Classic and follicular variant of papillary thyroid microcarcinoma:2 different phenotypes beyond tumor size

Context: Despite the wide revision of current guidelines, the management of papillary thyroid microcarcinoma (mPTC) still has to be decided case by case. There is conflicting evidence about the role of more frequent histological subtypes, and no data about potential differences at presentation. Objective: Our aim was to compare the phenotype of the 2 most frequent mPTC variants, namely, classical papillary thyroid microcarcinoma (mPTCc) and the follicular variant of papillary thyroid microcarcinoma (mFVPTC). Methods: Retrospective observational study, from January 2008 to December 2017 of a consecutive series of patients with mPTCc and mFVPTC. All cases were classified according to the 2015 American Thyroid Association (ATA) risk classification. Clinical and preclinical features of mPTCc and mFVPTC at diagnosis were collected. The comparison was also performed according to the incidental/nonincidental diagnosis and differences were verified by binary logistic analysis. Results: In total, 235 patients were eligible for the analysis (125 and 110 mPTCc and mFVPTC, respectively). Compared with mPTCc, mFVPTCs were more often incidental and significantly smaller (4 vs 7mm) (P <. 001 all), possibly influenced by the higher rate of incidental detection. mFVPTC and incidental (P <. 001 both) tumors were significantly more often allocated within the low-risk class. A logistic regression model, with ATA risk class as the dependent variable, showed that both mFVPTC (OR 0.465 [0.235-0.922]; P =. 028]) and incidental diagnosis (OR 0.074 [0.036-0.163]; P <. 001) independently predicted ATA risk stratification. Conclusion: mFVPTC shows some differences in diagnostic presentation compared with mPTCc, and seems to retain a significant number of favorable features, including a prevalent onset as incidental diagnosis.