Pembrolizumab and Involved Site Radiation Therapy for Early Stage Relapsed or Primary Refractory Hodgkin Lymphoma

Inclusion Criteria: * Three populations of patients are eligible for enrollment: (1) Patients with early stage disease at diagnosis (stage I-II) who were treated with chemotherapy alone and relapsed with early stage disease (stage RI-II). (2) Patients with early stage disease at diagnosis (stage I-II) who were treated with chemotherapy alone and have early stage (stage RI-II) primary refractory disease (residual disease on a scan 1 month after the completion of initial therapy) without B-symptoms and with each area of disease less than 10 cm in size. (3) Patients with early stage disease at diagnosis (stage I-II) who were treated with combined modality therapy (chemotherapy and radiation) who relapse with early stage disease (stage RI-II) outside the prior radiation therapy field. * Histologic confirmation of classical Hodgkin Lymphoma after imaging documenting primary refractory or relapsed disease * Age 12 or older * ECOG Performance Status of 0-1 or Lansky Performance Status of ≥ 50 (Pts 12-15 years of age) * Adequate organ function. Screening labs should be performed within 14 days of treatment initiation. Hematologic * Absolute neutrophil count (ANC) ≥1,000 /mcL * Platelets\* ≥75,000 / mcL * Hemoglobin\* ≥8 g/dL Renal * Creatinine OR Measured or calculated\*\* creatinine clearance CrCl (GFR can also be used in place of creatinine or ≤1.5 X upper limit of normal for age (ULN) OR ≥60 mL/min for subject with creatinine levels \> 1.5 X institutional ULN Hepatic *…