by the single high IORT dose, when compared to standard external beam fractionation (12, 31). Moreover, there is evidence to suggest that high dosage of radiation in a single fraction can eradicate cancer stem cells that would have been radioresistant at standard fractionation and has even been theorized to have implications in unleashing favorable immune responses such as the abscopal effect (32, 33). Therefore, IORT has a promising role from a radiobiology perspective, in improving LC after resection of primary or recurrent HNCs.

### MODALITIES FOR IORT

Intraoperative radiation therapy can be delivered using several techniques and modalities that optimize target dose while minimizing it to the surrounding tissues. HDR brachytherapy, and electron and photon IORT are methods for this localized delivery of dosage. While harboring several similarities, the physics and radiobiology involved generally display a broad variance among the modalities and allow for suitable selections tailored for different HNC patients. Employing these tools correctly plays a particularly critical role in HNCs where surgical management of certain territories may be constrained by essential tissues or adjacent vascular components.

High-dose rate IORT allows the administration of focused radiation in regions where an EBRT cone is not appropriate (34). Application consists of placing a high activity source in close physical proximity to the post-surgical tumor bed while retracting or shielding adjacent structures. Treatment times typically elapse 15–60 min, allowing for treatment during a surgical procedure in a shielded room (35). HDR IORT offers strict spatial restriction of the administered dose, owing to the sharp fall in the inverse square function at short distances from the HDR source, resulting in relatively very little dose delivered beyond the prescribed 100% isodose line (36).

Although HDR IORT offers several advantages in tumor bed management, electron (IOERT) or photon IORT on the other hand provide optimal flexibility for a wide range of treatment sites. These forms of IORT may be delivered using radiation at different energies (37). At the present time, external beam IORT is administered by a dedicated linear accelerator with parallel electron beams of 3–12 MeV kinetic energy or isotropic photon fields with energies between contact and superficial therapy of 50 kV X-rays (38). Such low photon energy beams (e.g., Intrabeam® by Carl Zeiss AG, Germany) have demonstrated favorable outcomes in some sites but with many questions remaining unanswered (39, 40).

### CHALLENGES AND FUTURE DIRECTIONS IN IORT FOR HNC

Although IORT has emerged as a feasible modality in management of HNC, several challenges warrant further investigation. First, the efficacy of IORT needs to be further evaluated in randomized phase III trials. Due to paucity of radiotherapy centers with IORT equipment, this is best conducted *via* multicenter cooperative groups such as International Society of Intraoperative Radiation Therapy. Second, there needs to be professional guidelines describing IORT workflow and coordination between surgical and radiation specialties. Third, the therapeutic window for IORT in HNC patients needs to be further improved by using small molecule adjuncts that radiosensitize tumor cells and/or further protect normal tissues. Recent research efforts cast a promising future for HNC IORT (**Figure 1**) (28). For instance, the introduction of IOERT treatment planning system (41) affords accurate documentation of target and normal tissues dose distribution. This is anticipated to yield improved target coverage and better documentation of normal tissue doses.

Finally, assuring radiation safety when using IORT in the OR is a major concern. Typical stray doses from IORT at a 1 m

distance from the patient are around 6 µSv per Gy of patient dose (42). Radiation safety regulations usually require shielding which reduces dose below the permitted constraints at any position around the OR under the assumption that the highest dose of stray radiation occurs in the direction of the beam during the complete workload. This need for structural shielding could be reduced by the possible use of mobile shielding walls (43). For example, in a study from Poland by Rabin et al., they calculated that it is sufficient to have mobile lead shield parameters of 1 cm × 140 cm × 150 cm between the accelerator in the OR and the control room to have the dose distribution in the patient plane meet radiation protection requirements (44). However, some authorities might object to mobile shielding due to the lack of control of correct placement by the personnel. A possible future solution might be the development of interlocked systems, which permit irradiation only if the crucial directions around the accelerator are adequately protected by correctly positioning the mobile shields (43).

### CONCLUSION

Intraoperative radiation therapy has emerged as an effective modality for HNC patients at high risk for local failure. To date, most of the scientific literature on head and neck IORT remains by and large limited to single institutional experiences. Recent technological advances and the advent of new IORT platforms predict an expansion of radiotherapy centers offering IORT. This is anticipated to accelerate opening of multi-institutional clinical trials in order to refine indications for IORT in HNC patients.

### AUTHOR CONTRIBUTIONS

LH and YZ outlined the manuscript. LH added **Table 1**. LH, KF, IG, PR, J-PO, and YZ wrote the sections of the manuscript. FG, BY, and WJ reviewed and edited the manuscript.

