BACKGROUND Among follicular variant papillary thyroid carcinomas (FVPTCs), the noninvasive encapsulated subtype has an excellent prognosis. For this reason, reclassification of noninvasive encapsulated FVPTC (EFVPTC) as a new entity called "noninvasive follicular thyroid neoplasm with papillary-like nuclear features" (NIFTP) has been proposed, but controversy remains. To characterize noninvasive EFVPTC in an Asian population, the clinicopathologic features of each FVPTC subtype were compared in a Korean population.

METHODS FVPTC patients (n = 142) who underwent thyroidectomy between 2009 and 2014, and whose tumor size was >1 cm, were included in the study. The surgical pathology of each patient was reevaluated by two independent expert pathologists.

RESULTS The percentages of noninvasive and invasive EFVPTC and infiltrative FVPTC (IFVPTC) in the study were 30%, 31%, and 39%, respectively. There was no difference in preoperative cytological diagnosis or the extent of surgery between noninvasive and invasive EFVPTC. However, the proportion of Bethesda category IV was lower in IFVPTC (16%) than in noninvasive and invasive EFVPTC (35% and 36%, respectively). Therefore, thyroid lobectomy was more common in noninvasive or invasive EFVPTC (54% or 48%, respectively) than in IFVPTC (16%). Noninvasive EFVPTC showed lower multiplicity, extrathyroidal extension, and BRAF^V600E mutation frequency (three cases; 8%) than did invasive EFVPTC, but other pathological characteristics were similar. However, IFVPTC showed significant differences in tumor size, extrathyroidal extension, lymph node metastasis, Tumor Node Metastasis stage, and American Thyroid Association high-risk category compared with noninvasive and invasive EFVPTC. In the noninvasive EFVPTC group, there were six (14%) cases with multifocality and three (7%) cases with lymph node metastasis. However, only two cases with multifocality and one case with lymph node metastasis originated from noninvasive FVPTC, while the other cases were from coexisting conventional PTCs.

CONCLUSIONS Noninvasive EFVPTC has favorable pathological features, but lymph node metastasis or BRAF^V600E mutations were observed in some patients. Therefore, in order for the distinction between noninvasive EFVPTC and invasive EFVPTC to have more clinical significance, the criteria for NIFTP need to be more strictly revised.