Tislelizumab for the Treatment of Recurrent Mismatch Repair Deficient Endometrial Cancer

Inclusion Criteria: * Able to provide written informed consent and can understand and agree to comply with the requirements of the study and the schedule of assessments * Age \>= 18 years on the day of signing the informed consent form (or the legal age of consent in the jurisdiction in which the study is taking place) * All patients with recurrent endometrial carcinoma, of any histology including clear cell, serous papillary carcinomas, and carcinosarcoma, whose disease is not amenable to definitive local therapy (including surgery and/or radiation therapy) * Patients must have deficient mismatch repair as demonstrated by lack of expression of at least 1 mismatch repair protein by immunohistochemistry, or evidence of microsatellite instability (MSI) high, or evidence of Lynch syndrome * The patient must have a lesion that is amenable to safe biopsy and the patient must agree to pre-and post-treatment biopsies * Patients may have received radiation for the treatment of endometrial cancer * Patient must have recovered from toxicity related to prior treatment to grade 2 or less * At least two weeks should have elapsed since completion of prior chemotherapy or 5 half-lives (whichever is shorter), or 4 weeks from radiation involving the whole pelvis or over 50% of the spine * At least 1 measurable lesion as defined per RECIST v1.1 that is amenable to biopsy * Eastern Cooperative Oncology Group (ECOG) performance status =\< 1 * Absolute neutrophil count (ANC) \>= 1.5 x 10\^9/L (m…