Initial ablation ratio predicts the recurrence of low-risk papillary thyroid microcarcinomas treated with microwave ablation: a 5-year, single-institution cohort study

Objective To assess the long-term efficacy and safety of microwave ablation (MWA) in treating low-risk papillary thyroid microcarcinomas (PTMC) and to identify predictive factors for the postoperative local tumor progression of PTMC. Methods A total of 154 low-risk PTMC patients treated with MWA who were followed up for at least 3 months were retrospectively recruited. Ultrasonography was performed after MWA to assess the local tumor progression. Adverse events associated with MWA were recorded. The ablated volume (Va) and initial ablation ratio (IAR) were measured to assess their influences on the recurrence risk of PTMC. Results The mean tumor volume of PTMC before MWA was 0.071 (0.039, 0.121) cm3, with a maximum diameter of 0.60 ± 0.18 cm. All PTMC patients were followed up for 6 (3, 18) months. Va increased immediately after MWA, then gradually decreased over time, till significantly smaller at 12 months than that before MWA (P < 0.05). The median volume reduction ratio at 24 months reached 100%, which was maintained during a 60-month follow-up. A total of 7 (4.55%) cases of local tumor progression were recorded during the follow-up. Kaplan–Meier survival analysis revealed that the rate of local tumor progression was significantly lower in PTMC patients with a maximum tumor diameter < 0.70 cm than in those with ≥0.70 cm (P = 0.031). A significant better prognosis was achieved in PTMC patients with IAR ≥ 15 than in those with IAR < 15 (P = 0.015). Sex, age (<55 years) and preoperative thyroid-stimulating hormone (>2.0 mU/L) of PTMC patients were not correlated with local tumor progression. Conclusion MWA is an effective therapeutic strategy for low-risk PTMC with high safety. The maximum tumor diameter and IAR are predictive factors for the local tumor progression of PTMC after MWA.


Introduction
With the widespread application of ultrasonography in recent decades (1), the prevalence of thyroid cancer has gradually risen to the 11th and 5th ranks in all cancers and those in women, respectively (2). Papillary thyroid carcinomas (PTCs) are the most common type of thyroid cancer, especially papillary thyroid microcarcinomas (PTMCs) with a diameter ≤ 10 mm. According to 'Thyroid Cancer Incidence and Mortality Trends in the United States: 2000-2018', the overall incidence of thyroid cancer in the United States is 11.95/100,000, of which 89.1% and 35.3% are PTCs and thyroid cancers with a diameter ≤ 1 cm, respectively (3). The incidence of thyroid diseases in China has increased significantly as well. A sharp increase in the proportion of PTMC in PTC cases has been observed: from 0 in 1997-1985 to 32.1% in 2000-2014 (4). A retrospective study in Shanghai, China, showed that PTC (98%, 2440/2490) accounts for the majority of thyroid cancers, and more than 50% (1295/2440) of them are diagnosed as PTMCs (5).
Thyroid cancer is characterized by a favorable prognosis and low tumor-specific mortality (6). Low-risk PTMC patients usually have an acceptable prognosis, with 10-year and 15-year survivals of 94.6% and 90.7%, respectively, and the disease-specific survival is up to 99.5% (7). Surgical treatment is preferred for PTMC patients. A recent study has reported that the recurrence rate after unilateral lobectomy in PTMC patients is only 3.8% (8). A meta-analysis of 21,329 person-years of follow-up showed that the postoperative recurrence of nonincidental PTMC is as low as 7.9% (9). Furthermore, the recurrence of nonincidental PTMC is not correlated with age, sex, tumor size, tumor multifocality, lymph node involvement, and therapeutic strategies. Interestingly, a meta-analysis involving 13 clinical studies has demonstrated that male gender, multifocality, tumor size > 5 mm, and extrathyroidal extension are reliable predictors of central lymph node metastasis in CN0 PTMC (clinical node-negative PTMCs) (10). Multiple cohort studies conducted in Japan (11), the United States (12), and Korea (13) have proven that active surveillance (AS) is an alternative to surgical treatment for low-risk PTMC. A Japanese research group has reported that 5 and 10% of PTMC patients experience a minimum increase of 3 mm in the maximum tumor diameter at 5 and 10 years, respectively (14,15). A multi-center cohort study has shown that tumor volume increase and lymph node metastases are observed in 23.2 and 4.6% of PTMC patients during the AS period. Age is believed to be a prognostic factor for thyroid cancer, that is, younger age is correlated with tumor growth (16). Whether sex is a risk factor for lymph node metastasis in low-risk PTMC, however, is controversial (17).
Thermal ablation is an alternative to low-risk PTMC patients who refuse to be managed by surgery or AS. It may emerge as radiofrequency ablation (RFA), laser ablation (LA), and microwave ablation (MWA), all of which are highly effective and safe for low-risk PTMC. Yan et al. (18) have analyzed 884 PTMC patients intervened by surgery or RFA via propensity score matching. They have found that the rate of local tumor progression in PTMC patients treated with RFA is 2.0% (9/460), which is comparable to that in surgically treated patients. However, they did not analyze the relevant risk factors for the local tumor progression of PTMC. Kim et al. (19) have followed up on 90 PTMC patients after LA for 10 years. The 12-month volume reduction ratio (VRR) and recurrence rates are 100 and 5.5%, respectively. One patient developed lymph node metastasis shortly after LA. Besides, the recurrence of PTMC is not correlated with age, sex, tumor size, tumor volume, tumor location, presence or absence of adjacent organs, ultrasound features, ablation volume, and ablation time. Wei et al. (20) have reported that the disease progression rate of PTMC patients after MWA is 3.4%, and they did not identify any independent risk factors for the progression of T1N0M0 PTC. Our previous study has consistently validated the safety of MWA in the treatment of PTMC, with a 2-year tumor progression rate of 5.48% (4/73) (21). Nevertheless, the risk factors for the recurrence of PTMC after thermal ablation still remain unclear.
The present single-center cohort study analyzed the efficacy and safety of MWA in the treatment of PTMC. Through a 5-year follow-up, we identified that the maximum tumor diameter was a risk factor for the local tumor progression of PTMC, and initial ablation ratio (IAR) was a predictive factor. Our findings support the efficacy of thermal ablation in treating PTMC and preventing postoperative tumor progression.

