Factors associated with recurrence of BTN after ablation

To the Editor,

We thank Baek et al. for their comments on our work. We agree with Baek et al that the initial volume of benign thyroid nodule and the experience of the operator might affect the treatment results.

The US-guided thermal ablation, including microwave ablation, radio frequency and laser ablation, has shown safety and efficacy for the treatment of benign solid thyroid nodules [1,2]. We found several recurrences during one-year follow-up period, which is consistent with other studies [3]. The recurrence was defined by the identification of new blood flow in the nodules in the total ablation area or/and >50% increase in nodule volume compared to the previous US examination [3,4].

In our study, tumour vascularity was one of the important factors affecting the treatment responses. The peripheral blood flow of the nodules might be a significant factor influencing the treatment results and a cause of re-growth and insufficient volume reduction during the follow-up period. Previous studies suggested that the presence of vascularity in a solid component is an independent factor of recurrence [5,6]. In order to reduce the recurrence rate, the vasculature of the thyroid nodule margin should be totally damaged [7]. Hence, we need a more sensitive indicator of blood flow, especially for small blood vessels. Contrast-enhanced US (CEUS) is developed by taking advantage of the propensity of microbubbles that create strong nonlinear harmonic echoes. Accordingly, CEUS may enhance the visibility of small blood vessels. Thus, CEUS may provide both microcirculation and macrocirculation information of the thyroid nodules and accurately evaluate the post-treatment efficacy of MWA for BTNs [8]. In addition, the use of US contrast medium is recommended for the evaluation of treatment response after ablation procedure according to the AACE/ACE/AME Guideline [9]. In this study, we employed both the CDFI and CEUS to evaluate the local treatment efficacy after MWA. We found that some cases showed complete necrosis of the lesions immediately after MWA evaluated by both CDFI and CEUS. However, the nodules showed peripheral nodular enhancement on CEUS scan without vascular signal on CDFI 24 h after MWA. The heat may induce vasoconstriction that affects the results of CEUS and CDFI. Therefore, CEUS results at 24 h after ablation may provide a more precise evaluation for tumour necrosis [10,11].

In our study, the mean initial volume of thyroid nodule was 12.6 ml, and the nodules are mainly solid nodules (solid portion >80%). Most patients who underwent MWA ablation were suffered from cosmetic problems. The mean volume of the recurrence group was significantly larger than that of the non-recurrence group.

We agree with Baek et al. that the experience of the operators may have an effect on the treatment result. Ablation effect of BTN is associated with a distinct learning curve [12]. A well-trained radiologist can validate the efficacy and safety of thermal ablation of BTN treatment [13]. Our results were based on the MWA experience of 110 patients, and the local recurrence rate (14.5%) was relatively higher than that of other researchers. Recurrence rates reported after RFA and laser ablation are 5.6% and 9%, respectively [3,14]. We believe the recurrence rate will decrease dramatically as the operator's skill and experience levels increase. According to our recent multicentre data, the recurrence rate was only 7.7% for more than 700 BTNs treated by MWA (mean follow-up period: 13.7 ± 6.2 months).

Since results were summarised at the early stage of the MWA of thyroid nodules, the local recurrence was relatively higher. Following the improvement of the operator's skill and experience levels, the lower recurrence rates are expected in the future.

Disclosure statement

No potential conflict of interest was reported by the authors.