Pembrolizumab in Combination With Decitabine and Hypofractionated Index Lesion Radiation in Pediatrics and Young Adults

Criteria: Inclusion Criteria: * Age: greater than or equal to 12 months and less than or equal to 40 years. * Diagnosis: Patients must have had histologic verification of malignancy at original diagnosis or relapse. Eligible diagnoses include: * Relapsed or refractory solid tumors (excluding primary CNS tumors) * Lymphoma in second or greater relapse or with refractory disease * Disease Evaluation: Patients must have evaluable or measurable disease (patients with evaluable disease must have at least one lesion that is amenable to radiation as below). * Patients with stable non-brainstem CNS metastases may be eligible. * Therapeutic Options: Patient's current disease state must be one for which there is no known curative therapy or therapy proven to prolong survival with an acceptable quality of life. * Performance Level: Lansky/Karnofsky greater than or equal to 50. * Prior Therapy: Patients who have previously received inhibitors of PD-1, PD-L1, CTLA4 or other immune checkpoint inhibitors, regardless of response, are eligible as long as they had not experienced a medically significant immune related adverse event that required treatment with supraphysiologic steroids or other immunomodulatory drug. * Patients must have recovered from the acute toxic effects of all prior anti-cancer chemotherapy and meet the following: * Myelosuppressive chemotherapy: At least 21 days after the last dose of myelosuppressive chemotherapy (42 days if prior nitrosourea). * Hematopoiet…