SUN-351 Clinical Features And Outcomes Of Medullary Thyroid Cancer Based On Different Bethesda Cytology Categories

Abstract Disclosure: S. Puri: None. M.Y. Roth: None. K. Madani: None. T.A. Tylee: None. M. Endo: None. Background: The Bethesda scoring system is a standardized risk assessment for malignancy based on features of thyroid FNA results. Categories range from I (nondiagnostic) to VI (malignant). This project aims to compare the presence of aggressive features of medullary thyroid cancer (MTC) between Bethesda categories. Methods: A cohort of patients at The University of Washington with a history of MTC was identified using Epic LEAF tool. Relevant data including Bethesda category, presence of lymph node metastases, cancer staging information, pre-op carcinoembryonic antigen (CEA) and calcitonin levels, and CEA and calcitonin levels at last follow up were extracted via chart review. Data was analyzed using JMP Pro 17. Wilcoxon two sample t-tests were performed, comparing data between Bethesda II/III/IV and V/VI groups. N=73 (Bethesda II=1, III=14, IV=3, V=10, VI=45). Results: Lymph node metastasis was significantly higher in Bethesda V/VI than II/III/IV [II/III/IV (N0 vs N1A vs N1B= 44% vs 11 vs 17%); V/VI (N0 vs N1A vs N1B= 20% vs 9 vs 60%)], (p=0.01). Structural disease was higher in Bethesda V/VI than II/III/IV (56% vs 27%), but not statistically significant (p=0.1). The presence of germline mutations was significantly higher in Bethesda V/VI [II/III/IV: 0% germline, 100% somatic; V/VI: 28% germline, 72% somatic (p=0.04)]. There was no statistically significant difference in tumor grading, metastases, lymphovascular invasion (LVI), and extrathyroidal extension (ETE) between Bethesda groups. Pre-op calcitonin was significantly higher in Bethesda V/VI [II/III/IV vs V/VI= 506 vs 1550, median value (p=0.02)]. There was no statistically significant difference in pre-op CEA levels, but these were higher in V/VI [II/III/IV vs V/VI= 20 vs 35, median value (p=0.39)]. CEA at the last follow-up was significantly higher in Bethesda V/VI [II/II/IV vs V/VI = 1.3 vs 6.5, median value (p=0.03)]. However, there was no statistically significant difference in calcitonin at last follow-up between Bethesda categories [II/III/VI vs V/VI= 8 vs 78, median value (p=0.09)]. Conclusion: Patients with Bethesda V/VI cytology tended to have more aggressive features of MTC such as lymph node metastases and higher tumor marker levels than those with II/III/VI cytology. Thus, initial FNA cytology may play a role in risk stratification and guiding future management of MTC. Presentation: Sunday, July 13, 2025