Cytological features of "noninvasive follicular thyroid neoplasm with papillary-like nuclear features" and their correlation with tumor histology

EFVPTC have very low risk of recurrence or other adverse events and have been recently proposed to be designated as "Non-invasive follicular thyroid neoplasm with papillary-like nuclear features" or "NIFTP", thus eliminating the term "carcinoma". This proposal is expected to significantly impact the risk of malignancy associated with the currently used diagnostic categories of thyroid cytology. In this study, we analyzed the fine needle aspiration biopsy (FNAB) cytology features of 96 histologically proven NIFTP and determined how the main nuclear features of NIFTP correlate between cytological and histological samples. Blind review of FNAB cytology from NIFTP nodules yielded the diagnosis of "follicular neoplasm" (Bethesda category IV) in 56% of cases, "suspicious for malignancy" (category V) in 27%, "atypia of undetermined significance/follicular lesion of undetermined significance" (category III) in 15%, and "malignant" (category VI) in 2%. We found good correlation (k=0.62) of nuclear features between histological and cytological specimens. NIFTP nuclear features (size, irregularities of contours and chromatin clearing) were significantly different from those of benign nodules, but not from those of invasive EFVPTC. Our data indicate that most of NIFTP nodules yield an indeterminate cytological diagnosis in FNAB cytology and nuclear features found in cytology samples are reproducibly identified in corresponding histology samples. Because of the overlapping nuclear features with invasive EFVPTC, NIFTP cannot be reliably diagnosed preoperatively, but should be listed in differential diagnosis of all indeterminate categories of thyroid cytology.