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

This is a single center phase I clinical trial assessing the safety of combination therapy of TSEBT and pembrolizumab for treatment of Stage IB-IV relapsed/refractory MF and SS. Primary Endpoint: • Primary endpoint will be maximum tolerated dose (MTD). Secondary Endpoints: * Efficacy of the combination of TSEBT with pembrolizumab therapy is measured. * CTCAE v4.0 toxicity beyond the 30 day period following the second therapy in the combination protocol treatment and up to 30 days following last dose of pembrolizumab. * Skindex-29 patient-reported HRQOL survey. Sample Size and Accrual: 18 patients will be enrolled. Statistical Analysis: Time to event will be estimated using the Kaplan-Meier approach along with the 95% confidence interval.