Active surveillance (AS) is accepted as an alternative strategy to immediate surgery for low-risk papillary thyroid microcarcinoma (PTMC)(1-3). As the mortality is very low in PTMC (4-6), some tumors may progress during AS (4-7). Thus, the appropriate selection of candidates and timely management of patients with disease progression during AS are important clinical issues.

Several patient and tumor characteristics have been associated with tumor progression (4, 8-10). Young age at diagnosis is consistently associated with an increased risk of tumor progression (4, 11). Elevated thyroid stimulating hormone (TSH) levels have also been associated with tumor growth (12, 13). 

US features, which provide intrinsic imaging phenotypes of tumor biology, have also been used as predictors of recurrence or LNM in PTMC in previous studies (14-16). However, the results of previous studies examining the significance of US characteristics in PTMC under AS are inconsistent (17-19).

Given that participants with PTMC undergo serial US examination as part of surveillance, it is of clinical relevance to know whether US features have prognostic value. In this study, we assessed if US features could predict tumor progression in participants with low-risk PTMC and AS.