Thyroid Fine Needle Aspiration Cytology with Histologic Correlation and Risk of Malignancy for Bethesda Categories: An Institutional Retrospective Review

Summary: Introduction: The Bethesda system for Reporting Thyroid Cytopathology (TBSRTC) has standardized the way cytopathologists report thyroid fine needle aspiration cytology (FNAC) and provides implied risk of malignancy (ROM) for the established diagnostic categories. Clinical management recommendations in TBSRTC (3rd edition) are based on ROM from surgically resected nodules with prior cytologic diagnosis. Materials and Methods: A retrospective review of thyroid FNAC with histologic correlation was performed at our institution (main campus and satellite locations) for the year of 2023 (January to December) as a part of annual Quality Assurance. In September 2023, our institution added reflex molecular testing (Afirma or ThyroSeq) for all thyroid FNAC cases categorized as Atypia of Undetermined Significance (AUS, TBSRTC category III) and Follicular/Oncocytic Neoplasm (FN/OFN, TBSRTC category IV). Results: A total of 2345 thyroid FNAC were performed. Of these, 207 cases had histologic follow-up. ROM was calculated and correlated with each diagnostic category from TBSRTC 3rd edition (Table 1). Conclusions: In our cohort, the ROM was higher in cases categorized as AUS (34%, n=11/32) and FN/OFN (54%, n=14/26) compared to the reported ranges in the TBSRTC (AUS: 13-30%; FN/OFN: 23-34%). The use of reflex molecular testing may have contributed to this shift. Ancillary molecular testing avoids multiple follow-up FNAs and may change the management, enabling triaging of suspicious or positive cases on molecular testing for surgery while cases with negative molecular testing may be clinically followed. This may lead to a selection bias of resected cases and thereby increase in rate of malignancy in the resected specimen.