Inclusion Criteria:
* Histologically or cytologically confirmed mucosal melanoma that has undergone surgical resection. Patient must not have received prior radiation therapy within the area of interest.
* At least 16 years of age.
* Eastern Cooperative Oncology Group (ECOG) performance status ≤ 2 (Karnofsky ≥ 70%)
* Normal bone marrow and organ function as defined below:
* Absolute neutrophil count ≥ 1,200/mcL
* Platelets ≥ 100,000/mcL
* Total bilirubin ≤ 1.5 x institutional upper limit of normal (IULN)
* AST(SGOT)/ALT(SGPT) ≤ 3.0 x IULN
* Creatinine clearance \> 30 mL/min by Cockcroft-Gault
* The effects of pembrolizumab on the developing human fetus are unknown. For this reason, women of childbearing potential and men must agree to use adequate contraception (hormonal or barrier method of birth control, abstinence) prior to study entry, for the duration of study participation, and 6 months after last dose of pembrolizumab. Should a woman become pregnant or suspect she is pregnant while participating in this study, she must inform her treating physician immediately. Men treated or enrolled on this protocol must also agree to use adequate contraception prior to the study, for the duration of the study, and 6 months after last dose of pembrolizumab.
* Ability to understand and willingness to sign an IRB approved written informed consent document (or that of legally authorized representative, if applicable).
Exclusion Criteria:
* A history of other malignancy with the exception of malignancies for which all treatment was completed at least 2 years before registration and the patient has no evidence of disease.
* Received radiation therapy within the area of interest.
* Currently receiving any other investigational agents.
* Metastatic disease.
* A history of allergic reactions attributed to compounds of similar chemical or biologic composition to pembrolizumab or other agents used in the study.
* Uncontrolled intercurrent illness including, but not limited to, ongoing or active infection, symptomatic congestive heart failure, unstable angina pectoris, or cardiac arrhythmia.
* Has a diagnosis of immunodeficiency or is receiving systemic steroid therapy or any other form of immunosuppressive therapy within 7 days prior to the first dose of study treatment. The use of up to 10 mg/day of prednisone or equivalent is approved and does not exclude the patient from the trial.
* Active autoimmune disease that has required systemic treatment in past 2 years (i.e., with use of disease modifying agents, \> 10 mg of prednisone per day, or immunosuppressive drugs). Replacement therapy (e.g., thyroxine, insulin, or physiologic corticosteroid replacement therapy for adrenal or pituitary insufficiency, etc.) is not considered a form of systemic treatment. The use of up to 10 mg/day of prednisone or equivalent is approved and does not exclude the patient from the trial.
* Has a history of (non-infectious) pneumonitis/Interstitial lung disease that required maintenance steroids (\>10 mg of prednisone) or current pneumonitis/interstitial lung disease.
* Has received a live vaccine or live-attenuated vaccination within 30 days of planned treatment start. Administration of killed vaccines is allowed.
* Pregnant and/or breastfeeding. Women of childbearing potential must have a negative pregnancy test within 14 days of study entry.
* Patients with HIV are eligible unless their CD4+ T-cell counts are \< 350 cells/mcL or they have a history of AIDS-defining opportunistic infection within the 12 months prior to registration. Concurrent treatment with effective ART according to DHHS treatment guidelines is recommended.
