Impact of thyroid Bethesda category IV (follicular neoplasm) terminology unification on atypia of undetermined significance reporting patterns in thyroid fine-needle aspiration.

INTRODUCTION: The 2023 edition of the Bethesda System for Reporting Thyroid Cytopathology (TBSRTC) unified Category IV terminology by eliminating "suspicious for a follicular neoplasm," retaining only "follicular neoplasm (FN)." Given that nearly 30% of FN diagnoses on fine-needle aspiration (FNA) prove to be benign follicular nodular disease upon resection, concerns arose that the singular use of "FN" could lead cytopathologists to under-call FN and instead assign the atypia of undetermined significance (AUS) category. This study investigates whether the terminology change in 2023 TBSRTC influenced the distribution of FN and AUS diagnoses in thyroid FNA cases at The Johns Hopkins Hospital.

MATERIALS AND METHODS: A retrospective review of 3173 thyroid FNAs (May 2022-May 2025) at Johns Hopkins Hospital was conducted. Cases were divided into 2 groups: "Pre-2023 TBSRTC" (May 2022-April 2024) and "Post-2023 TBSRTC" (May 2024-May 2025). AUS and FN incidences were compared using Z-tests.

RESULTS: Among 3173 FNAs, 491 (15.5%) were diagnosed as AUS. Overall AUS incidence showed no significant change post-2023 TBSRTC implementation (16.2% pre versus 14.2% post; P = 0.1554). Interestingly, the subcategory of AUS-nuclear atypia significantly increased (33.0%-50.3%, P = 0.0002), while AUS-Other (architectural or mixed atypia) significantly decreased (56.4%-41.0%, P = 0.0014).

CONCLUSIONS: Contrary to concerns the unification, of 'suspicious for a follicular neoplasm' and 'FN' into 'FN' alone might prompt an increased use of the AUS-Other category, our findings show the opposite. For unexplained reasons, there has been a significant decrease in AUS-Other diagnoses. The updated nomenclature and subclassification framework of 2023 TBSRTC did not increase AUS incidence.