How to Monitor and Manage Nodule Regrowth after Thermal Ablation of Benign Thyroid Nodules

Abstract

Dear Editor, With great interest, we read the article published by Negro et al. (1) entitled “Twelve-Month Volume Reduction Ratio Predicts Regrowth and Time to Regrowth in Thyroid Nodules Submitted to Laser Ablation: A 5-Year Follow-Up Retrospective Study” in the Korean Journal of Radiology. The authors evaluated the regrowth rate and predictive risk factors for the regrowth of solid thyroid nodules after laser ablation (LA) during a 5-year follow-up period in a relatively large population (104 patients) with a median nodule volume of 12.5 mL. They found that the regrowth rate was inversely related to the 12-month volume reduction ratio (VRR). In addition, non-spongiform type nodules had a higher regrowth rate. Among 37.5% (39/104) of patients who experienced nodule regrowth, 29.8% (31/104) had a 12-month VRR < 50%. Of these 39 patients, 17 (43.6%) underwent surgery and 14 (35.9%) underwent secondary LA. This is an important study investigating nodule regrowth after LA over a 5-year follow-up period (1). Although the present study made excellent observations during a long-term follow-up period, the authors mentioned several limitations, such as its retrospective design and technical limitations due to the use of the ‘pull back’ technique for complete LAs of the nodule margin (1). We agree that these limitations are pertinent. Below, we raise several other points that readers should be aware of when reading this report. First, the present study suggested a ‘12-month VRR’ to predict nodule regrowth, whereas a previous study suggested that the initial ablation ratio (IAR) should be used (2). IAR is a quantitative indicator one month after thermal ablation and is highly correlated with the long-term VRR (i.e., if IAR > 70%, a VRR > 50% is expected). Therefore, IAR can predict nodule regrowth earlier (one month), and thus the real clinical value of the 12-month VRR should be compared with that of IAR in a future study. Second, the present study described, as a technical limitation, that the ‘moving-shot’ and ‘vascular ablation’ techniques may achieve more complete ablation than the ‘pull back’ technique for LA. Since regrowth may appear at the margin of the ablated nodule, previous studies have recommended that the margin be completely ablated by using the ‘moving-shot’ and ‘vascular ablation’ techniques (3 4). The current radiofrequency ablation guidelines also recommend these techniques (3 5). Finally, we would like to address the study’s definition of regrowth. The present study defined regrowth as a nodule volume increase > 50.0% over the minimum recorded volume. However, the definition of nodule regrowth varies between studies (6 7). A majority of studies have defined regrowth as an increase in the nodule volume > 50.0% over the previously recorded volume (8 9 10). Others define regrowth as a follow-up nodule volume greater than the initial nodule volume (11) or a > 20.0% larger volume than that of one year after treatment (12). It is unclear which definition is most clinically meaningful; thus more work is needed to address this issue and arrive at a conclusive definition for future guidelines. In conclusion, the present study reports important clinical aspects of nodule regrowth after thyroid LA. We appreciate the observations and suggest several points that readers should consider when appraising the regrowth of benign thyroid nodules after thermal ablation.