Retrospective multicenter study of thermal ablation outcomes in subcapsular versus intraparenchymal papillary thyroid microcarcinoma

Objective To evaluate the clinical outcomes of thermal ablation (TA) for subcapsular papillary thyroid microcarcinoma (PTMC) and compared them with those of intraparenchymal PTMC.Methods Patients with unifocal, clinical node-negative PTMC who underwent TA at three institutions between July 2015 and December 2021 were enrolled. Patients were divided into the subcapsular group (PTMC ≤ 2 mm from the capsule) and the intraparenchymal group (PTMC > 2 mm from the capsule). Propensity score matching (PSM) was used to balance between-group confounders. Primary outcomes were disease progression and disease-free survival (DFS); secondary outcomes included post-ablation lesion volume, volume reduction rate (VRR), cumulative tumor disappearance rate, and complications.Results A total of 209 and 288 patients were included in the subcapsular and intraparenchymal groups, respectively. During a mean follow-up of 48.4 months, the overall disease progression was 3.4%. Multivariate Cox regression analysis showed no association between subcapsular location and disease progression after adjusting for tumor volume, thyroid-stimulating hormone level, and ablation modality (HR = 1.3 [0.5, 3.5], p = 0.59). After 1:1 PSM, no significant differences between the subcapsular and intraparenchymal groups were observed in primary outcomes (disease progression: 4.3% vs. 3.8%; 5-year DFS rate: 93.2% vs. 93.9%, p = 0.82) or secondary outcomes (complication rate: 1.6% vs. 1.1%; latest volume: 0 mm³ vs. 0 mm³; latest VRR: 99.8% vs. 99.9%; cumulative tumor disappearance rate: 98.9% vs. 99.5%) (all p > 0.05).Conclusion For eligible subcapsular PTMC patients, TA achieves 4-year clinical outcomes comparable to those of intraparenchymal PTMC cases, warranting further prospective investigations.