Papillary thyroid carcinoma measuring 1.0 cm or less: an epidemiological and clinicopathological study

The incidence of papillary thyroid microcarcinoma (PTMC) measuring 1 cm or less has noticeably increased over the past 30 years, particularly among clinically significant subgroups that are diagnosed chiefly preoperatively.

This retrospective study investigated the rate of occurrence and the pathological high-risk features of PTMC discovered incidentally during thyroidectomies for benign thyroid diseases, as well as those diagnosed preoperatively (non-incidental), in a sample of 1,408 thyroidectomies.

PTMC accounted for 30.5% of all resected malignant thyroid tumors, with 53% being incidental and 47% non-incidental. The incidence of incidental PTMC has increased twofold over the past seven years, from 4.5% to 9.0%, while the incidence of the non-incidental subgroup has increased tenfold, from 0.9% to 9.0%. Compared to incidental cases, non-incidental cases were more likely to affect males, exhibited significantly larger tumors (7.2 mm versus 3.7 mm), and had higher rates of multiplicity (49.4% vs. 23.6%) and bilaterality (36.7% vs. 16.7%). Non-incidental cases also had higher rates of lymph node metastasis (25.9% vs. 4.2%) and greater margin involvement (34.4% vs. 16.7%). Extrathyroidal extension occurred exclusively in approximately 2.5% of the non-incidental cases. The most aggressive PTMC subtype is the tall cell subtype (TCS), followed by the classic subtype, which is observed in 30% and 29% of non-incidental cases, respectively. Follicular subtypes exhibit indolent behavior, as observed in 11% of non-incidental cases. An account of the TCS morphology is also provided.

The incidence of PTMC has dramatically increased in the non-screened population. Male sex, tumor size >5 mm, and TCS subtype are risk factors for aggressive behavior.