The Efficacy and Safety of Radiofrequency Ablation for Bilateral Papillary Thyroid Microcarcinoma

Objective To investigate the long-term clinical results of radiofrequency ablation (RFA) for bilateral papillary thyroid microcarcinoma (PTMC). Materials and Methods From October 2014 to February 2018, 47 patients (37 females, 10 males, mean age 43.39 ± 9.26 years) with 100 bilateral PTMC (mean volume 75.22 ± 73.87 mm3) treated by RFA were included in this retrospective study. Bilateral PTMC was defined as at least one tumor located in the contralateral lobe. Patients were followed up at 1, 3, 6, 12 months and every 6–12 months thereafter. Volume, volume reduction ratio (VRR) and local tumor recurrence were evaluated during the follow-up period. Results After a mean follow-up period of 47.77 ± 11.54 months, the mean volume of bilateral PTMC decreased from 75.22 ± 73.87 mm3 to 0.09 ± 0.44 mm3. The mean VRR was 99.94 ± 0.28% and the complete disappearance rate was 92.00%. During the follow-up, one patient (2.13%) developed lymph node metastasis and two patients (4.26%) had recurrent PTMC. All the recurrent lesions underwent additional RFA and two of them disappeared completely. No life-threatening or delayed complications occurred. Conclusions With sufficient preoperative evaluation, RFA might be a promising alternative for bilateral PTMC patients who were unsuitable for surgery or refused surgery.


INTRODUCTION
The incidence of thyroid cancer has increased worldwide, ranking in ninth among all cancers (1)(2)(3)(4). Approximately 50% of the increase was attributed to the detection of papillary thyroid microcarcinoma (PTMC), which is a form of papillary thyroid cancer with a maximum diameter of 1 cm, with or without high-risk features (5). Bilateral lesions are very common in PTMC with an incidence of approximately 10-30% (6). It was previously correlated with a higher risk of locoregional recurrence. However, some studies have shown that tumor bilaterality was not associated with an increased risk of recurrence/persistence (7). According to the 8th AJCC/TNM risk of mortality system and American thyroid guideline (ATA) risk stratification, intrathyroidal bilateral PTMC was classified stage I with a low-risk of recurrence (8,9). Surgery is the first-line treatment for bilateral PTMC. However, its drawbacks include invasiveness, cosmetic problems and life-long thyroid hormone replacement, which affect the quality of life (10). Moreover, the incidence of transient vocal fold paralysis and transient hypocalcemia after surgery were 5.5 and 4.3%, respectively (11). Radiofrequency ablation (RFA) as a minimally invasive technique, might be a potentially alternative for bilateral PTMC who were unsuitable for surgery or are contraindicated for surgery.
RFA is a commonly used thermal technique, has been reported as an effective and safe treatment for low-risk PTMC patients who refuse surgery or active surveillance (12)(13)(14)(15)(16)(17)(18)(19). After ablation, the pooled proportion of volume reduction rate (VRR) was 98.1% (20) and the pooled proportion of complete disappearance rate was 57.6-76.2% (20,21). However, the results were based mainly on the clinical outcomes of RFA for unifocal PTMC. To the best of our knowledge, no study has reported the long-term clinical outcomes of ablation for bilateral PTMC.
Therefore, the purpose of this study was to evaluate the efficacy and safety of RFA for bilateral PTMC.

MATERIALS AND METHODS
The Institutional Review Board of our institution approved this retrospective study. All the patients were provided written information consent before RFA. The RFA informed consent emphasized that surgery was the routine treatment recommended by guidelines.

Patients
Bilateral PTMC was defined as at least one tumor located in the contralateral lobe (11). The inclusion criteria were: (1) PTMC lesions were confirmed by core-needle biopsy(CNB) or fineneedle aspiration(FNA); (2) no evidence of extrathyroidal extension(ETE) or lymph node(LN)/distant metastasis on US and chest CT; (3) patients who were unsuitable for surgery or refused surgery; (4) no neck irradiation history; (5) follow-up period was ≥24 months. The exclusion criteria of patients were: (1) no convincing evidence of aggressive disease by biopsy (9); (2) the maximum diameter of the tumor was ≥10 mm; (3) ETE was found; (4) LN metastasis or distant metastasis was detected; (5) patients with conscious disturbance or coagulation disorder or serious primary disease; (6) follow-up period was <24 months.
From October 2014 to February 2018, 468 patients with bilateral PTMC underwent treatment in this institution. Among them, 76 patients who were unsuitable for surgery or rejected surgery underwent RFA. Among them, patients with follow-up period less than 24 months (N = 29) were excluded. At last, a total of 47 patients with 100 low-risk PTMC were evaluated.