Subjects
It was a single-center cohort study involving low-risk PTMC patients treated with MWA in the Affiliated Hospital of Integrated Traditional Chinese and Western Medicine, Nanjing University of Chinese Medicine from August 2016 to October 2022 who were followed up for a minimum of 3 months (Fig. 1). Inclusion criteria were as follows: (i) PTMC with a single focus was diagnosed by thyroid fine-needle aspiration cytology (FNAC) or pathology by core needle biopsy; (ii) maximum tumor diameter ≤ 10 mm; (iii) aged 16-75 years; (iv) absence of clinical or imaging evidences of capsular invasion, extrathyroidal invasion, invasion of recurrent laryngeal nerve and trachea, lymph node metastasis, and distant metastasis; (v) subjects were clearly informed of the risk and willing to be treated with MWA. Exclusion criteria were as follows: (i) other types of thyroid cancers; (ii) PTMC with multiple foci; (iii) allergy to local anesthetics, painkillers, and hemostatics used in the present study; (iv) coagulation disorders (prothrombin time > 18 s, prothrombin activity < 40%); (v) pregnant and lactating women; (vi) clinical data were not available; (vii) combined with other malignant diseases. The study protocol was approved by the Ethics Committee of Affiliated Hospital of Integrated Traditional Chinese and Western Medicine, Nanjing University of Chinese Medicine. Written informed consent was obtained from all subjects.

Definition
VRR was calculated using the formula: VRR (%) = (initial tumor volume − final volume)/initial volume × 100%. Two indexes, ablated volume (Va) and IAR, were introduced in the present study (22). Va was defined as the ablation area measured by contrastenhanced ultrasonography on the day or the next day of MWA, which was calculated as follows: Va (cm 3 ) = πabc/6. IAR was defined as the ratio of Va to preoperative tumor volume, which was calculated as follows: IAR (%) = Va/preoperative tumor volume × 100%.
The primary outcomes included local tumor progression, recurrence-free survival (RFS), and delayed surgery. Local tumor progression was defined as any of the following (23,24): (i) malignancies persistently detected within the area where tumor ablation was confirmed by biopsy; (ii) newly formed PTMC lesions in the thyroid parenchyma confirmed by biopsy; (iii) cervical lymph node metastasis confirmed by ultrasoundguided puncture or surgery. RFS was defined as the duration from the primary treatment to the time of tumor relapse or the last follow-up visit. Delayed surgery was defined as when the patient having been initially treated with MWA received thyroidectomy due to tumor progression during the follow-up period or anxiety. The disappearance of tumor was defined as the absence of tumor in the ablation area confirmed by the follow-up ultrasonography.
Major and minor complications were recorded. The former mainly included recurrent laryngeal nerve injury and hypoparathyroidism, and the latter mainly included postoperative pain, bleeding, infection, and abnormal thyroid function. Recurrent laryngeal nerve  injury was defined as the impairment of unilateral or bilateral vocal cord movement examined by laryngoscopy. Hypoparathyroidism was diagnosed by a parathyroid hormone level < 15 pg/mL at 24 h postoperatively. Permanent recurrent laryngeal nerve injury and hypoparathyroidism were defined as no recovery of the impairment of unilateral or bilateral vocal cord movement and parathyroid hormone level, respectively, within 6 months.

