Two-tier subclassification of the Bethesda category III (atypia of undetermined significance/follicular lesion of undetermined significance) in thyroid cytology

Background: The Bethesda category III, AUS/FLUS, comprises a heterogeneous group of thyroid lesions with variable risk of malignancy (ROM). This study evaluates ROM in two subgroups of this category based on nuclear atypia and architectural atypia.Methods: Cases in Bethesda category III were reported based on nuclear atypia (AUS) and architectural atypia (FLUS). ROM was calculated by comparing the cytologic diagnosis to the follow-up histologic diagnosis.Results: Among the 610 Bethesda category III cases in this study, 306 (50.2%) and 304 (49.8%) cases were reported as AUS and FLUS, respectively. One hundred and eighty six of 306 AUS (60.8%) and 193 of 304 FLUS (63.5%) cases underwent surgical intervention. ROM of the cases in Bethesda category III was 12.8% if all cases were counted and 20.6% if only surgical cases were counted. When analyzing separately, ROM of AUS cases was 17.0% and 28.0% with all cases and surgical cases only, respectively. For FLUS cases, ROM was 8.6% and 13.5% with all cases and surgical cases only, respectively.Conclusion: In Bethesda category III, ROM in the cases with nuclear atypia was significantly higher than the cases with architectural atypia. Sub-classifying the Bethesda Category III cases with nuclear atypia and architectural atypia, respectively may better stratify the ROM.

Automated summary

In Bethesda category III, cases with nuclear atypia have a significantly higher risk of malignancy compared to cases with architectural atypia. Sub-classifying these two subgroups may better stratify the risk of malignancy.