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.