Long-term patient experience with online MR-guided radiotherapy: adaptive versus non-adaptive workflow

Purpose/objective: Magnetic resonance–linear accelerator (MR-Linac) systems enable high-precision radiotherapy through real-time MR guidance and daily online adaptive treatment planning. While online adaptation offers substantial dosimetric advantages, it extends treatment session durations on an already resource-intensive platform. This study aimed to evaluate patient-reported outcome measures (PROMs) and long-term toxicity profiles associated with MR-guided radiotherapy, with a particular focus on the impact of online adaptive workflows.

Materials and methods: This subgroup analysis of an ongoing prospective observational study comprises patients treated with the MRIdian Linac at the Department of Radiation Oncology at Heidelberg University Hospital between January 2019 and May 2021. Online plan adaptation was implemented in February 2020. A custom-designed in-house questionnaire (PRO-Q) was employed to assess patient experience with MR-guided treatment. Toxicity was classified according to the Common Terminology Criteria for Adverse Events (CTCAE v. 5.0).

Results: A total of 231 patients were included, comprising 130 non-adaptive and 101 adaptive treatments across 286 target volumes. Baseline patient characteristics, prior systemic therapy, and median planning target volumes (36.4 mL vs. 35.3 mL) were comparable between groups. Adaptive treatment was associated with significantly prolonged session durations (median 71 minutes vs. 36 minutes; p<0.01). During adaptive treatment, patients reported significantly higher discomfort in domains related to treatment duration, immobility, and sensory perceptions (e.g., tingling) as per PRO-Q responses. No statistically significant differences in overall toxicity were observed. However, patients undergoing adaptive therapy exhibited a faster return to baseline status post-treatment (6–8 weeks vs. 6–12 months).

Conclusion: Online plan adaptation at the MR-Linac increased treatment times and was associated with less favorable short-term patient-reported outcomes, yet it was delivered safely without compromising toxicity or oncologic outcomes. These results support adaptive MR-guided radiotherapy as a feasible and technically promising approach, while highlighting the need for further studies with validated PROMs and cost-benefit analyses to define its clinical value.

1 Background and purpose

Modern radiotherapy is increasingly benefiting from the growing possibilities of computer technology and the wider availability of sophisticated imaging techniques. It is now technically possible to modify a radiation treatment plan while the patient is still on the treatment couch, and MRI image guidance can be integrated into this process (1). Through its superior soft tissue contrast, online MR-guided radiotherapy allows for a more precise definition of tumor tissue and organs at risk (OARs). Moreover, with online treatment plan adaptation, changes in organ at risk (OAR) or tumor anatomy (e.g., bladder filling or tumor regression) can be accounted for on a daily basis (2–4). However, this adaptability comes at the cost of extended treatment times, imposing logistical and physical burdens on both patients and radiation oncology teams (5). The MR-Linac, as a hybrid device, is capable of both online MR guidance and online treatment plan adaptation. There is broad evidence for a dosimetric superiority of online MR-guided adaptive radiotherapy; however, there is currently limited robust randomized data available to demonstrate a translation into superior clinical outcomes (6–11). Dosimetric data for evaluating online adaptive radiotherapy are readily obtainable, as both adapted and non-adapted treatment plans are available for analysis without the need for a patient control group.

In our prior work, we showed that, among others, online adaptive MR-guided radiotherapy provides consistent dosimetric advantages across multiple tumor sites, including lung, liver, adrenal, lymphatic, and prostate tumors. We demonstrated that daily adaptive replanning enables reduced safety margins and improves dose conformity while maintaining or safely escalating ablative dose levels. Our results showed that up to 32.9% of non-adapted plans exhibited OAR constraint violations, and up to 99.4% demonstrated planning target volume coverage violations, both of which could be resolved through plan adaptation. The dosimetric benefit was most pronounced for tumors in high-risk or anatomically complex locations characterized by significant interfractional anatomical variation and motion. Collectively, these findings establish a robust dosimetric foundation supporting the clinical outcomes reported in the present study (11–17).

