Pembrolizumab and Radiotherapy for Patients With NK/T Cell Lymphoma

Inclusion Criteria: * Confirmed histological diagnosis of NK/T Cell Lymphoma * No previous anti-lymphoma treatment * Age ≥ 18 years * Ann Arbor stage I-II * At least one measurable/evaluable site after diagnostic biopsy before treatment start * At least one of the following high-risk features: age \> 60 years, elevated LDH, stage II, primary tumor invasion * Patient ineligible to receive full dose standard chemotherapy * ECOG performance status of 0-1 * Signed Informed consent * Ability to comply with the protocol * Adequate hematological and organ function; * Tumor tissue (fresh preferred, archival tissue is also acceptable) * For women of childbearing potential a negative pregnancy test on day 1 of cycle 1 and agree to adopt an adequate measure to avoid pregnancy during study treatment and for at least one year from end of treatment * For men agreement to remain abstinent or to use barrier contraception Exclusion Criteria: * Advanced stage disease (AA stage III-IV) * Extranasal type NKTCL * History of autoimmune disease * History of other(s) infiltrating cancer(s) in the previous 3 years that were not treated with curative intent or who are still receiving anticancer therapy (including hormone therapy for breast or prostate cancer). * History of (non-infectious) pneumonitis that required steroids; evidence of interstitial lung disease or active, non-infectious pneumonitis * Active infection requiring systemic therapy * Significant cardiovascular disease, myocardial infarction in the previous 3 months, unstable arrhythmias, or unstable angina. * Prior therapy with an anti-PD-1, anti-PD-L1, or anti-PD-L2 agent * HBsAg, HCV or HIV positivity. Positive serology is admitted for HBV and HCV but DNA/RNA test must be negative * Administration of a live attenuated vaccine within 4 weeks before cyle 1 day 1. Patients must not receive live, attenuate vaccines, including influenza vaccines at any time during study. * Treatment with systemic immunosuppressive medications, including prednisone, cyclophosphamide, azathioprine, methotrexate, thalidomide and anti tumor necrosis factor (anti-TNF) agents within 2 weeks prior to cycle 1 day 1; inhaled corticosteroids are allowed. * Evidence of suspect of CNS disease * Clinically significant hypersensitivity (e.g., anaphylactic or anaphylactoid reactions to the compound Pembrolizumab itself or to the excipients in its formulation). * Has had an allogenic tissue/solid organ transplant * Known history of active TB (Bacillus Tuberculosis)