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

PRIMARY OBJECTIVE: I. To describe the progression-free survival (PFS) of stereotactic body radiation therapy (SBRT) + pembrolizumab (MK-3475) compared to pembrolizumab (MK-3475) alone in advanced/metastatic Merkel cell carcinoma (MCC) patients. SECONDARY OBJECTIVES: I. To describe the PFS of SBRT + MK-3475 compared to pembrolizumab (MK-3475) alone across Response Evaluation Criteria in Solid Tumors (RECIST) measurable (including both radiated and non-radiated) cancer deposits. II. To describe the overall response rate of SBRT + pembrolizumab (MK-3475) compared to pembrolizumab(MK-3475) alone in both radiated and in non-radiated deposit(s). III. To determine the PFS at 6 months of SBRT + pembrolizumab (MK-3475) compared to pembrolizumab (MK-3475) alone across all cancerous deposits by RECIST. IV. To determine the rate of grade \> 3-4 adverse events, by organ system, by Common Terminology Criteria for Adverse Events (CTCAE) version (v) 5.0. V. To determine the local control of SBRT treated tumors. VI. To calculate delivered radiation dose using cone-beam computed tomography (CT) images collected on the radiation treatment table in the final treatment position. CORRELATIVE SCIENCE OBJECTIVES: I. To test the utility of CT-based radiomics to predict radiation-induced pneumonitis and true delivered dose of SBRT based on cone beam collected imaging and diagnostic scans. II. Biobanking for future correlative science projects. OUTLINE: Patients are randomized to 1 of 2 groups. GROUP I: Patients receive pembrolizumab intravenously (IV) over 30 minutes on day 1. Cycles repeat every 21 days for up to 2 years in the absence of disease progression or unacceptable toxicity. GROUP II: Patients receive pembrolizumab IV over 30 minutes on day 1. Cycles repeat every 21 days for up to 2 years in the absence of disease progression or unacceptable toxicity. Patients also undergo SBRT for 3 doses during cycle 1. After completion of study treatment, patients are followed up every 6 months for up to 5 years.