A Trial Assessing the Effect of Pembrolizumab Combined with Radiotherapy in Patients with Relapsed, Refractory, Specified Stages of Cutaneous T-cell Lymphoma (CTCL) Mycosis Fungoides (MF)/Sezary Syndrome (SS)

Inclusion Criteria: * Age ≥ 18 years * Diagnosis of Stage IB-IVB CTCL mycosis fungoides (MF)/Sézary Syndrome (SS) * Have relapsed, are refractory or progressed after at least 1 systemic therapy * Skin biopsy at the time of or within 6 months prior to study entry * Patients must have a total mSWAT (modified Severity Weighted Assessment Tool) score of ≥10 OR have 2 or more measurable tumours of any size. Of this area: there should be at least 1 cutaneous lesion (MF) or a defined area of involved skin (erythrodermic MF or SS) which is an appropriate target for palliative radiotherapy. There should be an area of skin involved by measurable Mycosis Fungoides/SS that will not be irradiated (To assess the abscopal effect of radiotherapy) * Have a minimum wash-out and adverse event (AE) recovery period from previous treatments (e.g. topical therapy, phototherapy, local radiotherapy, monoclonal antibody, systemic cytotoxic anticancer treatment or other novel agents) prior to the first dose of pembrolizumab * Have ECOG performance status of 0 or 1 * Life expectancy of at least 6 months * Demonstrate adequate organ function * Female patients of childbearing potential must have a negative urine or serum pregnancy test at pre-registration. If the urine test is positive or cannot be confirmed as negative, a serum pregnancy test will be required * Willing to comply with the contraception requirements * Written informed consent •Exclusion Criteria: * Received chemotherapy or targeted sma…