Association Between MMR Status and Prognostic Pathological Factors in Endometrioid Endometrial Cancer—A Single-Center Retrospective Study

<b>Background/Objectives</b>: Prognostic assessment in endometrial cancer (EC) is based on clinical and pathological features such as histological type, FIGO stage, tumor grade, LVSI, P53 status, and hormone receptor expression. Recent molecular research has distinguished four EC subtypes, with MMR status (pMMR vs. dMMR) providing clinically relevant stratification due to its predictive value for immunotherapy. The present study aims to compare dMMR and pMMR tumors in terms of the prevalence of adverse histopathological prognostic factors. <b>Methods</b>: This retrospective study included 179 patients with endometrioid endometrial carcinoma (EEC) treated at the authors' institution (1 January 2023-31 August 2025). Patients were classified by MMR status (pMMR vs. dMMR) based on immunohistochemistry, and clinicopathological variables, including FIGO stage, myometrial invasion depth, tumor grade, LVSI, ER/PR expression, and P53 status, were analyzed. Normality was assessed using the Shapiro-Wilk test. Categorical variables were tested with chi-square or Fisher's exact tests, reporting odds ratios with 95% CI, while continuous variables were compared using the Mann-Whitney test and presented as median (IQR) with the Hodges-Lehmann difference and 95% CI. Multivariable logistic regression with Wald tests was performed. <b>Results</b>: dMMR tumors accounted for 29.05% of all cases. Patients in the dMMR group were significantly more likely to present with FIGO stage III/IV disease (<i>p</i> = 0.036) and to exhibit LVSI (<i>p</i> = 0.008). No differences were observed between the groups with respect to tumor grade, estrogen receptor positivity, progesterone receptor positivity, or the prevalence of deep myometrial invasion. The most frequent pattern of protein loss in the dMMR population was concurrent loss of MLH1 and PMS2. <b>Conclusions</b>: In the studied population, dMMR tumors more frequently exhibited adverse prognostic features of EC, such as advanced stage of disease and lymphovascular space invasion. This suggests the potential for effective immunotherapy in this patient group.