Young investigator challenge: The morphologic analysis of noninvasive follicular thyroid neoplasm with papillary-like nuclear features on liquid-based cytology: Some insights into their identification.

Background: Noninvasive follicular thyroid neoplasm with papillary-like nuclear features (NIFTP) represents a challenge for the diagnosis and management of thyroid carcinoma. Some authors have proposed histological criteria that are able to distinguish NIFTPs from invasive follicular variant of papillary thyroid carcinoma (I-FVPTC). Hence, NIFTPs may have repercussions in the diagnostic categories on fine-needle aspiration. In the current study, the authors evaluated the criteria for NIFTPs on liquid-based cytology samples.

Methods: The authors recorded all 61 liquid-based cytology samples proved to be histological FVPTC between January 2013 and March 2016 and analyzed the architectural, cytoplasmic, and nuclear parameters. They compared them with a cohort of 40 PTC cases and 20 follicular adenoma cases.

Results: The authors reported 37 NIFTP cases and 24 I-FVPTC cases at histology. The cytological diagnoses of follicular nodules in the NIFTP cases were twice those found in the I-FVPTC cases (54.1% vs 29.2%). The number of positive for malignancy cases among the NIFTPs were approximately half those of I-FVPTC cases. When compared with I-FVPTCs, 70% of the NIFTP cases demonstrated a nuclear size <20 μm (P = .025) and rarely exhibited grooves (13% vs 42%; P = .009). The authors found 100% of cases with wild-type BRAF gene in NIFTP cases versus 38.4% in mutated I-FVPTC cases (P = .046). The cytoplasmic features might help to discriminate NIFTPs from follicular adenomas but not from I-FVPTCs (P>.05). A predominant microfollicular pattern was recognized in both NIFTPs and I-FVPTCs (97.3% vs 100%).

Conclusions: The majority of NIFTPs appear to be devoid of nuclear pseudoinclusions and papillary structures, thereby allowing the inclusion in the follicular nodule cases. Nuclear size and microfollicular clusters may suggest the discrimination between NIFTPs and I-FVPCs. Cancer Cytopathol 2016;124:699-710. © 2016 American Cancer Society.