Background: The "atypia of undetermined significance/follicular lesion of undetermined significance" (AUS/FLUS) category in the Bethesda System for Reporting Thyroid Cytopathology is a heterogeneous category of cases that are not clearly benign or malignant.
Methods: We conducted an analysis of cytologic and histologic evaluations of thyroid nodules that had been interpreted as AUS/FLUS on fine-needle aspiration (FNA) at a single institution from April 2011 to April 2012. Those cases were classified into 2 subgroups according to the predominance of nuclear atypia (AUS) or microfollicular architecture (FLUS). In addition, for a number of these cases, BRAF gene mutation analyses were performed.
Results: Of 6402 thyroid FNAs performed, 431 cases were diagnosed as AUS and 120 as FLUS. Follow-up cytologic or histologic outcome data were available for 315 AUS cases and 73 FLUS cases. Among AUS cases, 52.7% were malignant on repeat FNA or histologic diagnosis. In contrast, for FLUS, 6.8% were malignant on repeat FNA or histologic diagnosis. Among AUS/FLUS cases, 147 had adequate BRAF mutation analysis, which accompanied the histologic diagnosis. BRAF mutations were found in 87 AUS cases, 86 of which were papillary carcinoma. In contrast, there was only 1 case of BRAF mutation in FLUS. Correlating molecular results with histologic outcome revealed a 98.9% cancer probability for AUS cases with BRAF mutation.
Conclusions: The AUS subcategory indicates a higher risk of malignancy than the FLUS subcategory. Furthermore, BRAF molecular testing is helpful in stratifying the malignant risk of AUS cases into high-risk and low-risk groups.
Keywords: BRAF mutation; Bethesda System for Reporting Thyroid Cytopathology; atypia of undetermined significance; fine needle aspiration; follicular lesion of undetermined significance.
© 2014 American Cancer Society.