### REFERENCES


radiotherapy in advanced head-and-neck cancer. *Int J Radiat Oncol Biol Phys* (2001) 51:571–8. doi:10.1016/S0360-3016(01)01690-X


experience with the intrabeam delivery system. *Cancer Control* (2012) 19:295–308.


44. Wysocka-Rabin A, Adrich P, Wasilewski A. Radiation protection studies for a new mobile electron accelerator for intraoperative radiation therapy (IORT). *Prog Nucl Sci Technol* (2014) 4:298–302. doi:10.15669/pnst.4.298

**Conflict of Interest Statement:** The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

*Copyright © 2017 Hilal, Al Feghali, Ramia, Abu Gheida, Obeid, Jalbout, Youssef, Geara and Zeidan. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) or licensor are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.*

# Future Directions of Intraoperative Radiation Therapy: A Brief Review

### *Tatjana Paunesku and Gayle E. Woloschak\**

*Department of Radiation Oncology, Feinberg School of Medicine, Northwestern University, Chicago, IL, United States*

The use of intraoperative radiation therapy (IORT) is increasing with the development of new devices for patient treatment that allow irradiation without the need to move the patient from the surgical table. At the moment, ionizing radiation in the course of IORT is supported most often by the use of mobile devices that produce electrons, kilo voltage X-rays, and electronic brachytherapy and the development of applicators suitable for delivery of radionuclides for short-term brachytherapy. The establishment of new treatment devices and protocols that can be foreseen in the future, e.g., the development of proton or heavy ion sources suitable for IORT or the establishment of new treatment protocols such as the use of IORT in combination with immune system modulators or radiosensitizing nanoparticles, could lead to a significant increase in the use of IORT in the future. This review discusses the still limited use of IORT at this point in time and hypothesizes about possible future approaches to radiotherapy.

### *Edited by:*

*Tarita O. Thomas, Loyola University Medical Center, United States*

#### *Reviewed by:*

*Valdir Carlos Colussi, UH Seidman Case Medical Center, United States Michael Wayne Epperly, University of Pittsburgh, United States*

### *\*Correspondence:*

*Gayle E. Woloschak g-woloschak@northwestern.edu*

#### *Specialty section:*

*This article was submitted to Radiation Oncology, a section of the journal Frontiers in Oncology*

*Received: 21 September 2017 Accepted: 22 November 2017 Published: 12 December 2017*

#### *Citation:*

*Paunesku T and Woloschak GE (2017) Future Directions of Intraoperative Radiation Therapy: A Brief Review. Front. Oncol. 7:300. doi: 10.3389/fonc.2017.00300*

Keywords: intraoperative radiation therapy, immunotherapy, nanotechnology, radiosensitizers, normal tissue injury

## INTRAOPERATIVE RADIATION THERAPY (IORT) TODAY

By definition, IORT is delivered to the tumor and tumor bed (preferably to tissues at the 10 mm depth) as they are exposed during surgery to the patient; often, this also includes surgical distancing of normal tissues that could be injured by radiation (e.g., retraction of skin away from the IORT source). Regardless of the dose of radiation delivered, the critical feature of IORT is its precision and minimal exposure to the surrounding healthy tissues. In this manner, IORT fulfills one of the key promises of targeted radiation therapy—it accomplishes minimization of systemic side effects by limiting the irradiated normal tissue volume and increasing the therapeutic index. Thus, IORT has found its use in those situations where surgical intervention provides an opportunity to increase radiation dose to the tumor (dose escalation) and/or decrease total dose to normal tissues (dose de-escalation).

The use of radiation in combination with surgery has a long history. Before the 1960s, the preferred approach to combine these treatment modalities was to do a presurgical radiation treatment. For example, the delivery of 35–40 Gy to the chest wall followed at 6 weeks by mastectomy was a frequent approach for breast cancer patient care at MD Anderson before 1960s (1). Technical difficulties of delivering radiation at the time of surgery were probably the primary reasons why such treatment was attempted for the first time only after 1960. The first IORT work similar to current practice was done in Japan at Kyoto University (2–4); it was soon followed by similar work around the world (5). The successful results of this early work generated much excitement; the first group of IORT gastric cancer patients included two examples of posttreatment 5-year survival of patients with only partially resected gastric cancer who received a 40 Gy intraoperative dose (4). Further developments in the field followed, rapidly at first, with surgical suites built in radiation

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rooms in some cases (5), and much instrument manipulation done collaboratively between hospitals and manufacturers of IR devices (6). IORT quickly became adopted worldwide despite the technical challenges (2, 7–10) but became less used once IR equipment manufacturers ceased engaging in customized instrument changes. A recent resurgence of this treatment modality can probably be credited to the development of new types of IORT dedicated radiation devices such as mobile linear accelerators (11) and specially designed applicators for positioning of high activity radioactive sources (12). Thus, although current medical device regulations prevent in-house development of IORT accelerators—a problem that slowed this field until recently, the availability of commercial devices obviated this necessity.

