RecruitingDrug trialTrial of Anti-PD-1 Immunotherapy and Stereotactic Radiation in Patients With Recurrent GlioblastomaThe purpose of this study is to assess the safety/tolerability/feasibility of pembrolizumab and radiation therapy before surgical resection in patients with recurrent glioblastoma as defined by treatment-related AEs and the number of patients who do not necessitate a delay in surgical resection, and to assess overall survival. The secondary objectives are to assess progression free survival, and to assess the T cell clonality, CD8 T cell activation and Tumor Infiltrating Lymphocyte (TIL) score after treatment