Should atypia of undetermined significance be subclassified to better estimate risk of thyroid cancer?

Background Atypia or follicular lesion of undetermined significance (AUS) is a cytologic category of thyroid aspirates with a wide range of reported malignancy. We aimed to determine whether specific cytologic features are associated with different rates of thyroid malignancy. Methods All thyroid fine needle aspiration biopsies with AUS from 2010 to 2012 were reanalyzed. Cytologic features were correlated with final pathology. Cytopathologists were blinded to the original cytologic interpretation and final diagnosis. Results Seventy-six patients had AUS; 39 (54%) underwent surgery with a malignancy rate of 18%. Specimens with moderate or large amount of thin colloid and absent or few nuclear inclusions had a >88% rate of benign disease. More than rare nuclear inclusions or grooves were associated with a higher rate of cancer (75% vs 9%, P =.005; 45% vs 7%, P =.003). Conclusions Patients with AUS and more than rare nuclear inclusions or grooves are at higher risk for cancer and should forego repeat fine needle aspiration biopsy and undergo thyroidectomy.