Early translational studies, conducted by Abe before the first patient IORT and subsequently by many who worked in this field, found a relatively high resilience of different tissues to IORT treatment. For example, experimental studies in a canine model established IORT doses for abdominal surgeries incompatible with different types of postsurgical healing (13). In a study that included nearly 70 animals, it was determined that the intact large blood vessels tolerate doses up to 50 Gy, the urethra up to 30 Gy, and bile ducts up to 20 Gy. Postsurgery the same tissues became less able to cope with radiation injury. Intestinal sutures and arterial anastomoses were found to heal after doses of 45 Gy although not without occasional fibrotic complications. These and subsequent studies in animal models and data from human patients led to the general conclusion that IORT doses as high as 25 Gy could be tolerated by the majority of normal (not surgically treated) tissues without significant toxicity (14, 15). Among the late effects of IORT, delayed progressive ischemia was found to be the complication of most concern. Most of these radiobiological studies were followed-up with patient work that helped drive the IORT field in general.

While some of the illustrative examples of IORT are discussed here, this document is not intended to provide a thorough overview of IORT in current practice. Several exhaustive reviews of IORT were published in recent years demonstrating obstacles and successes of this field (5, 12, 16, 17), and the diverse group of treatment approaches and cancer types treated by IORT in the past few decades. Conclusions from those studies were positive in all cases—if used in appropriate patient groups and controlled correctly, IORT is a life saving procedure. Patient selection criteria, according to these sources, include the following:


The last of these criteria (no evidence of distant metastases) is the basis for most successful IORT treatments. Immediate, single dose irradiation after surgery, in low-risk patients who are most likely to be free of metastasis further assures long-term disease free survival of such patients. For example, an IORT study with women stratified into low-risk and high-risk cohorts as defined by the ELIOT trial, found a greater survival and greatest benefits from IORT in the low-risk group (18). While it may be disappointing that IORT is not of greater benefit for high-risk patients (in whom it does not seem to be of much effect), the fact that this treatment improves disease free survival in low-risk patients is one of the most significant arguments in favor of this therapy.

Nevertheless, it should also be recognized that IORT has found its place in other cancer related scenarios. These include supplemental treatment to recurrent cancers (not eligible for external beam radiotherapy), as well as in palliative care. For example, IORT may be used in combination with kyphoplasty in patients with vertebral metastases; few studies using IORT for metastatic disease documented good pain control (16).

A recent review by Pilar and others (12) provides an excellent resource for review of surgical situations known to employ IORT and lists the possible outcomes achieved by IORT under these different circumstances. In this, most recent review of IORT literature, cancer types treated by IORT include following: primary and recurrent head and neck cancers (IORT doses between 10 and 22 Gy, 2-year overall survival 20–60%), breast cancers (wide spectra of cancer types and outcomes), locally advanced colorectal cancers (IORT doses between 10 and 20 Gy, 5-year overall survival up to 75%), soft tissue sarcomas (IORT doses between 7.5 and 30 Gy, 5-year overall survival up to 7–40%), pediatric tumors (mostly neuroblastoma) (IORT doses between 7.5 and 20 Gy, 10-year overall survival up to 74%), gynecological tumors (IORT doses between 8 and 30 Gy, 5-year overall survival up to 47%), bladder and renal cancers (IORT doses between 9 and 20 Gy, 5-year overall survival up to 73%), prostate cancers (IORT doses between 10 and 30 Gy, 5-year overall survival up to 100%), gastric cancers (IORT doses between 12 and 35 Gy, 2-year overall survival up to 47%), and pancreatic cancers (IORT doses between 10 and 33 Gy, 5-year overall survival up to 35%).

## FUTURE DIRECTIONS IN IORT

First, it should be mentioned that the potential combined use of IORT and immunotherapy was recognized in a recent review by Herskind and others (19). Increasing evidence suggests that high single doses of ionizing radiation may result in tumor-directed immune reactions, locally and systemically (20). While no conclusive data about immune reactions and IORT are available, high single doses delivered during IORT may be expected to cause immunogenic cell death (21) of tumor cells remaining *in situ* after surgery. The greatest concern lies with the fact that the numbers of tumor cells remaining after surgery may be too low to trigger a favorable immune system reaction. (In the same vein—it could perhaps be interesting to investigate the data from IORT cases with incomplete surgical resection, such as, for example, unexpected survivors from Abe's initial studies.) The possibility that IORT may lead to immunogenic cell death of tumor cells may provide an additional reason to investigate IORT anew, perhaps in combination with immune checkpoint blockade treatments (19).

