Epigenetic silencing of MLH1 in endometrial cancers is associated with larger tumor volume, increased rate of lymph node positivity and reduced recurrence-free survival

•MLH1 methylated endometrial tumors have poor prognostic features including larger size.•Tumor volume and mismatch repair class are associated with lymph node involvement.•Women with MLH1 methylated tumors have reduced recurrence-free survival.•Recurrence rate by MMR class differed dramatically in advanced stage endometrial cancer.•MMR defective tumors with MLH1 methylation may exhibit chemoresistance. To determine the relationship between mismatch repair (MMR) classification and clinicopathologic features including tumor volume, and explore outcomes by MMR class in a contemporary cohort. Single institution cohort evaluating MMR classification for endometrial cancers (EC). MMR immunohistochemistry (IHC)±microsatellite instability (MSI) testing and reflex MLH1 methylation testing was performed. Tumors with MMR abnormalities by IHC or MSI and MLH1 methylation were classified as epigenetic MMR deficiency while those without MLH1 methylation were classified as probable MMR mutations. Clinicopathologic characteristics were analyzed. 466 endometrial cancers were classified; 75% as MMR proficient, 20% epigenetic MMR defects, and 5% as probable MMR mutations. Epigenetic MMR defects were associated with advanced stage, higher grade, presence of lymphovascular space invasion, and older age. MMR class was significantly associated with tumor volume, an association not previously reported. The epigenetic MMR defect tumors median volume was 10,220mm3 compared to 3321mm3 and 2,846mm3, for MMR proficient and probable MMR mutations respectively (P