Pembrolizumab and Total Skin Electron Beam Radiotherapy in Mycosis Fungoides and Sézary Syndrome

Inclusion Criteria: * Biopsy confirmed Mycosis Fungoides or Sézary Syndrome * Stage IB-IV by ISCL/EORTC 2007 Revision Staging (See Appendix Section 13.3). Maximal stage since diagnosis will determine eligibility. * Failed or intolerant to at least one prior line of systemic therapy * Life expectancy \> 6 months * Eastern Cooperative Oncology Group (ECOG) Performance Status ≤ 2 and able to stand for TSEBT * Baseline measurable disease by the mSWAT criteria * Acceptable baseline laboratories: * Leukocytes ≥ 2,000/mcL * Absolute neutrophil count ≥ 1,500/mcL * Platelets ≥ 100,000/mcl * Hemoglobin ≥ 9g/dL * Total bilirubin ≤ 1.5 X institutional upper limit of normal * AST(SGOT)/ALT(SPGT) ≤ 2.5 X institutional upper limit of normal * INR/PT ≤ 1.5 X institutional upper limit of normal (\*unless on anticoagulation therapy and therapeutic) * Serum creatinine ≤ 1.5 X institutional upper limit normal OR ≥ 60 mL/min calculated creatinine clearance ≥60 mL/min for subject * Women of child-bearing 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 for 90 days following completion of therapy. Should a woman become pregnant or suspect she is pregnant while participating in this study, she should inform her treating physician immediately. * A female of child-bearing potential is any woman (regardless of sexual orientation, having undergone a tub…