Pre-Ablation Evaluation
Before RFA, patients all underwent thorough examinations, including complete blood count, thyroid function tests, coagulation tests and imaging evaluation, including ultrasound and chest CT (12)(13)(14). The volume of PTMC was calculated with the equations: V is the volume, while a is the largest diameter, b and c are the other two perpendicular diameters. US were performed using a Siemens Acuson Sequoia 512 Ultrasound System (Siemens) or a Philips iU22 Ultrasound System (Philips Healthcare) or a Mindray M9 Ultrasound System (Mindray). CNB and RFA were all performed using a Siemens Acuson Sequoia 512 Ultrasound System. Contrastenhanced ultrasound (CEUS) was used to assess the tumor before and immediately after RFA procedure. The ultrasound contrast agent was sulfur hexafluoride (SonoVueR). After injection of 2.4 ml SonoVue followed by a 5 ml of normal saline flush, CEUS was applied to observe the real-time microbubble perfusions within the tumor and the surrounding thyroid tissues.

Ablation Procedures
All RFA procedures were performed by an experienced US physician. A bipolar RFA generator (CelonLabPOWER) and an 18-gauge bipolar RF applicator with 0.9 cm active tip (CelonProSurge micro 100-T09) were used.
Patients lay on an operating table in the supine position with the neck hyperextended. The targeted tumor was evaluated by multi-angle scanning to determine a practical and proper approach. Doppler ultrasound was used to access the detailed vascular anatomy along the approach route to prevent bleeding. Local anesthetic (1% lidocaine) was injected at the subcutaneous puncture site and the thyroid anterior capsule. During the RFA procedure, the smaller lesion was ablated first.
RFA procedure was performed using trans-isthmic approach and moving-shot technique. To prevent thermal injury, hydrodissection technique was performed by injection of normal saline to separate the target tumor from critical structures (trachea, carotid artery, jugular vein, esophagus and recurrent laryngeal nerve). Normal saline was injected using another needle (23 gauge) to form at least 1 cm distance between the tumor and the critical structure (13,14).
The initial RFA power was 3 W and was increased to 5 to 9 W if a transient hyperechoic zone did not form at the electrode tip in 10 s. To prevent marginal residue and recurrence, we enlarged the ablation area which exceeded the tumor edge (at least 3-5 mm) (21,22). CEUS was performed immediately after the RFA procedure to assess the ablation area. If any enhancement existed, a complementary RFA should be applied.

Post-Ablation Assessment
Patients were followed up at 1, 3, 6 months and every 6-12 months thereafter by US, CEUS and chest CT. At 3 or 6 months after RFA, the ablated area was evaluated by CNB, which was performed to the central zone, the peripheral zone and surrounding thyroid parenchyma (22)(23)(24). The development of metastatic LNs and the suspicious new lesions were submitted to biopsy. The volume reduction rate (VRR) was calculated by the equations: RFA was considered to successful when one of the criteria were met (25): (1) the ablated tumor was completely disappearance; (2) the ablated tumor remained as scar-like on US but absence of enhancement on both arterial and venous phase on CEUS; (3) If the ablation area still existed, no malignant cells was confirmed by CNB. Local tumor recurrence was defined to include two situations (26): (1) new lesion was confirmed to be PTMC after CNB; (2) cervical LN metastasis was confirmed after biopsy. Distant metastasis was diagnosed by CT, positron emission tomography or bone scan when there were suspicious symptoms. Delayed surgery is defined as that patients received surgery due to tumor progression or anxiety during the follow-up.

Statistical Analysis
Statistical analysis was performed using the SPSS statistical software (Version 25.0). Continuous data were presented as mean ± SD (range). Non-parametric Wilcoxon signed-rank tests were used to compare the pre-ablation with post-ablation volume as each follow-up period. A P <0.05 was considered as statistically significant.

RESULTS
Clinical characteristics of patients before RFA are shown in Table 1. Among the 47 patients, 41 had two tumors and six had three tumors. The mean diameter was 4.82 ± 1.57 mm and the mean volume was 75.22 ± 73.87 mm 3 . The volume of dominant tumor was 108.77 ± 87.08 mm 3 .
For each tumor, the mean power was 3.92 ± 0.98 W. The mean RFA time was 224.09 ± 156.68 s and the mean energy was 853.64 ± 614.39 J.

Local Tumor Recurrence
Because 13 tumors disappeared in the first 3 months and another six tumors disappeared at 6 months, a total of 81 tumors underwent post-ablation CNB and the results were all negative.
The incidence of LN metastasis and of recurrence PTMC was 2.13% (1/47) and 4.26% (2/47), respectively. No distant metastasis was detected. No patient underwent delayed surgery during the follow-up. The clinical characteristics and outcomes of PTMC patients with local tumor recurrence are showed in Table 3. One patient developed LN metastasis in the central compartments at 12 months after RFA with a volume of 100.53 mm 3 . Two patients had recurrent PTMC and the volume was 18.85 mm 3 and 153.93 mm 3 , respectively. All of these three patients underwent additional RFA. Two recurrent PTMCs were completely disappeared during the follow-up. The metastatic LN shrunk to 37.70 mm 3 after 1-year follow-up. This lesion underwent CNB and demonstrated no malignancy.

