Testing the Addition of the Immunotherapy Drug, Pembrolizumab, to the Usual Radiation Treatment for Newly Diagnosed Early Stage High Intermediate Risk Endometrial Cancer

Inclusion Criteria: * Patients must have: * Stage I endometrioid endometrial cancer and a combination of age and risk factors as listed below: * Age \>= 70 and 1 or more risk factors * Age 50 - \< 70 and 2 or more risk factors * Age \< 50 and 3 risk factors * Risk factors: * Myometrial invasion \>= 50% * Lymphovascular space invasion * Grade 2 or 3 OR * Stage II endometrioid endometrial cancer * Note: Patients with isolated tumor cells in sentinel lymph nodes are eligible (considered N0i) as long as there is no evidence of micro- or macro-metastases in any lymph nodes * CT or MRI abdomen or pelvis and either chest X-ray or CT chest demonstrating no evidence of disease outside of the uterus. Imaging can be performed pre-operatively or post-operatively. CT with contrast is the preferred modality. Positron emission tomography (PET)/CT is NOT to be used for any disease assessment or reassessment unless there is documentation that PET/CT is of diagnostic quality equal to CT with contrast * Patients must have deficient mismatch repair as demonstrated by lack of expression of at least one mismatch repair protein by immunohistochemistry (IHC) and/or evidence of microsatellite instability (MSI) high. The institutional pathology report documenting MMR deficiency must be submitted * Patients must have undergone surgical staging with at least hysterectomy, removal of cervix, bilateral (if both are present) salpingo-oophorectomy, and…