Pathological characteristics of low-risk papillary thyroid microcarcinoma with progression during active surveillance.

Papillary thyroid microcarcinoma (PTMC) is generally an indolent disease and active surveillance is conducted for low-risk cases. This study was carried out to clarify the pathological characteristics of PTMC cases that exhibited enlarged nodules or nodal metastasis during the surveillance period. A total of 188 PTMC cases that underwent surgery after active surveillance for ≥ 1 year were examined. Ki-67 labeling indices of > 5% and > 10% were detected in 50.0% and 22.2% of enlarged cases, respectively, values that were significantly higher than those in non-enlarged cases. Intraglandular dissemination and psammoma bodies in normal thyroid tissue were associated with new occurrence of nodal metastasis. Ultrasonographic macrocalcification and follicular variants were observed in 13.8% and 10.6% of non-enlarged cases, respectively, but not in enlarged or nodal metastatic cases. Intraglandular dissemination and psammoma bodies were ultrasonographically detected in 50.0% and 40.0% of cases, which was confirmed by microscopy. Thus, high Ki-67 labeling index, intraglandular metastasis, and psammoma bodies in normal thyroid tissue are indicators of progressive PTMC, and may be identified cytologically or ultrasonographically. In PTMC cases with ultrasonographic macrocalcification, active surveillance can be proactively implemented.