Safety
All the 47 patients were tolerable to RFA. Four patients underwent local pain or discomfort and they all resolved spontaneously within 3 days. No patients had major or delayed complications.  Bilaterality has been observed in approximately 10-30% of patients with PTMC (6). It refers to the presence of multiple synchronous primary tumors, arising from independent clones, instead of intraglandular metastasis from a single primary tumor via intraglandular lymphatics (27,28). However, the prognostic value of bilateral PTMC remains controversial. Studies recommended more aggressive treatments such as total thyroidectomy with central neck dissection and subsequent radioactive iodine ablation therapy for bilateral PTMC, which has an increased loco-regional recurrence rate (29,30). In contrast, Choi et al. (7) found that bilateral PTMC was not an important prognostic factor in PTMC patients, but in the non-PTMC patients. Zhou et al. (6) also reported that bilateral PTMC was significantly associated with central LN metastasis on the univariate analysis. However, this was not an independent risk factor after multivariate logistic regression analysis. The conflicting results were likely due to the early detection and treatment of bilateral lesions, which did not fully exhibit its biological behavior.
Although thermal ablation is not the first-line treatments for PTMC, it has been applied to unifocal low-risk PTMC patients  concerned about active surveillance or complications after surgery (17,25,(31)(32)(33)(34)(35)(36)(37)(38). A few patients with multifocal or bilateral PTMC were also treated with ablation, but the clinical outcomes were not reported separately (14)(15)(16)39). The viability of RFA as an alternative for bilateral PTMC patients who refuse surgery or are unsuitable for surgery is unknown. This study included 100 bilateral PTMC lesions in 47 patients who were unsuitable for surgery or refused surgery. This was the first study to evaluate the clinical results of RFA for bilateral PTMC. After a mean follow-up period of 47.77 ± 11.54 months, the VRR was 99.94 ± 0.28% and 92% of tumors completely disappeared. This was consistent with previous studies on RFA for unifocal PTMC, which reported a VRR of 90-100%, with a complete disappearance rate of 15.22-100% (17,25,(31)(32)(33)(34)(35)(36)(37)(38). This demonstrated that the efficacy of RFA for bilateral PTMC was comparable to that of unifocal PTMC.
In terms of safety, a meta-analysis reported that the pooled proportion of overall complications after thermal ablation was 3.1% and major complications occurred in 0.7% (20). The pooled proportion of complications after RFA was 1.7% (21). In this study, only four patients experienced local pain or discomfort. None of the patients experienced major complications. Several strategies were related to the low incidence of complications. First, RFA was performed by an experienced physician. A detailed preoperative evaluation and adequate knowledge of neck anatomy on US could minimize the incidence of complication (40). Second, during the RFA procedure, smaller lesion was usually treated first and the needle tip was monitored continuously and cautiously via US. Third, safe precautions against thermal injuries, such as the moving-shot technique, trans-isthmic approach and hydrodissection technique, were also performed during the RFA procedure (40).  The local tumor recurrence rate after ablation for unifocal PTMC is low. A previous study showed that the incidence of LN metastasis and recurrent PTMC following unifocal PTMC ablation were 0.84-2.98 and 1.19-2.78%, respectively (25,(34)(35)(36)(37)(38). A similar incidence of LN metastasis (2.13%) was observed in this study. However, the incidence of recurrent PTMC for bilateral PTMC was 4.26%, which was higher than that for unifocal PTMC. This was likely due to the low sensitivity of US in detecting multiple tumors. Thus, occult tumor foci may have been missed by preoperative evaluation. Although all recurrent lesions were successfully treated by additional RFA, sufficient preoperative evaluation was crucial in identifying the number and location of tumors and formulating an appropriate treatment strategy. Surgery remains the first-line treatment for bilateral PTMC. However, patients who refuse or are ineligible for surgery can opt for RFA for palliative purposes, with fully informed consent and thorough follow-up management.
This study had some limitations. First, it was a single-center retrospective study. Second, the sample size of bilateral PTMC cases was relatively small. Third, the follow-up period was relatively short. Given the good prognosis of PTMC, further studies with a larger number of bilateral PTMC patients and a more extended follow-up period are needed. Fourth, this study did not compare RFA with surgery for the treatment of bilateral PTMC.
In conclusion, with sufficient preoperative evaluation, RFA might be a promising treatment for bilateral PTMC patients who were unsuitable for surgery or refused surgery.

DATA AVAILABILITY STATEMENT
The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.

ETHICS STATEMENT
The studies involving human participants were reviewed and approved by The Institutional Review Board of Chinese PLA General Hospital. The patients/participants provided their written informed consent to participate in this study.

AUTHOR CONTRIBUTIONS
LY interpreted the patient data and drafted the manuscript. YKL performed RFA procedure, and conceived of the study and coordination. MBZ, QS, JX, and YZ collected and analyzed the patient data. All authors contributed to the article and approved the submitted version.