Objective There is uncertainty about the prognostic significance of mismatch repair (MMR) deficiency in endometrial cancer. The objective was to evaluate clinical characteristics and outcomes of endometrial cancers based on MMR status within a population-based study.

Methods This was a retrospective population-based cohort study of all endometrial cancer cases from the Vancouver Coastal Health Authority region, evaluated for 4 MMR proteins using immunohistochemistry from 2012 to 2015. Patients were classified as MMR deficient (dMMR, any MMR protein absent) or MMR proficient (pMMR), Demographics, tumor characteristics, recurrences, and survival rates were compared according to MMR status.

Results There were 892 patients, with 650 pMMR (72.5%) and 242 dMMR tumors. The dMMR group had more endometrioid tumors (87.6% vs 74.0%, P < 0.001), lymphovascular space invasion (43.8% vs 30.8%, P = 0.001), and dedifferentiation (5.9% vs 1.5%, P < 0.001), but fewer grade 1 tumors compared with the pMMR group (31.8% vs 40.8%, P < 0.001). Median progression-free survival and overall survival have not been reached. After a median follow-up of 31 months (1–99 months), there was no difference in progression or recurrence rates between pMMR and dMMR tumors (19.5% vs 16.5%; P = 0.31). However, among those with nonendometrioid tumors, recurrence and mortality rates were significantly higher for pMMR than dMMR tumors (42.0% vs 10.0%, P = 0.001, and 36.1% vs 13.1%, P = 0.01, respectively), despite similar stage and lymphovascular space invasion distributions.

Discussion In this population-based study, there were no significant differences in recurrence or survival outcomes according to MMR status in endometrial cancer. However, among those with nonendometrioid tumors, there were lower recurrence and mortality rates associated with MMR-deficient compared with MMR-proficient tumors.