ctDNA Testing to Inform Standard-of-Care Treatment Decisions in People With Endometrial Cancer

Inclusion Criteria: * Patients must have pathologically confirmed endometrial cancer * Patients with advanced endometrial cancer * Stage III with residual disease * Stage IV * Recurrent endometrial cancer after adjuvant therapy only * Patients can have primary or planned interval surgery * MMR-D on immunohistochemistry OR MSI-H using any commercially available test * Patients with treated brain metastases are eligible if follow up brain imaging after CNS directed therapy shows no evidence of progression. * Eligible for standard of care chemotherapy with immune checkpoint inhibitor per treating investigator with no clinical contraindications * For the up to 10 patients who are allowed to enroll after C1D1 of standard of care treatments, they need to be on treatment with no clinical evidence of disease progression. * Patients may have received prior radiation therapy for treatment of endometrial cancer. Prior radiation therapy may have included pelvic radiation therapy, extended field pelvic/para aortic radiation therapy, intravaginal brachytherapy, and/or palliative radiation therapy. * Patients may have received prior hormonal therapy for treatment of endometrial cancer. * Patients may not have received prior therapy with an anti-PD-1, anti-PD-L1 or anti-CTLA-4 therapeutic antibody or other similar agents. * Except for the up to 10 patients who are allowed to enroll after C1D1 of standard of care treatments (chemotherapy and/or immune checkpoint inhibitors) per protocol. * Age ≥ 18 * Not Pregnant and Not Nursing