Factors related to the recurrence of benign thyroid nodules after thermal ablation

Dear Editor,

We read the paper entitled ?Factors related to recurrence of the benign non-functioning thyroid nodules after percutaneous microwave ablation? in the International Journal of Hyperthermia by Wang et al. with great interest [1]. The authors suggested four factors related to the recurrence of benign thyroid nodules after microwave ablation (MWA): large initial volume, irregular blood vessels, low microwave energy and nodules adjacent to vital structures. They achieved an acceptable volume reduction ratio of 48.4% at 6months and 74.6% at 1year. Marginal regrowth is a major cause of recurrence after thermal ablation using radiofrequency or MWA; therefore, these findings are important to achieve better results after MWA for benign thyroid nodules.

We appreciate that the authors showed results that predicted recurrence, as well as tried to follow the standard treatment strategies according to the current guidelines. First, they enrolled their patients using standard inclusion criteria, as suggested by the 2012 Guidelines of the Korean Society of Thyroid Radiology [2]. They enrolled patients with solid thyroid nodules (solid component more than 80%) who complained of symptomatic and/or cosmetic issues. Second, they used standard techniques for thermal ablation of the thyroid: local anaesthesia, hydrodissection and the moving-shot technique [2]. Moreover, the definition of recurrence in this study is reasonable based upon the previous long-term follow-up results [3]. They used contrast-enhanced ultrasound (CEUS) to detect the recurrent portion. As a result, the authors reported that the non-recurrent group showed a larger initial nodule volume, less energy applied, higher vascularity and nodules closer to vital structures. However, the issue on colour Doppler US versus CEUS was not completely resolved in this study. The majority of patients in the thyroid RFA groups have undergone colour Doppler US examination. Based on our experience, we can successfully analyse the under-treated nodule margin using combined US findings (echo change, usually decreased echo in the treated area) and positive colour Doppler US. Moreover, one must consider the cost and invasiveness (IV injections) of CEUS. Therefore, the value of CEUS should be the focus of future work.

We agree with their suggestions on factors related to recurrence; however, they did not include several well-known factors related to recurrence or incomplete treatment after thermal ablation of benign thyroid nodules. Ha et al. compared two thermal ablation modalities, radiofrequency and laser, using a Bayesian network meta-analysis to allow for indirect comparison of the methods in two groups [4]. To compare the two groups, factors related to outcomes were evaluated: initial nodule volume [3,5,6], proportion of the solid component [3,7], number of treatment sessions [3,5] and follow-up period [3,6,8]. The initial volume is a well-known factor related to outcome and recurrence [4]. However, this study analysed only small-sized thyroid nodules (mean volume, 6.35 ml). Future studies should include larger thyroid nodules (at least medium-sized nodules, 10–20 ml). Second, for solidity, the present study enrolled only solid thyroid nodules (solid component more than 80%); therefore, solidity is reasonable in this study. The numbers of treatment sessions and follow-up period in this study raise some concerns [1]. In this study, the authors [1] treated the patients using two sessions in some cases (which was not clearly defined in the manuscript) and only achieved limited volume reduction [1]. In addition, recurrence after thermal ablation should be observed for at least 2 years, based upon previous studies [6,8], but the follow-up period of the present study is only one year. A one-year follow-up period is not long enough to evaluate recurrence. Wang et al. may report their long-term results (with recurrence rate) using this cohort data in the future. Finally, the experience of the operators is an important factor to minimise incomplete ablation or recurrence. Recently, the Korean Society of Thyroid Radiology suggested that 50–100 cases for an operators’ experience is necessary for both efficacy [9,10] and safety [11] of thyroid RFA. In the present study, the experience of the operator was not described.

In conclusion, this study showed significant value in minimising the recurrence of benign thyroid nodules after thermal ablation. They addressed several points on recurrence. We agree with the authors’ opinion and suggest several points that should be considered for the readers to better understand the recurrence of benign thyroid nodules after thermal ablation.

Disclosure statement

No potential conflict of interest was reported by the author.