RecruitingDrug trialPembrolizumab, Ibrutinib and Rituximab in PCNSLThis research study is evaluating a combination therapy of 3 drugs as possible treatments for recurrent primary central nervous system lymphoma (PCNSL). The three drugs being used in the study are: * Pembrolizumab (MK3475) * Ibrutinib * Rituximab (or biosimilar)