As far as workflow descriptions on the MR-Linac are concerned, studies on several hundred patients are now available (18, 19). Even without treatment adaptation, MR-guided stereotactic radiotherapy of liver metastases, for example, has a median session duration of 39 minutes (20). However, patient-reported outcome measures (PROMs) are especially important for resource-intensive treatment techniques such as the MR-Linac, since it requires a high degree of patient compliance (21). Preliminary international PROMs of MR-Linac treatment and early toxicity rates were positive (22–26). However, comprehensive assessments of long-term toxicity are sparse, and, to date, no direct comparison of PROMs between adaptive and non-adaptive MR-Linac treatments has been reported. In our previous work, we were able to demonstrate the acceptance and good clinical tolerability of MR-guided radiotherapy. However, that work was conducted at a time when adaptive radiotherapy was not yet established in our center (10, 20). We sought to evaluate the PROMs and long-term toxicity associated with MR-guided radiotherapy, with a particular focus on the impact of online adaptive workflows.

2 Methods

This is an analysis from a prospective observational trial comprising all cancer patients referred to our institution for online MR-guided radiotherapy. Patients were treated with MR-guided radiotherapy at the MRIdian Linac (ViewRay Inc., Mountain View, CA) at the Department of Radiation Oncology at Heidelberg University Hospital between January 2019 and May 2021. Radiotherapy dose and fractionation schemes were prescribed in accordance with tumor-specific standard operating procedures (SOPs) established at our institution. SBRT was defined according to the guideline of the working group “Stereotactic Radiotherapy” of the German Society of Radiation Oncology (single fraction dose ≥ 4Gy and number of fractions ≤12) (27). The implementation of online treatment plan adaptation commenced in February 2020. Online plan adaptation was implemented to account for anatomical changes, improve target volume coverage and enhance sparing of organs at risk. Since evidence on which anatomical treatment sites benefit most from adaptation was limited at the time, we recommended a broad inclusion framework. However, we subsequently discontinued treating patients with bone metastases on the MR-Linac, as no relevant anatomical changes were expected for this specific anatomical site. A subset of patients who received MR-guided radiotherapy for liver and lymph node metastases had previously been reported on in relation to dosimetric parameters and clinical outcomes. These patients (20 patients with liver metastases and 29 patients with lymph node metastases) were included in the present analysis with extended follow-up (10, 20). A comprehensive description of the MR-Linac treatment protocols, including patient selection, simulation, planning, and delivery workflows, has been published previously (28).

The simulation MR images, which served as the primary treatment planning modality, were acquired either during breath-hold in deep inspiration or during free breathing using the TrueFISP sequence. Different in-plane resolutions were used for the 3D simulation MRI (either 1.5 × 1.5mm² or 1.6 × 1.6mm² and slice thickness of 3mm with different fields of view). Sagittal-plane cine MRI images obtained with the TrueFISP sequence were utilized to assess movement. No contrast agent was administered. CT simulation scans were additionally obtained (with or without contrast agent) with the same specifications as the MR simulation and then deformably registered. Clinical target volume margins ranged from 1 mm for pelvic lymph nodes to 5 mm for liver metastases, depending on the anatomical region. In general, a 3 mm margin was added as the planning target volume. Following this, step-and-shoot IMRT treatment plans were generated using the integrated MRIdian planning system. The Monte Carlo dose calculation accounted for the static magnetic field.

To systematically evaluate patient experience with MR-Linac-based radiotherapy, we used an in-house-designed patient-reported outcome questionnaire (PRO-Q) (28). This PRO-Q dates back to the early days of implementing MR-Linac therapy at our center. It is a non-validated tool designed by us for internal quality control. The instrument employs a 5-point Likert scale, with scores ranging from 1 (very positive experience) to 5 (very negative experience). To complement the patient-reported data and provide additional insight into treatment complexity, MR-Linac staff were also asked to assess each patient’s treatment course using a 10-point scale, where 1 indicated minimal difficulty and 10 represented a level of effort approaching unacceptability. The resulting scores were evaluated descriptively for each patient.

Before the first and last treatment session, as well as at first follow-up, each patient was specifically assessed for the presence of fatigue, nausea, vomiting, diarrhea, constipation, dyspnea, cough, skin disorder, and pain. Follow-up started 6–8 weeks after the last treatment day together with a clinical examination and a contrast-enhanced MRI or CT scan if indicated according to the respective institutional guidelines. Further follow-up was not part of the prospective study. Hence, assessment at later follow-up points was performed retrospectively. The Response Evaluation Criteria in Solid Tumors (RECIST 1.1) and the Common Terminology Criteria for Adverse Events (CTCAE v. 5.0) were used for tumor response and toxicity assessment, respectively.

