Testing the Addition of Radiation Therapy to Immunotherapy for Merkel Cell Carcinoma

Inclusion Criteria: * Patients must have pathologically (histologically or cytologically) proven diagnosis of MCC by local pathology review * Have measurable disease based on RECIST 1.1 including at least two cancerous deposits; at least one deposit must be RECIST measurable while at least one deposit must meet criteria for SBRT; non-radiated tumor will be identified prior to randomization on the protocol * Patients must have advanced or metastatic MCC defined as evidence of distant metastasis(es) on imaging * Patients with locoregionally confined disease are not eligible * No prior immunotherapy for advanced/metastatic MCC * Patients with known or suspected central nervous system (CNS) metastases, untreated CNS metastases, or with the CNS as the only site of disease are excluded; however, subjects with controlled brain metastases will be allowed to enroll; controlled brain metastases are defined as no radiographic progression for at least 4 weeks following radiation and/or surgical treatment (or 4 weeks of observation if no intervention is clinically indicated), and off of steroids for at least 2 weeks, and no new or progressive neurological signs and symptoms * Patients having received palliative radiotherapy for extracranial metastasis(es) are eligible as long as there are 2 cancerous deposits that have not received prior radiation therapy (RT) and they meet the following criteria * No prior radiation therapy (\> 5 Gy) to the metastasis intended to be treated with S…