Cytologic and clinical features of NIFTP: Can we diagnose based on preoperative fine-needle aspiration.

Background: Noninvasive follicular thyroid neoplasm with papillary-like nuclear features (NIFTP) is considered an indolent neoplasm of the thyroid. Currently, this entity presents a diagnostic challenge on preoperative fine-needle aspiration (FNA) cytology.

Methods: Cases of NIFTP and noninvasive encapsulated follicular variant of papillary thyroid carcinoma from January 1994 to August 2018 were retrieved from our institution's pathology databases, and their clinical and cytopathologic features were reviewed.

Results: A total of 45 patients with NIFTP were identified. Thirty-nine of 45 patients had presurgical thyroid FNAs, 27 of which showed abnormal cytology (27/39). NIFTPs were most often classified in the indeterminate diagnostic categories on presurgical FNAs (78%), including suspicious for follicular neoplasm (8/27), atypia of undetermined significance (8/27) and suspicious for malignancy (5/27). Six patients had a cytologic diagnosis of papillary thyroid carcinoma (6/27) on thyroid FNAs. Fourteen out of 27 cases with abnormal FNAs had in-house cytology available for review. The most frequent cytologic features observed in NIFTPs included architectural abnormalities, such as crowded clusters (13/14), nuclear overlapping (13/14), predominance or presence of individual microfollicles (11/14), and abundant to scattered isolated cells (10/14). The papillary-like nuclear features observed included nuclear enlargement (14/14), eccentric nucleoli (13/14), nuclear membrane irregularity (12/14), pale chromatin (12/14), nuclear grooves (10/14), and elongated nuclear contour (8/14).

Conclusions: The NIFTPs were most frequently categorized as atypia of undetermined significance and suspicious for follicular neoplasm on cytology. Recognition of the architectural and nuclear features of NIFTP can be helpful in distinguishing NIFTPs from other entities, such as papillary thyroid carcinoma and follicular neoplasms.