MWA procedures
Preparations before MWA Multi-angle cross-sectional ultrasound data of thyroid nodules were recorded using the Siemens ACUSON S2000 ultrasound system before MWA by experienced sonographers, including tumor size, volume, location, and surrounding tissues. These data were considered to determine the optimal approach to MWA.

Microwave ablation
Ultrasound-guided MWA was performed by an experienced physician using an MWA system (KY-2000, Canyon Medical, Nanjing, China) consisting of a generator, a power distribution system, and an antenna as previously reported (21). A cooled shaft antenna with 1.6-mm diameter, 10-cm length, and 3-mm distance from the electrode to the needle tip was used to puncture into the thyroid nodule. The MWA system produced 2450 MHz and 35 W output power to induce coagulative necrosis of thyroid tumors and the surrounding tissues, which were continuously ablated until the entire tumor and the surrounding area were hyperechoic. The residual heat of the antenna was used to coagulate the puncture approach to prevent the proliferation and implantation of tumor cells. We intermittently communicated with the patient during the MWA procedure, aiming to reduce the potential damage to the recurrent laryngeal nerve.

Follow-up
Thyroid function tests and ultrasound elastography were performed at 1, 3, 6, and 12 months, then every 6 months. Thyroid hormone suppression therapy was not given to PTMC patients after MWA. Recurrence, cervical lymph node metastasis, and distant metastasis were assessed, and Va, IAR, and VRR were calculated.

Statistical analysis
Statistical analysis was performed using SPSS 20.0 and Prism. Measurement data in a normal distribution were expressed as mean ± standard deviation, otherwise as the median and quartile. Categorical data were expressed as the percentage. Differences between groups were analyzed by the Chi-square test or Fisher's exact test. The cut-off value was identified by plotting ROC curves. The Kaplan-Meier curve was plotted to analyze the survival, followed by the comparison using the log-rank test. Univariable logistic regression analysis was performed to screen factors that were correlated with the recurrence risk of PTMC, and those with P < 0.2 were submitted to the multivariable logistic regression analysis to identify risk factors. P < 0.05 was considered statistically significant.

Baseline characteristics
From August 2016 to October 2022, a total of 186 patients with PTMC were treated with MWA in our hospital. After the exclusion of 32 patients according to the inclusion criteria, 154 patients with PTMC were finally included in this study (Fig. 1). Patients were followed up for a median of 6 (3,18)

Local tumor progression after MWA and risk factors
During the follow-up period, there were 7/154 (4.55%) cases of local tumor progression. Six patients received delayed surgery, and one was managed by AS. Suspicious nodule and/or lymph node metastasis were discovered by ultrasonography at 1 year of follow-up in Patients 1, 2, and 5, who were treated with salvage surgery. Patient 3 was treated with thyroidectomy due to the suspicious malignant findings obtained by the FNAC of the hypoechoic nodule primarily located in the thyroid isthmus at 1 year following MWA, which was pathologically diagnosed as PTMC with ipsilateral    central lymph node metastasis. Abnormal findings were not detected on ultrasound images of Patient 4. However, Patient 4 was surgically treated because of anxiety. Tumor residue was not found in the ablation area, while lymph node metastasis was detected. Lymph node metastasis was detected in Patient 6 at 8 months, who was closely followed up. Patient 7 received salvage surgery due to the suspicious malignant findings obtained by the FNAC of the hypoechoic nodule located in the thyroid isthmus at 7 months. Generally, there were two cases of local recurrence, two cases of lymph node metastasis, and three cases of local recurrence combined with lymph node metastasis after MWA. Distant metastasis was not detected (Table 3).
We further performed Kaplan-Meier survival analysis based on the common risk factors for local tumor progression, including sex, age, and maximum tumor diameter (Fig. 4). Subgrouped by sex, RFS was comparable between male and female PTMC patients (P = 0.68). None of the PTMC patients older than 55 years suffered from tumor recurrence, although no significant difference in RFS was detected between PTMC patients younger and older than 55 years (P = 0.41). Based on the cut-off value of 0.70 cm obtained by the plotted ROC curves, a significantly lower RFS was detected in PTMC patients with a maximum tumor diameter < 0.70 cm than those with ≥0.70 cm (P = 0.031). However, the area under the curve was relatively small (AUC = 0.565), suggesting the poor potential of the maximum tumor diameter in diagnosing PTMC. RFS was comparable between PTMC patients subgrouped by Va with a cutoff value of 1.88 cm 3 (P = 0.092). PTMC patients with IAR ≥ 15 had a better prognosis than those with IAR < 15 (P = 0.015, AUC = 0.672). After excluding PTMC patients with a history of hyperthyroidism or hypothyroidism (n = 4) and those lacking follow-up data (n = 4), the remaining were classified into two groups based on the preoperative TSH level of 2.0 mU/L. We did not detect a significant difference in RFS between PTMC patients with TSH ≤ 2.0 mU/L and >2.0 mU/L (P = 0.77).

