Introduction: Subclassification of atypia of undetermined significance/follicular lesion of undetermined significance (AUS/FLUS) is encouraged in the Bethesda System. In our practice, we subclassified AUS/FLUS into 3 subcategories: atypical follicular cells of undetermined significance (ACUS) for cases with cytologic atypia; follicular lesion (FL) for cellular cases with follicular cells with-minimal or no atypia, arranged in a macro- and micro-follicular pattern with scant colloid; and indeterminate follicular lesion, favor benign (IFL-FB) for cases with few clusters of follicular cells without atypia associated with minimal or no colloid. The objective of our study was to evaluate the prevalence, clinical management, and risk of malignancy for each subcategory.
Materials and methods: We retrospectively identified ultrasound-guided fine-needle aspiration (US-FNA) of thyroid cases that were subcategorized as ACUS, IFL-FB, and FL at our-institution during 2014-2016. The results of US-FNA were correlated with clinical outcome in the subsequent 2 years including repeat US-FNA, thyroid surgery, and clinical/imaging follow-up.
Results: Of 3207 thyroid US-FNA cases, 718 (22.4%) cases were included in the study. Of these 718 cases, 104 (14.5%) were subcategorized as ACUS, 166 (23.1%) as FL, and 448 (62.4%) as IFL-FB. The surgery rate was 39.4% (41 of 104) for ACUS, 13.6% (61 of 448) for IFL-FB, and 27.1% (45 of 166) for FL. The risk of malignancy (ROM) was 25% (26 of 104) for ACUS, and 2.9% (13 of 448) for IFL-FB, 6.0% (10 of 166) for FL. The surgery rate and ROM was significantly higher for ACUS in comparison to IFL-FB (P < 0.05) and FL (P < 0.05).
Conclusions: Subclassification of AUS/FLUS into 3 groups based on cytopathologic findings alone not only improved the triage of patients for subsequent clinical management but also effectively stratified the risk of malignancy.
Keywords: AUS/FLUS; Fine-needle aspiration; Risk of malignancy; Subclassification; Thyroid.
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