Trial of Anti-PD-1 Immunotherapy and Stereotactic Radiation in Patients With Recurrent Glioblastoma

Inclusion Criteria: * Age 18 years or older * Confirmed histologic diagnosis of WHO Grade IV, Glioblastoma Multiforme * GBM recurrence or progression with planned standard of care surgical resection and repeat radiation * Tumor size less than 6 cm * ECOG performance status of 0-1 * Adequate laboratory values Exclusion Criteria: * Contraindication to additional radiation * Prior therapy with an anti-PD-1, anti-PD-L1, or anti-PD-L2 agent or with an agent directed to another stimulatory or co-inhibitory T-cell receptor * Immunodeficiency diagnosis or receiving chronic systemic steroid therapy (exceeding 10 mg daily of prednisone) or any other form of immunosuppressive therapy * Severe hypersensitivity to pembrolizumab Complete inclusion/exclusion criteria are detailed in the protocol.