The risk of malignancy in the atypia of undetermined significance/follicular lesion of undetermined significance (AUS/FLUS) category subgroups: a Thai institute experience

Abstract

This study investigated the rate of reporting and the risk of malignancy (ROM) for atypia of undetermined significance (AUS) subgroups in a Thai population. AUS, which is category III of the Bethesda System for Reporting Thyroid Cytopathology, is a problematic diagnosis for thyroid nodule management because the risks of malignancy are diverse. Patients who underwent thyroid fine needle aspirations between January 2015 and December 2019 were included in this retrospective study. Gender, age, and nodule features were described, and all slides were re-evaluated and categorized into 2 subgroups: AUS-Nuclear (including cytology atypia and cytologic and architectural atypia) and AUS-Other (including architectural atypia, oncocytic atypia, and atypia not otherwise specified). The lower and upper limits of ROM were calculated for each subgroup. Of total, 258 out of 2995 fine needle aspirations (8.6%) were diagnosed as AUS. The patients were predominantly female (88.9%), with a mean age of 54.1 years. The average nodule size was 2.5 cm. Of the 258 AUS patients, 81 (38.9%) had histological correlations. The ROM for the AUS category was 9.1% to 23.5%. The ROM of the AUS-Nuclear and AUS-Other were 11.1% to 27.3% and 2.2% to 6.7%, respectively. Features of pseudonuclear inclusions had the highest ROM (33.3%-42.9%), followed by pale chromatin (28.57%-47.06%). Less than ten percent of our interpretations were AUS, which is acceptable in our practice. Cytological atypia harbored the highest ROM. Studies of associations between cytology and histology may aid in improving diagnostic criteria for this population.