Patient outcomes following surgical management of thyroid nodules classified as Bethesda category III (AUS/FLUS)

Introduction: The Bethesda classification system for reporting thyroid cytopathology is the standard for interpreting fine needle aspirate (FNA). Because of its heterogeneity and inconsistent reporting, atypia of undetermined significance or follicular lesion of undetermined significance (AUS/FLUS), known as Bethesda category III, is the most controversial category. Thyroid nodules that fall within Bethesda categories III-IV have an overall risk of malignancy of between 15 and 40%. The aim of this study was to determine the malignancy rate in Bethesda III nodules. Material and methods: A retrospective study was performed for 1166 patients who underwent thyroid surgery for multinodular goitre (MNG) or solitary nodular goitre (SNG) in our institution between June 2010 and May 2020. Data retrieved included demographic characteristics of the patients, FNB cytology, thyroid function test results, type of thyroidectomy, and final histology results. Results: During the study period, 29.5% (344/1166) of patients with an FNA categorized as AUS/FLUS underwent thyroid surgery. Of these 344 patients, 190 were diagnosed with MNG and 154 with SNG. Incidental malignancy was found in 35 of 190 cases of MNG (18.42%) and 31 of 154 cases of SNG (20.13%). The most common malignant tumour type in either category was the follicular variant of papillary thyroid carcinoma. Conclusions: The current study demonstrates that patients with a FNA categorized as AUS/FLUS may have a higher risk of malignancy than traditionally believed. Reconsideration may be necessary to guidelines that recommend observation or repeat FNA in this category of patients.