Additional Study Information: The purpose of this study is to learn the effects of the investigational new drug, tirabrutinib, when given for the treatment of PCNSL. Investigational new drug means a drug that has not been approved as a marketed product (that is, available to be prescribed or sold) by the United States (US) Food and Drug Administration (FDA). Tirabrutinib acts by blocking an enzyme called Brutons tyrosine kinase or BTK. Some cancer cells use BTK to grow and thrive in your body. Tirabrutinib blocks BTK and may interfere with the growth of cancer cells. This study is divided into 2 parts; Part A of the study is for patients with relapsed or refractory (R/R) disease (disease that has not responded or progressed on their last prior therapy) and Part B is for patients with newly diagnosed disease who have not yet been treated. The purpose of Part A is to study the effects of the study drug when given alone for the treatment of PCNSL in study participants who have already received at least one prior chemotherapy regimen. The purpose of Part B is to study the effects of the study drug when given in combination with methotrexate based therapy for the treatment of newly diagnosed PCNSL in study participants who have not received any treatment. This study is being conducted to look at the safety (whether an investigational drug will cause any side effects) and efficacy (how effective an investigational drug is at producing a desired effect) of tirabrutinib in both Parts A and B.