Patient and treatment characteristics were analyzed descriptively (IBM SPSS Version 28.0). Group comparisons were performed using the chi-square test or the Mann–Whitney U test. Multiple responses were taken into account to analyze toxicity. When first- and second-grade toxicity were reported simultaneously, the higher grade was counted for clarity. However, the severity of toxicity was documented at each grade to obtain a complete picture. Toxicity was compared using the chi-square test. To assess PROMs, the Wilcoxon signed-rank test was used to compare patient experiences after the first and last radiation fraction. The Mann–Whitney U test was used to compare patient groups with and without online adaptation. Local control (LC) and overall survival (OS) were estimated starting from the first day of the irradiation. LC was calculated based on each irradiated lesion. OS was calculated per patient. LC and OS were estimated using the Kaplan–Meier method, and significance was tested using the log-rank method. A significance level of α=5% was utilized. The MR-Linac observational study was approved by the Ethics committee of the University Hospital Heidelberg (S-543/2018, S-862/2019).

3 Results

Between January 2019 and May 2021, 231 patients were treated at our MR-Linac (n=130 before and n=101 after the implementation of adaptive radiotherapy 02/2020). In total, n=286 tumor lesions were targeted and n=2020 fractions were applied. Both treatment groups (non-adaptive/adaptive) were similar in terms of age (65 years, range: 28–84 years vs. 66 years; range: 19–89 years), sex, body mass index, performance score, and comorbidity index (Table 1). Median planning target volumes (36.4 mL vs. 35.3 mL) were comparable between both groups. The predominant primary tumors were lung cancer, prostate cancer, and breast cancer, which did not significantly differ between the two treatment groups (Table 2). The most prevalent targets were lymph node and liver metastases (Table 3). Most patients did not receive systemic therapy four weeks before or after MR-guided radiotherapy. Table 4 provides an overview of the systemic therapy that was administered. The treatment characteristics (Table 5) were comparable between the two groups, but the median treatment duration increased from 36 minutes to 71 minutes after the implementation of adaptive radiotherapy. Toxicity was low in general, with no grade 4 or higher toxicity at all and only one case of grade 3 toxicity at any time. There was no significant difference between the treatment groups (Table 6), but descriptive analysis revealed that grade 1 toxicity already returned to baseline at first follow-up in the adaptive treatment group. Figure 1 depicts the toxicity over time. Table 7 provides an overview of the symptoms reported. Fatigue was by far the most common symptom and was already present at baseline in 28% of the patients.

Table 1

Table 1. Patient characteristics.

Table 2

Table 2. Primary tumor.

Table 3

Table 3. Localization of target volumes.

Table 4

Table 4. Systemic therapy 4 weeks before and after irradiation.

Table 5

Table 5. Treatment characteristics.

Table 6

Table 6. Comparison of toxicity (CTCAE).

Figure 1

Figure 1. Comparison of long-term toxicity (no adaption vs. adaption).

Table 7

Table 7. Specification of toxicity.

Three items within the PRO-Q were rated significantly less favorably by patients in the adaptive treatment group (Table 8). Specifically, patients in the adaptive cohort reported greater dissatisfaction with the overall treatment duration, the requirement to remain immobile during the procedure, and the occurrence of tingling sensations in the extremities, with mean scores of 3.3 vs. 2.5, 2.6 vs. 2.2, and 2.2 vs. 1.8, respectively. In addition, treatment complexity, as assessed by MR-Linac personnel, was rated significantly higher in the adaptive group, with a mean score of 4.8 compared to 4.1. The median follow-up across the cohort was 35 months. In an exploratory analysis, treatment duration showed no additional association with patient-reported experience or acute toxicity beyond the effects of adaptive versus non-adaptive workflows. No statistically significant differences in local control (LC) or overall survival (OS) were observed between the adaptive and non-adaptive groups (Figure 2).

Table 8

Table 8. Results of the patient-reported outcome questionnaire (positions 1-17) and the staff questionnaire (position 18).

