This trial is testing a combination of two different types of cancer treatment: checkpoint inhibitors and stereotactic body radiation therapy (SBRT). Checkpoint inhibitors help the immune system fight cancer, while SBRT is a type of radiation therapy that gives high doses of radiation to tumors with high precision. The use of checkpoint inhibitors in combination with SBRT has been suggested to improve the immune response against cancer, but this combination has not been tested in a formal clinical trial before. Up to three lesions can be treated with SBRT in this study.