AbstractPurpose:Hysterectomy with bilateral salpingo-oophorectomy (BSO) is the cornerstone of uterine cancer treatment. The Surveillance, Epidemiology, and End Results (SEER)-Medicare linked database provides two sources of surgery information: medical record abstraction (SEER) and billing claims (Medicare). Concordance between these two sources has not been evaluated among uterine cancer patients. Given the increasing use of SEER-Medicare for assessment of treatment-related uterine cancer survival effects, an understanding of the concordance of these two sources is needed.Methods:Patients diagnosed with uterine cancer between 2000 and 2019 were identified using the SEER-Medicare linked database. We calculated sensitivity, specificity, positive predictive value, negative predictive value, and Kappa statistics for receipt of hysterectomy with or without BSO and receipt of hysterectomy with BSO using Medicare claims as the presumed gold standard. Analyses were repeated stratified by race, histology, and stage.Results:A total of 34, 904 uterine cancer patients [9% Black, mean (standard deviation) age at diagnosis: 74.6 (6.7) years] were included in the comparison of surgery derived in SEER vs. Medicare claims, after excluding 39.7% with unknown surgery status from SEER. A total of 30, 943 patients were identified as having hysterectomy with or without BSO in both the claims and SEER, resulting in a moderately strong sensitivity [78.4%, 95% confidence interval (CI)= 77.1%, 79.8%]. A total of 29, 733 patients were identified as having hysterectomy with BSO in both the claims and SEER, resulting in a sensitivity of 69.5% (95% CI= 68.1%, 70.8%). For both procedures, lower sensitivity was noted for non-Hispanic Whites, those diagnosed with grade 1 endometrioid, or those diagnosed with stage 1 disease. Specificity was high for both treatment types and across race, histology, and stage categories (>96.9%). In cases where there was discordance between claims and SEER, claims were more likely to report a corresponding insurance claim when there was no corresponding surgery indication in SEER.Conclusions:In the subset of uterine cancer patients with known SEER surgery, SEER data reliably identified receipt of hysterectomy with or without BSO and hysterectomy with BSO; however, variations according to patient characteristics were observed. Of concern is the large proportion of unknown surgery data in SEER, which has implications for research aimed at estimating the population prevalence of surgery and investigations comparing outcomes between surgically and non-surgically treated patients. Investigations of the origins of unknown SEER surgery data are warranted.Citation Format:Ashley S. Felix, Caitlin E. Meade, Qinhan Hu, Anusha M. Vable, Tasleem J. Padamsee, Marcarius Donneyong, Kemi M. Doll, Jennifer A. Sinnott. Ascertainment of uterine cancer surgery in the Surveillance, Epidemiology, End Results (SEER)-Medicare linked database: Comparison of SEER and claims data [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2025; Part 1 (Regular Abstracts); 2025 Apr 25-30; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2025;85(8_Suppl_1):Abstract nr 2296.