Next, we would like to consider the fact that IORT could easily be combined with other therapies that are also more potent with local delivery and could, in turn, increase radiation sensitivity. IORT is inherently a treatment approach designated for local cancer control. Local delivery is achieved "mechanically"—by placing the source of radiation close to tumor or tumor bed. New therapeutic modalities, still at preclinical testing such as nanoparticle therapies, could also benefit from using a postsurgery scenario to secure targeted local delivery. IORT would then be complemented with localized delivery of nanoparticles that could be designed, e.g., for a controlled release of cargo. Many nanomaterials have the capacity to respond differentially to different temperatures, pH conditions and more; for an extensive review of porous nanomaterials fine tuned for "gated" cargo delivery see, for example Ref. (22). While it may be difficult to guarantee that a given nanoparticle type may not encounter a "cargo-release signal" if delivered systemically, it is much easier to be sure that cargo delivery will be controlled if nanomaterial is to be delivered locally, to tumor or tumor bed physically revealed by the surgery.

It is also not a difficult next step to imagine a potential synergy between radiosensitizing nanoparticles and IORT. In its simplest form—one can envision combined use of IORT with nanoparticles with radiosensitizing small molecule cargo, increasing the efficiency of radiation therapy additively or synergistically. A more complex, and possibly more exciting situation could be accomplished by using nanoparticles with inherent and novel radiosensitizing capacity such as ability to intensify radiation effects by localized release of electrons or reactive oxygen species (23). An overview of different possible radiosensitizing nanoparticle materials (24) details how many Auger electrons, Compton electrons, and photoelectrons are expected for nanoparticles made of elements of different Z and lists the anticipated total energy absorbed and released by each material upon exposure to keV and MV ionizing radiation. Relative biological effectiveness calculated based on these data suggests that keV radiation may be more effective than MV radiation in the presence of nanomaterials rich in Auger electrons. Similarly, several predictions about radiosensitization that can be expected by combining gold nanoparticles with brachytherapy radionuclides (I-125, Pd-103, and Yb-169) and low-energy (50 keV) X-rays were also recently published (25, 26); as well as investigation of potential effects of combination of high Z nanomaterials and proton therapy (27). To all these considerations, one may also add that nanoparticle uptake by cells leads to radiosensitization by biological, cellular, and molecular mechanisms as well (23).

A significant concern in IORT is the dosimetry, and a 10 mm depth margin surrounding the tumor cavity is considered as tissue that must be treated. The current "standard" in IORT linacs that produce MeV electrons are devices called the Mobetron, the Liac, and the Novac; while IORT low-energy X-ray irradiators most frequently used at present are the Intrabeam System and the

### REFERENCES


Papillon System (6). The linacs used for IORT produce electron beams with energies between 4 and 12 MeV with 2–3 MeV steps increase and corresponding penetration increase of 7–10 mm per step; use of higher energies is avoided to prevent neutron contamination that would require additional shielding (6). IORT low-energy X-ray irradiators produce 30 and/or 50 keV X-ray photons and the dose delivered with these devices decreases rapidly with distance; and a prescription at 10 mm depth is considered as best to ensure that desired dose arrives at this tissue depth for all applicator sizes (28). Based on discussion about high Z nanoparticles above, it is clear that they could significantly increase effect of IORT exposures and do so locally—which is the exact prerequisite for IORT.

Finally, it should be mentioned that it might be possible that IORT devices that produce protons or even heavy ions may become possible in the future. These radiation modalities could extend the postsurgical radiation treatment deeper and further than 10 mm from resection margin. While treatment distances further than 10 mm were not considered in IORT practice so far, it is possible that such devices would find their use in clinical practice as well.

### CONCLUDING THOUGHTS

Intraoperative radiation therapy was originally an innovative and clever approach for delivery of radiation during the course of surgery to reduce normal tissue toxicity and at the same time improve tumor treatment. When the modality was first established, limitations revolved predominantly around the availability of instrumentation, the design of facilities and other physical concerns. Radiobiology done to understand constraints of the technology was limited and focused on available systems. With the deeper understanding of the importance of tumor immunology and the development of new tools in nanotechnology, it is hopeful that IORT will be rediscovered. Additional radiobiology experiments designed to probe possible new uses of IORT are essential to understand potential benefits and limitations of these treatments.

## AUTHOR CONTRIBUTIONS

TP and GW selected the references and cowrote the text.

### FUNDING

This work was supported by internal funds from Northwestern University and by NCI grant number R01CA221150.


for intraoperative radiotherapy after resection. *Front Oncol* (2017) 7:147. doi:10.3389/fonc.2017.00147


**Conflict of Interest Statement:** The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

*Copyright © 2017 Paunesku and Woloschak. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) or licensor are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.*