Longitudinal Ultrasonographic Analysis of Involutional Changes after Radiofrequency Ablation of Papillary Thyroid Microcarcinoma.

Abstract

Background: Radiofrequency ablation (RFA) has emerged as an effective nonsurgical treatment for low-risk papillary thyroid microcarcinoma (PTMC), with widely variable rates of complete disappearance reported. The purpose of our study was to characterize the longitudinal ultrasonographic (US) changes following RFA of PTMC and to propose imaging-based criteria to guide posttreatment surveillance.

Methods: This retrospective cohort study included 98 low-risk unifocal PTMC patients (82 females; age range, 27-59 years) who underwent RFA with a median follow-up of 48 months (range, 12-74 months). Comprehensive longitudinal US analysis was performed to characterize temporal changes in the ablation zone and surrounding parenchyma. Factors associated with postablational nodular remnant at 1 year after RFA were also evaluated.

Results: All RFA procedures achieved 100% technical success. By 1 year, the volume of the ablation zone had decreased to a median of 0 mL (range, 0-0.41 mL). However, ultrasonography demonstrated complete disappearance in only 70.4% of PTMCs, while 29.6% exhibited postablational nodular remnants despite volumetric resolution. These nodular remnants progressively regressed and eventually disappeared during subsequent follow-up, except for a single case of a tiny macrocalcified residue persisting at 5-year follow-up after RFA. Characteristic postablation US features gradually emerged during the first year after RFA, with incidence at 1 year of capsular retraction, postablational strands, and postablational parenchymal hypoechogenicity reaching 61.2%, 60.2%, and 51.0%, respectively. These findings remained relatively stable through the primary follow-up period of up to 36 months, although postablational strands showed a further significant increase in incidence throughout this period. Without major complications, disease progression occurred in 4.1% (4/98) patients: Three contralateral PTMC cases were treated with additional RFA, and one case with lymph node metastasis underwent surgery. No clinico-radiological or procedural factors were significantly associated with 1-year postablational nodular remnant after RFA.

Conclusions: RFA of low-risk PTMC achieves an excellent final complete disappearance rate while frequently inducing characteristic US features, including capsular retraction, postablational strands, and postablational parenchymal hypoechogenicity. Recognition of these US involutional changes is crucial for accurate posttreatment monitoring and for avoiding misinterpretation of benign healing changes as pathological findings, and may inform imaging-based post-RFA surveillance.