Thyroidectomy with CCND has been used for decades of years for PTC with CLNM, and the therapeutic effect of it has been demonstrated in several studies[15]. However, considering the surgical trauma of CCND and the negative effect on life quality in case of complications, it's difficult to perform CCND on patients with multiple underlying diseases who are at high risk of death due to complications. It is reported in many studies that CLNM was common in PTC patients with clinically node-negative neck [16–18], and there was no difference in recurrence rates between patients undergoing thyroidectomy and thyroidectomy with prophylactic CCND[15, 18, 19]. Therefore, it is reasonable that the removal of imaging visible PTC metastatic lymph nodes alone may also be feasible for the prevention of recurrence.
Several studies on the thermal ablation of PTC have shown promising result regarding the safety and effectivity[8, 10]. A few other studies have a preliminary proof on the effectivity of thermal ablation of metastatic lymph nodes after surgery[13, 20]. These above results have demonstrated that thermal ablation could treat PTC and CLNM effectively.
In the present study, 20 PTC patients with CLNM was enrolled and treated with MWA, with technical success rates of 100%. The primary finding of this study was that MWA could treat PTC with CLNM invasion without serious complications. At the end of follow-up, all the volume of ablated area decreased and was essentially absorbed in 9 cases. No PTC local recurrence was found at the end of follow-up. One patient with a large PTC nodule developed metastatic lymph nodes 6 months after initial MWA and underwent a second MWA procedure. No other metastatic lymph node was suspected at the 24th month follow-up after the second MWA. No major complications were encountered in all the cases. There were no delayed complications during the follow-up, either. During the follow-up period, metastatic lymph nodes recurred only in one case. Moreover, the successful second MWA of recurrence CLNM demonstrated that there had been no postoperative adhesion due to first ablation which could influence further MWA procedure or affect the patient's life quality.
According to our experience, the following factors are important for the successful ablation. First, high-frequency US has high diagnostic efficiency in diagnosing PTC and CLNM[21–23]. The relationship among PTC nodule, metastatic lymph nodes and important neck structures such as trachea, carotid vessels and nerves could be clearly revealed by US. The clear display of PTC and metastatic lymph nodes could ensure the accurate ablation of metastatic lymph nodes rather than normal lymph nodes. In contrast, it is difficult to identify all the metastatic lymph nodes precisely during neck exploration and CCND is the most effective way to clear all the suspected lymph nodes despite the surgical trauma. Second, the spatial resolution and time resolution of high-frequency US have advantages in the guidance during the procedure of MWA, including the precisely insertion of core needle and microwave antenna, evaluation and adjustment of the isolating fluid thickness during ablation, and evaluation of the thermal field as well as observation of the possible hematoma around the ablation zone. Third, for MWA of PTC with CLNM, safety and effectivity of the procedure is the most important. During the ablation, the thermal field could be revealed by US to ensure complete ablation and avoid carotid vessel injury at the same time. Heat injury could be prevented through different strategies. On one hand, low power and short radiation time were used to limit the heat conduction. On the other hand, isolating fluid around the ablation zone could protect the surrounding structures from heat injury effectively. After ablation, CEUS could evaluate the necrosis band and ensure the complete ablation of cancer and lymph nodes.
Our study has several limitations. First, the number of patients involved in this study is still small, and a further study enrolling more cases will lead to a more definite result. Second, although most tumor recurrence/persistence occur at the second year after operation according to the previous study[24], the follow-up time is still short. Third, a comparison study between MWA and surgery for PTC with CLNM should be carried out.