ODP474 Critical Analysis of the Thyroid Cytopathologic Diagnosis of Atypia of Undetermined Significance and Associated Clinical Outcomes

INTRODUCTION: FNA is the optimal test for classifying concerning thyroid nodules. Approximately 15-30% of FNAs are indeterminate, representing diagnostic challenge with intra- and inter-observer variability among cytologists. The Bethesda System (BS) for cytopathology was created in 2007 to standardize thyroid cytology with corresponding malignancy risk. Our goal is to identify the incidence of atypia of undetermined significance/follicular lesion of undetermined significance (AUS/ FLUS) and the percent of malignant AUS/FLUS cases at our institution. METHODS: Chart review of 3068 adult patients with FNA performed from 2003 to 2015 was completed, with attention to reports considered "indeterminant". BS for reporting thyroid cytology was not in use at our institution during that time. Cytopathology reports were evaluated for key descriptors concerning for AUS, FLUS, or follicular neoplasm. RESULTS: 461 cytology reports included language consistent with possible "indeterminate" cytology. These reports were reviewed with our institution's cytopathologist and retrospectively assigned an equivalent BS category. 241 cases were consistent with BS category III (7.86% of all thyroid nodule FNAs done during this time period). Molecular marker studies were not being performed at our institution on indeterminate thyroid cytology specimens during this time period. Of the patients who had indeterminate cytology consistent with BS category III, 66.3% (160/241) went for surgery. The malignancy rate was 27.5% (44/160) in the BS category III group. CONCLUSION: Our data demonstrated that category III nodules in some practice settings may have a higher risk of malignancy than traditionally believed, and that guidelines recommending repeat FNA or observation merit reconsideration. Molecular marker testing on indeterminate thyroid cytology specimens has been shown to be helpful in determining cases for surgery and is now being utilized at our institution. Presentation: No date and time listed