Anti-PD-1 +/- RT for MSI-H Solid Tumors

To determine if the out-of-field ORR is improved with the addition of radiation therapy to anti-PD-1 for patients with MSI-H/dMMR metastatic solid tumors. Determine the rates of in-field tumor control, disease control (stable disease, partial response, complete response), durability of disease response, progression-free survival, overall survival, and to assess quality of life and toxicity. Determine the chronology and profile of the radiation-associated immune response.