Inclusion Criteria:

          -  Biopsy proven Merkel cell carcinoma which is unresectable or metastatic, stage III or
             IV

          -  Prior first-line treatment with aPD1 monotherapy (defined as at least one dose of
             pembrolizumab, avelumab, nivolumab, etc.) with evidence of progression of disease ≥10
             weeks after starting therapy, in the absence of significant clinical deterioration

               -  Patients with progression in only one of several responding metastases will not
                  be eligible

               -  Patients with clinical deterioration during aPD1 monotherapy are eligible ≥6
                  weeks after starting aPD1 therapy

               -  Criteria for clinical deterioration to be determined and agreed upon by treating
                  physician and Principal Investigator

          -  All detectable sites of MCC are amenable to comprehensive ablative radiation therapy
             in opinion of treating radiation oncologist and principal investigator

          -  ≥18 years of age

          -  Performance status ≤2 on the Eastern Cooperative Oncology Group Performance Scale

          -  Able to provide valid written informed consent

          -  Normal organ and marrow function

               -  Hematologic: Lymphocyte count ≥800/mm^3, neutrophil count ≥1500/mm^3, platelet
                  count ≥75,000/mm^3, leukocyte count ≥3000/mm^3, hemoglobin ≥9 g/dL

               -  Hepatic: Total bilirubin ≤ 1.5 times the upper limit of normal, unless Gilbert's
                  syndrome; aspartate transaminase and alanine transaminase ≤ 2.5 times the upper
                  limit of normal (in the absence of hepatobiliary metastases); ≤ 3.0 times the
                  upper limit of normal (in the presence of hepatobiliary metastases)

               -  Renal: Estimated creatinine clearance ≥ 30 mL/min according to the
                  Cockcroft-Gault formula.

        Exclusion Criteria:

          -  Prior systemic therapy for MCC other than first-line aPD1 monotherapy (ie,
             chemotherapy)

          -  Pregnancy or breastfeeding

          -  Adverse events due to prior cancer therapy which are grade 3 or higher and have not
             resolved

             °Patients with prior grade 3 or higher immune related adverse events are not eligible,
             even if they have resolved

          -  Prior severe hypersensitivity reaction (CTCAE version 5.0 grade ≥3) to avelumab

          -  Prior radiotherapy which precludes the ability to safely deliver comprehensive
             ablative radiation therapy in the opinion of the treating radiation oncologist and
             principal investigator

             °Institutional guidelines for reirradiation will be used when making this
             determination

          -  Known central nervous system metastases

          -  Known clinically significant cardiovascular disease, defined as:

               -  Stroke or myocardial infarction within 6 months of first dose of avelumab

               -  Symptomatic congestive heart failure (New York Heart Association Class 2 or
                  higher)

               -  Serious arrhythmia requiring anti-arrhythmic agents

          -  Known Human Immunodeficiency Virus infection

          -  Known Hepatitis B or C infection requiring ongoing treatment

          -  Vaccination within 4 weeks of first dose of avelumab

             °Inactivated vaccines are permissible

          -  Iatrogenic immunosuppression with daily systemic corticosteroid equivalent of >10 mg
             of prednisone

          -  Active autoimmune disease that may cause clinical deterioration during immunotherapy

             °Including, but not limited to:

          -  Inflammatory bowel disease or immune colitis

          -  Immune mediated pneumonitis or pulmonary fibrosis

          -  History of solid organ or hematopoietic transplant

          -  Active infection requiring systemic therapy

          -  Active suicidal ideation or behavior

          -  Comorbid or diagnostic abnormalities which would interfere with interpretation of
             study results

          -  Known hematopoietic cancer or dysfunction (i.e., leukemia, lymphoma)

          -  Known non-MCC solid tumor with known metastasis or estimated risk of metastasis >20%
             within 3 months