Influencing factors for tumor disappearance after MWA
Clinical data of PTMC patients were introduced into the univariable logistic regression model, and the follow-up time, tumor volume, maximum tumor diameter, ablation time, free thyroxine, and IAR were found to be significantly correlated with the disappearance of PTMC after MWA. These factors were further entered into the multivariable logistic regression model. It is shown that the follow-up time was an independent protective factor for the disappearance of PTMC after MWA (P = 0.044; OR, 0.940; 95% CI (0.886, 0.998)), while IAR was a risk factor (P = 0.036; OR, 1.029; 95% CI (1.002, 1.057)) ( Table 4).

Complications
No patient experienced intolerable pain or significant discomfort which would have required treatment  suspension during MWA procedures. Four patients suffered hoarseness during the follow-up period, with three having recovered within 1 week and one after 6 months. Preoperative abnormal thyroid function was detected in 19 PTMC patients who recovered after the corresponding treatment. The incidences of abnormal thyroid function at 3, 6, 12, and 24 months after MWA were 12.84% (14/109), 15.19% (12/79), 18% (9/50), and 4.5% (1/22), respectively.

Discussion
Surgery or AS is the conventional treatment for low-risk PTMC, although the former may induce overtreatment and complications, and AS is only suitable for patients with certain indications. The application of AS is still controversial, especially in developing countries (25). The therapeutic efficacy of MWA on PTMC has been previously validated as a reliable alternative to surgery  and AS. Nevertheless, it is difficult to predict local tumor progression.
Our results consistently confirmed that MWA was an effective treatment for low-risk PTMC. The tumor volume was significantly reduced at 12 months after MWA. At 24 months, the median VRR achieved 100%, which was maintained during a 60-month follow-up. A total of 154 patients were followed up for more than 3 months. The rate of local tumor progression was 4.55%, slightly higher than that previously reported in other centers (0-4.2%) (26,27,28). Kim et al. (19) have reported that the rate of local tumor progression after LA is 5.5%, and local tumor progression occurs in most cases within 5 years. Therefore, we recommend a close follow-up of 5 years for MWA. In the present study, suspicious tumor lesions or lymph node metastases were detected by the follow-up ultrasonography within 1 year of MWA in five PTMC patients with local tumor progression, all of which were further confirmed by FNA or postoperative pathological examination. As a result, a close follow-up by cervical ultrasonography within 1 year of MWA is also recommended.
Sex, age, and tumor size are considered as risk factors for lymph node metastasis in PTMC patients (29,30,31). Our results showed that the RFS was comparable between male and female PTMC patients, although most of the recruited patients were women. Age < 55 years is believed as an independent risk factor for lymph node metastasis in PTMC (30). Ito et al. (15) have found that during AS in PTMC patients, older patients have a lower risk of tumor progression, especially those older than 60 years. According to the AJCC/UICC 8th edition, the cut-off value of 55 years was adopted. It is shown that PTMC patients ≥ 55 years had a better prognosis, although no significant difference was detected in the RFS between those younger and older than 55 years. It is suggested that the risk of local tumor progression was relatively low in elderly patients with PTMC during the treatment of MWA and AS. FNA is not recommended for patients with a maximum tumor diameter < 0.5 cm by the American Thyroid Association (ATA) (32). Therefore, the maximum tumor diameter of almost all recruited PTMC patients in the present study was larger than 0.5 cm. Subgrouped by the cut-off value of 0.7 cm, PTMC patients with a maximum tumor diameter < 0.7 cm presented a better prognosis. Sim et al. (22) have calculated the IAR to quantitatively assess the therapeutic efficacy of thermal ablation on benign thyroid nodules, finding that IAR is highly correlated with VRR. As a larger ablation area is more likely to completely inactivate the tumor lesion, we introduced Va and IAR to evaluate the RFS. It is shown that PTMC patients with IAR ≥ 15 had a better prognosis. Therefore, a larger ablation area was favorable to reduce the postoperative recurrence risk of PTMC. Thyroid hormone suppression therapy after thyroid lobectomy significantly improves the prognosis of high-risk