Molecular data from The Cancer Genome Atlas classified endometrial cancer into four distinct subtypes that are prognostic for outcome: p53 abnormal (p53abn); mismatch repair protein deficient (dMMR), POLE exonuclease domain ultramutated (POLEmut), and p53 wildtype with no specific molecular profile (NSMP). MMR proteins are involved in DNA repair following radiotherapy (RT), and may have prognostic importance in treatment response. This study evaluates if dMMR is associated with recurrence risk in women with FIGO stage IIIC endometrial cancer treated with hysterectomy and adjuvant RT +/- chemotherapy (CT). Medical records of 143 women with FIGO stage IIIC endometroid-type endometrial adenocarcinoma treated at two institutions between 2000 and 2016 were retrospectively analyzed. MMR status was determined from the pathology record with immunohistochemical staining of the 4 MMR proteins (MLH1, MSH2, MSH6, and PMS2). Tumors with the absence of staining of at least 1 MMR protein were considered dMMR. Recurrence-free survival (RFS) was calculated by Kaplan-Meier method and predictors of RFS were identified by Cox regression model. Of 143 patients, 46 (32%) had known MMR status, including 19 (41%) with dMMR. Absence of staining by MMR protein was: MLH1(13, 68%), PMS2 (15, 79%). MSH2 (2, 11%) and MSH6 (3, 16%). Baseline characteristics, including patient age, tumor grade, myometrial invasion and lymphovascular invasion were similar for dMMR and MMR-proficient tumors (Table 1). With a median follow-up of 33.6 months (IQR: 20.5 - 62.4), 2 of 19 patients (11%) with dMMR tumors had recurrence and 5 of 27 (19%) patients with MMR-proficient tumors relapsed. 3-year RFS for dMMR and MMR-proficient patients was 82.0% and 91.2%, respectively (log-rank p-value = 0.9184). On univariate analysis, MMR was not associated with RFS while age and stage were. Given the few number of events, multivariate analysis could not be performed and, therefore, adjusted hazard ratios for dMMR and MMR-proficient tumors could not be assessed. MMR deficiency was not prognostic for recurrence among patients with FIGO stage IIIC endometrial cancer patients treated with adjuvant radiation with and without chemotherapy. However, further study in the larger retrospective cohorts and the prospective setting is warranted to better understand the molecular subtypes of endometrial cancer, treatment response and outcome.Abstract 3096; TableBaseline patient and tumor characteristics stratified by MMR status.MMR-intact (n = 27)MMR-deficient (n = 19)Age: median, range57 (33-74)56 (44-84)%MMI: median, range60 (25-100)67 (5-100)Total Involved nodes: median, range1 (0-4)1 (1-8)Involved pelvic nodes: median, range1 (0-4)1 (0-8)Involved paraaortic nodes: median, range0(0-2)0 (0-5)LVIYes15 (79%)20 (74%)No4 (21%)6 (22%)missing01 (4%)Gradegrade 1&220 (74%)13 (68.4)grade 37 (26%)6 (32%)StageIIIC122 (81%)13 (68%)IIC25 (19%)6 (32%) Open table in a new tab