Inclusion Criteria: 

• Age ≥ 18 years.
• Patients must have an ECOG performance Status: 0, 1, or 2; however, those patients with a performance state of 3 because they are wheel chair bound due to congenital or traumatic events more than one year before the diagnosis of Merkel cell carcinoma are eligible).
• Women must not be pregnant or breast-feeding due to the unknown effects of the study drug in this setting.
• All women of childbearing potential must have a blood test or urine study within 2 weeks prior to registration to rule out pregnancy.
• A female of childbearing potential is any woman, regardless of sexual orientation or whether they have undergone tubal ligation, who meets the following criteria:
  • has achieved menarche at some point;
  • has not undergone a hysterectomy or bilateral oophorectomy; or 
  • has not been naturally postmenopausal (amenorrhea following cancer therapy does not rule out childbearing potential) for at least 24 consecutive months (i.e., has had menses at any time in the preceding 24 consecutive months).
    • Female of child bearing potential? ______ (Yes or No)
    • Date of blood test or urine study: ___________
• Women of childbearing potential and sexually active males on Arm A MK-3475 (Pembrolizumab) must use accepted and effective method(s) of contraception or abstain from sex from time of registration, while on study treatment, and continue for 120 days after the last dose of study treatment. For patients on Arm B only receiving radiation therapy, contraception use should be per  institutional standard.
• Patient must have a histological confirmation of diagnosis of Merkel cell carcinoma (MCC), pathologic stages (AJCC version 8) I-IIIb:
  • Stage I patients with negative sentinel lymph node biopsy are ineligible;
  • Patients who have a positive biopsy or for whom no biopsy was done are eligible;
  • Patients with distant metastatic disease (stage IV) are not eligible;
  • The primary tumor must have grossly negative margins. (Microscopically positive margins are allowed);
  • Cancers of unknown primary that have regional disease only can be included;
  • Complete nodal dissection is not required for eligibility.
• Patients with all macroscopic Merkel cell carcinoma (either identified by physical exam or imaging) have been completely resected by surgery within 16 weeks before registration.
• All patients must have disease-free status documented by a complete physical examination and conventional imaging studies within 8 weeks prior to registration.
• Patient must have the following required values for initial laboratory tests obtained within 4 weeks prior to registration:
  • White Blood Count ≥ 2000/uL:
    • WBC: __________ Date Obtained: __________
  • Absolute neutrophil count (ANC) ≥ 1000/uL:
    • ANC: __________ Date Obtained: __________
  • Platelets ≥ 75 x 103/uL:
    • Platelet: __________ Date Obtained: __________
  • Hemoglobin ≥ 8 g/dL (≥ 80 g/L; may be transfused):
    • Hemoglobin: __________Date Obtained: __________
  • Creatinine ≤ 2.0 x ULN:
    • Creatinine: __________ Institutional ULN : __________
  • AST and ALT ≤ 2.5 x ULN:
    • AST: _______ Institutional ULN: _________
    • Date Obtained: ________________________
    • ALT: _______ Institutional ULN: _________
    • Date Obtained: ________________________
  • Total Bilirubin ≤ 2.0 x ULN, (except patients with Gilbert's Syndrome, who must have a total bilirubin less than 3.0 mg/dL)
    • Bilirubin: __________ Institutional ULN: _____________
    • Date Obtained:____________________________
• Patients who are HIV+ with undetectable HIV viral load are eligible provided they meet all other protocol criteria for participation.
• Patients with HBV or HCV infection are eligible provided viral loads are undetectable.
• Patients on suppressive therapy are eligible.
• Patients must not be on active immunosuppression, have a history of life threating virus, have had other (beside non-melanoma skin cancers, or recent indolent cancers e.g.: resected low grade prostate cancer) invasive cancer diagnoses in the last two years, or have had immunotherapy of any kind within the last 2 years.
• Operative notes from patient’s surgical resection must be accessible.

Exclusion Criteria:

• Patient may not have a history of distant metastatic disease.
  • NOTE: loco-regional recurrent disease is acceptable, as long as this is not metastatic (prior surgery with or without radiation therapy is acceptable).
• For patients with initial presentation of Merkel cell carcinoma, patient must have no previous systemic therapy or radiation therapy prior to surgery for Merkel cell carcinoma and cannot have completed adjuvant radiation therapy for Merkel Cell Carcinoma more than 6 weeks prior to registration. Patients actively undergoing radiation therapy or having completed adjuvant radiation therapy within 6 weeks of registration are eligible, as long as resection date is within 16 weeks of registration.
• Patients must not have a history of (non-infectious) pneumonitis that required steroids or has current pneumonitis.
•