differentiated thyroid cancer and prevents its progression or recurrence. Whether low-risk patients can benefit from it, however, remains unclear (33). The ATA guidelines recommend that the TSH level in low-risk PTC patients treated with thyroid lobectomy maintains in the middle or low reference range (0.5-2mU/L) but maintains in a low reference range in patients managed by AS. Li et al. (23) have reported that there is no significant difference in the local tumor progression between low-risk PTC patients who have high and low TSH levels after RFA (9.4 vs 4.3%). Consistently, our data revealed that the RFS was similar between PTMC patients with preoperative TSH ≤ 2.0 mU/L and >2.0 mU/L. Recent studies have shown that the postoperative TSH level is not correlated with the recurrence in low-risk PTC patients after thyroid lobectomy (34,35). Therefore, thyroid hormone suppression therapy, or even keeping TSH ≤ 2.0 mU/L, is not necessary for low-risk PTMC patients after MWA, as long as normal thyroid function is maintained.
During the follow-up period, the rate of complete disappearance of tumor was 17.01% (25/147), which was lower than the previously reported 57.6-95.2% (36,37). It may be attributed to the definition of tumor disappearance in the present study as the complete disappearance of the ablated area confirmed by ultrasonography, excluding the scar-like changes (38). A relatively short follow-up period during which the ablation area had not disappeared could be another cause. Logistic regression analysis showed that the follow-up time was a protective factor for the complete disappearance of tumor, which is consistent with the changing rate of tumor disappearance. IAR was identified as a risk factor for the disappearance of the ablation area, which may be attributed to the coagulation necrosis of the tumor due to MWA-induced vibration of water molecules, and denatured proteins caused by thermal deposition of surrounding tissues that were not easily absorbed (39).
MWA may cause subcutaneous neck hematoma, fever, pain, voice change, skin burns, edema, and hypothyroidism. Severe complications of MWA include Y Ren et al. permanent recurrent laryngeal nerve damage and hypoparathyroidism. Through the literature review, the complication rate of PTMC treated with MWA ranges from 0 to 5.2% (18,28). In our study, the incidence of hoarseness after MWA was 2.60% (4/154), and hypoparathyroidism was not reported. The incidence of abnormal thyroid function after MWA was significantly high, which may be attributed to the exclusion of patients with lymph node metastasis by preoperative enhanced CT. The use of iodinated contrast media would increase the risk of thyroid dysfunction (40).
Some limitations in the present study should be noted. First of all, it was a single-center retrospective study. Second, the follow-up time of most PTMC patients was not long enough to effectively evaluate the long-term efficacy and safety of MWA. Third, most of the recruited PTMC patients were women and younger than 55 years, which may cause potential biases. Fourth, the number of local tumor progression was small, which may influence the statistical power of the analysis of its risk factors. Therefore, our findings should be further validated in multi-center studies with a large sample size and a long follow-up period.
Collectively, our study validated that ultrasoundguided MWA is a safe and effective therapeutic strategy for low-risk PTMC. The maximum tumor diameter before MWA and IAR are predictors of local tumor progression, and the follow-up time and IAR are significantly correlated with the complete disappearance of the tumor in the ablation area.

Declaration of interest
The authors declare that they have no competing interest.

Availability of data and material
All data are made available by the authors upon request without undue reservation.

Author contribution statement
Yujie Ren, Lin Jiang, and Shuhang Xu developed the research questionnaire and drafted the protocol for this study. Yujie Ren, Xue Han, Yujiang Li, Shuhang Xu, and Chao Liu were responsible for data collection and analysis. Guofang Chen and Shuhang Xu participated in the diagnosis. Shuhang Xu performed microwave ablation. Xue Han was responsible for the perioperative management. Yujie Ren and Yujiang Li drafted the manuscript. Lin Jiang and Shuhang revised the manuscript critically for important intellectual content. All authors agreed to take responsibility for the integrity of the data and the accuracy of data analysis, and approved the final version of the manuscript.