Background: Atypia of undetermined significance (AUS) or follicular lesion of undetermined significance (FLUS) is the most controversial category of the Bethesda System. The present study was conducted to compare the histological findings in a series of thyroid nodules diagnosed with AUS/FLUS after single or repeat fine needle aspiration (FNA) cytology.
Methods: Retrospective analysis of our institution's series of 514 patients with an initial diagnosis of AUS/FLUS between 11/2011 and 02/2020.
Results: Of 4887 FNA samples, 11.8% were classified as AUS/FLUS. Of patients with an initial AUS/FLUS diagnosis, 11.5% (59/514) underwent surgery after a single FNA, 55.4% (285/514) had a repeat FNA, and 32.7% (168/514) were either observed or lost to follow-up. Surgical pathology was available in 123 cases (23.9%), and malignancy was confirmed in 32.5% (40/123) cases, with similar rates in the single 32.2% (19/59) and repeat FNA 32.8% (21/64) groups. Repeat FNA reclassified 78.9% of the AUS/FLUS cases to a different category: 57.2% were reclassified as benign, 10.5% as follicular neoplasm, and 5.6% as suspicious for malignancy or malignant. The rates of nonneoplastic benign lesions were 52.5% (31/59) and 31.2% (20/64) in the single and repeat FNA groups, respectively (P = .018). The rates of follicular adenomas were higher when repeat FNA was performed (23/64, 35.9%) compared with a single FNA (9/59; 15.2%) (P = .013).
Conclusion: In this series, a repeat FNA in cases of AUS/FLUS increased detection of follicular adenomas but not the detection of malignancy. Repeat FNA reduced the rate of benign nonneoplastic lesions by 40% in the surgical samples.
Keywords: atypia of undetermined significance; follicular adenoma; follicular lesion of undetermined significance; thyroid cancer; thyroid cytology; thyroid nodule.
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