Figure 2

Figure 2. Local control (p=0.544) and overall survival (p = 0.875) compared between no adaption and adaption.

4 Discussion

This analysis presents patient-reported outcomes and long-term toxicity data from a cohort of 231 cancer patients treated with MR-guided radiotherapy on an MR-Linac system between January 2019 and May 2021. To our knowledge, this represents the largest single-institution series to date that specifically evaluates these parameters in the MR-Linac setting. The mid-study implementation of online adaptive radiotherapy in February 2020 afforded a unique opportunity to compare adaptive and non-adaptive workflows within a consistent institutional framework. Nearly half of our patients were treated for liver or lymph node metastases - disease sites particularly well suited for MR guidance given its superior soft-tissue contrast (29, 30).

The primary objective of this study was to evaluate long-term patient experience and acceptance of online MR-guided radiotherapy workflows, comparing adaptive and non-adaptive approaches, rather than to perform a definitive clinical efficacy comparison. Although baseline patient characteristics, prior systemic therapy, and median planning target volumes were comparable between groups, the non-adaptive MR-guided cohort reflects treatments delivered before routine implementation of online adaptation and therefore represents a heterogeneous group influenced by temporal, technical, and workflow-related factors. Moreover, adaptive treatments enabled delivery of significantly higher doses, further limiting the feasibility of a direct, uniform clinical comparison. Consequently, clinical outcome and toxicity data are reported descriptively and should be interpreted with caution, as acknowledged.

Consistent with other early clinical reports, overall treatment experience in our cohort was rated favorably (22, 23, 31, 32). However, the transition to online adaptation resulted in a doubling of median treatment session duration (36 vs. 71 minutes), which was associated with less favorable patient-reported outcomes in some domains. This effect is plausibly linked to the longer immobilization time and cooler treatment environment. Importantly, levels of anxiousness and overall treatment satisfaction did not deteriorate, suggesting that patients generally tolerated the increased burden. Our institutional experience also demonstrates that simple supportive measures, such as providing additional blankets, can improve patient comfort. Nevertheless, these findings highlight the need to balance technical precision against treatment burden and to prioritize workflow optimization and patient comfort when implementing adaptation (6–11, 15).

Treatment-related toxicity in our cohort was low, with no grade ≥4 events and durable safety confirmed after a median follow-up of 35 months - among the longest reported in MR-Linac literature (33–35). We found no significant differences in toxicity between adaptive and non-adaptive treatments. Interestingly, patients in the adaptive group tended to return to baseline symptoms earlier, although this trend did not reach statistical significance. Overall, our results indicate that adaptive therapy can be delivered safely without compromising oncologic outcomes, but measurable advantages in toxicity, local control, or survival were not observed in this retrospective comparison.

At first glance, adaptive radiotherapy may therefore appear less desirable from a patient perspective, as it prolongs treatment sessions and is associated with less favorable short-term patient-reported outcomes, without showing clinical differences in toxicity or survival. However, our study has several important limitations. First, the primary outcome instrument (PRO-Q) was developed internally and has not undergone formal external validation. Although it has proven useful for internal quality assurance, this limits the generalizability of our results. Second, the adaptive and non-adaptive groups were not concurrently assigned but defined by the timeline of implementation, introducing both temporal and selection bias. Third, structured prospective toxicity monitoring ended after the first follow-up, with long-term data collected retrospectively, raising the possibility of underreporting. These methodological constraints limit the strength of causal inference.

Despite these limitations, our findings are consistent with international experiences reporting favorable patient tolerance and low toxicity in MR-guided therapy (22). Reports from Tübingen, Amsterdam, Istanbul, Utrecht, and Toronto have similarly documented positive patient experiences, highlighting the global reproducibility of MR-Linac outcomes (23–26, 31, 32). Beyond patient experience, MR guidance itself provides independent advantages - including real-time tumor visualization, gating, and functional imaging- that are not contingent on adaptive workflows (36, 37). The MIRAGE trial, for example, demonstrated significantly reduced genitourinary and gastrointestinal toxicity and improved patient-reported bowel outcomes in prostate cancer patients treated with online MR guidance, despite not employing plan adaptation (38). This underscores that MR guidance alone confers important clinical benefits, and that the incremental contribution of adaptation must be carefully disentangled in future studies (7, 15, 39, 40).

