Noninvasive follicular thyroid neoplasm with papillary-like nuclear features (NIFTP): Implications for the risk of malignancy (ROM) in the Bethesda System for Reporting Thyroid Cytopathology (TBSRTC)

BACKGROUND: The introduction of noninvasive follicular thyroid neoplasm with papillary-like nuclear features (NIFTP) affects the risk of malignancy (ROM) mostly in the Bethesda System for Reporting Thyroid Cytopathology (TBSRTC) categories. In this multi-institutional, retrospective study, the authors investigated variations in the impact of an NIFTP diagnosis on the associated ROM for each TBSRTC category with an emphasis on the influence of pathologist and institutional diagnostic thresholds on the ROM. METHODS: Baseline data on cytology and histology diagnostic categories were collected over a 3-year period at 3 academic center hospitals (institutions A, B, and C). Histology slides for all cases diagnosed as follicular variant of papillary thyroid carcinoma (FVPTC) were re-reviewed at each institution, and those that qualifying as NIFTP were separated from other PTCs. RESULTS: The collective case cohort from the 3 institutions included 15,973 thyroid fine-needle aspiration cytology (FNAC) specimens and 5090 thyroid surgical resection specimens. Significant differences in baseline cytology and histology data were noted among the 3 institutions. The number of cases classified as NIFTP compared with FVPTC was highly variable (institution A, 14%; institution B, 39%; and institution C, 12%). For 3250 resected thyroid nodules with a previous FNAC diagnosis, the average decrease in ROM after the exclusion of NIFTP for all TBSRTC categories was as follows: institution A, 9.8%; institution B, 3.9%; and institution C, 1.3%. CONCLUSIONS: The institutional frequency of NIFTP histopathology diagnosis and cytology baseline data will impact the ROM associated with specific FNAC diagnoses, especially among the indeterminate TBSRTC categories. The range of ROM for each TBSRTC diagnostic category is reflective of the inherent diagnostic thresholds and interobserver and interinstitutional variability in the diagnosis of thyroid lesions. Cancer Cytopathol 2018;126:20-6.