Feasibility and Safety of Active Surveillance in Subcapsular Thyroid Nodules with High Suspicion for Malignancy

The safety of active surveillance (AS) in highly suspicious thyroid nodules classified as ACR TI-RADS 5, particularly those located close to the thyroid capsule, has not been fully established. This study aims to assess the feasibility and safety of AS in patients with subcapsular ACR TI-RADS 5 nodules compared with non-subcapsular nodules.

This retrospective study included 675 patients with 763 ACR TI-RADS 5 nodules who underwent AS at Ruijin Hospital between 2015 and 2024. Nodules were categorized according to the distance from the thyroid capsule as subcapsular (≤2 mm) or non-subcapsular (>2 mm). Disease progression was defined as an increase in maximum diameter of at least 3 mm, a volume increase of at least 50%, the appearance of new suspicious thyroid lesions, or lymph node metastasis. Clinical and ultrasound characteristics were compared between groups, and progression-free survival was analyzed using the Kaplan-Meier method.

Among the 763 nodules, 175 (22.9%) were subcapsular. The median follow-up duration was 55 months. Patients with subcapsular nodules were younger (33 ± 11 years vs 43 ± 12 years, p < 0.001) and more frequently had multifocal disease (21.6% vs 12.5%, p = 0.004). No significant differences were observed between the subcapsular and non-subcapsular groups in nodule enlargement (2.9% vs 2.7%), volume increase (25.7% vs 24.5%), new lesion development (5.1% vs 2.4%), or lymph node metastasis (2.3% vs 2.0%). No distant metastasis occurred. Progression-free survival did not differ significantly between groups (log-rank p = 0.78).

Subcapsular location was not associated with a higher risk of disease progression during AS. With appropriate selection and regular follow-up, AS appears to be a safe management option for ACR TI-RADS 5 nodules, including those near the thyroid capsule.