Subcategorisation: Improved Stratification of Thyroid Nodule with Cytological Results of Atypia of Undetermined Significance

Abstract

Aim: According to Bethesda Classification for cytology assessment of thyroid nodule, malignancy rate of 'atypia of undetermined significance' ranged from 5 to 15 %. However, AUS represented a heterogenous group of cytology features and reported rate of malignancy was up to 38%. We aim to subcategorise AUS nodules and determine their malignancy risk. Methods: 1075 thyroidectomies were performed between January 2013 and September 2015. 783 cytology specimens were obtained preoperatively, in which 158 (20.2%) were AUS. The cytology results were reviewed and subclassified into four subgroups: AUS cannot exclude papillary carcinoma (AUS-PTC), AUS cannot exclude follicular neoplasm (AUS-FN), AUS cannot exclude Hürthle cell neoplasm (AUS-HCN), and AUS not otherwise specified (AUS-NOS). The malignancy risk was determined with final pathology. Results: In 158 nodules with AUS, 35.4% (56/158) patients had malignant thyroid nodule. However, 11 of them were incidental finding. Therefore, only 45 patients' nodule with AUS was finally diagnosed as malignant. 14.5% (23/158) had repeat FNAC. Repeat cytology was unsatisfactory in 30.4% (7/23), benign in 17.4% (4/23), AUS in 43.4%(10/23), suspicious for malignancy in 8.7% (2/23). Nodules were subcategorised into AUS-PTC (43/158, 27.2%), AUS-FN (59/158, 37.3%), AUS-HCN (12/158, 7.6%) and AUS-NOS (44/158, 27.8%). Malignancy rate was 34.9% for AUS-PTC (15/43), 23.7% for AUS-FN (14/59), 33.3% for AUS-HCN (4/12) and 27.3% for AUS-NOS (12/44). Conclusion: Malignancy rate of AUS nodules is higher than previously reported. AUS-PTC has higher risk of malignancy than other subcategories. This suggests distinct cytomorphological feature may better predict malignancy and subcategorisation may have potential clinical implication in guiding management.