From a technical perspective, the dosimetric advantages of online adaptive radiotherapy are well established and are likely to translate into clinical benefit over time, as was the case during the transition from 3D-CRT to IMRT (41–43). However, the increased time required for adaptive workflows currently narrows the patient population who can feasibly benefit (44). Ongoing developments in workflow automation, deformable image registration, and artificial intelligence are expected to improve efficiency and patient tolerance. Furthermore, CT-based online adaptive systems are emerging and offer faster session times. These systems can also be coupled with weekly offline MR guidance (MARS) (45–47). We are currently investigating such approaches prospectively within our AIM-C1 trial for cervical cancer (48). These evolving strategies highlight the importance of tailoring adaptive and MR-guided workflows to patient-specific needs and treatment contexts.

5 Conclusion

The MR-Linac platform enables advanced radiotherapy delivery by combining real-time MR guidance with online plan adaptation. In this large single-institution series with long-term follow-up, we confirmed low treatment-related toxicity and overall favorable patient-reported outcomes. Adaptive therapy was associated with longer treatment times and less favorable ratings in certain patient-reported domains. The clinical relevance of the observed PROMs differences remains uncertain, but our findings underscore the importance of optimizing workflow efficiency and patient comfort when applying adaptive techniques. Future studies should clarify the unique contribution of adaptation, independent of the broader benefits of MR guidance, through randomized trials, validated PROMs instruments, and cost-benefit evaluations. Until such evidence is available, adaptive MR-guided radiotherapy should be applied selectively and particularly in patients where anatomical changes or organ-at-risk proximity make plan adaptation most likely to improve the therapeutic ratio.

Data availability statement

The data presented in this study will be available on reasonable request.

Ethics statement

The studies involving humans were approved by Ethics committee of the University Hospital Heidelberg. The studies were conducted in accordance with the local legislation and institutional requirements. The participants provided their written informed consent to participate in this study.

Author contributions

FW: Visualization, Writing – original draft, Writing – review & editing. CH-W: Visualization, Writing – review & editing. SR: Data curation, Software, Visualization, Writing – review & editing. JL: Conceptualization, Software, Visualization, Writing – review & editing. EM: Conceptualization, Software, Visualization, Writing – review & editing. PH-S: Software, Supervision, Visualization, Writing – review & editing. LK: Conceptualization, Supervision, Visualization, Writing – review & editing. KL: Software, Validation, Visualization, Writing – review & editing. CKR: Software, Supervision, Visualization, Writing – review & editing. CR: Software, Supervision, Visualization, Writing – review & editing. SK: Software, Supervision, Visualization, Writing – review & editing. JD: Software, Supervision, Visualization, Writing – review & editing. JH-R: Software, Supervision, Visualization, Writing – review & editing.

Funding

The author(s) declared that financial support was received for this work and/or its publication. PH-S was funded by the Physician-Scientist Program of Heidelberg University, Faculty of Medicine. The installation of the MR-Linac in Heidelberg was kindly funded by the German Research Foundation DFG (funding reference DE 614/16-1). For the publication fee we acknowledge financial support by Heidelberg University.

Conflict of interest

FW received speaker fees from AstraZeneca, Varian Medical Systems, Siemens Healthineers, Chulabhorn Royal Academy, and Merck Sharp & Dohme; travel support for attending meetings with AstraZeneca, Varian Medical Systems, Novocure GmbH, German Center for Lung Research DZL, Fraunhofer MEVIS, Chulabhorn Royal Academy, and Micropos Medical; and advisory board compensation from Novocure GmbH and Merck Sharp & Dohme. JD received grants from RaySearch Laboratories AB, Vision RT Limited, Merck Serono GmbH, Siemens Healthcare GmbH, PTW-Freiburg Dr. Pychlau GmbH, and Accuray Incorporated outside of the submitted work. PH-S received advisory board compensation from NovoCure GmbH outside of the submitted work. JH-R received speaker fees from Pfizer Inc., Sanofi, AstraZeneca, and Accuracy International Sàrl, as well as grants from IntraOP Medical and Varian Medical Systems outside the submitted work. SK received speaker fees von Siemens and participated in a clinical advisory board for Accuray International. SR holds shares in Siemens Healthineers.

The remaining author(s) declared that this work was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

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