Does mismatch repair (MMR) deficiency have prognostic significance in low-risk endometrioid endometrial cancers?

e17122 Background: Mismatch repair (MMR) deficiency is observed in 25-30% of all endometrial cancers. This can be detected by the absence of MMR protein staining on immunohistochemistry (IHC), and is used in many jurisdictions as a screen for an inherited mutation in one of the MMR genes (Lynch Syndrome). Only 10% of women with MMR deficiency (MMRd) have Lynch syndrome, but MMRd may still have prognostic significance. The objective of this study was to compare clinical outcomes between MMR deficient and proficient low-risk endometrioid endometrial cancers (stage IA, grade 1/2). Methods: This was a retrospective population-based cohort study of all low-risk endometrial cancers from Vancouver Coastal Health authority region from 2011 to 2016 that were assessed for MMR deficiency (MMRd). Primary outcome measures were recurrence rates expressed per person-years (py), progression free survival (PFS) and overall survival (OS) calculated using Kaplan-Meier method and log-rank tests. Cox proportional hazards model estimated the association between MMRd and recurrence and death after adjustment for covariates, expressed as hazard ratios (HR). Results: There were 475 low-risk patients, including 131 MMRd (27.6%) and 345 MMRp (proficient) patients. Women with MMRd tumors had higher recurrence rates (3.53p100py vs 1.21p100py) and worse PFS (p = 0.0082) compared to women with MMRp tumors. After adjustment for age, LVSI status, adjuvant therapy, and post-operative grade, MMRd status remained associated with a higher risk of recurrence (HR 2.99, 95% CI 1.27-7.04). There was no significant difference in OS between MMR groups (HR 1.38, 95% CI 0.57-3.33). Conclusions: In low-risk stage IA grade 1 or 2 endometrioid endometrial cancers, MMR deficiency is associated with a higher recurrence rate than in MMR proficient cases, after adjustment for covariates, implying that MMR deficiency reflects a different biology in endometrial cancer.