The Milan system atypia of undetermined significance: 5-year performance data

Background: The objective of this study was to evaluate the diagnostic performance of the category atypia of undetermined significance (AUS) at the authors' institution based on the Milan System for Reporting Salivary Gland Cytopathology. Methods: All AUS cases diagnosed at Fimlab Laboratories between January 1, 2018, and December 31, 2022, were included. Histologic verifications were checked until May 31, 2023. The upper-bound and lower-bound risk of malignancy and risk of neoplasm were calculated. The timelines between the pathology laboratory workflow and patient management were also calculated. Results: From 1157 fine-needle aspirations (FNAs), 162 (14.0%) AUS cases were diagnosed in 146 patients, with an average ± standard deviation age of 66.1 ± 14.9 years. There was variation in the AUS percentages, with higher values during the coronavirus disease 2019 pandemic years (15% and 17.5% in 2020 and 2021, respectively). Seventy-five cases (46.3%) had histologic follow-up: 16 were malignant neoplasms, and 36 were benign neoplasms. The upper and the lower bounds of the-risk of malignancy and risk of neoplasm were 21.3% and 69.3% and 9.9% and 32.1%, respectively. The average time from the first FNA with an AUS diagnosis to surgical resection ranged from 6 to 682 days, and the time to the first repeat FNA ranged from 10 to 691 days. Conclusions: The results indicated higher percentages of AUS cases compared with the reference value, which may be attributed to the impact of the coronavirus disease 2019 pandemic. The risk of malignancy calculated in this study was closer to the reference value from the first edition of the Milan System for Reporting Salivary Gland Cytopathology compared with the second edition.