This study is for patients with a type of brain cancer called Primary Central Nervous System Lymphoma (PCNSL). It will test a drug called tirabrutinib for safety and effectiveness. The study has two parts: Part A is for patients whose cancer came back or didn't respond to previous treatment, and Part B is for newly diagnosed patients. Patients in Part A will take tirabrutinib alone, while those in Part B will take it with other medications like methotrexate and rituximab.

• The study involves multiple visits and tests, as it closely monitors patients over time.
• Participants must be at least 18 years old and have a life expectancy of a few months.
• Those who join must not have certain health conditions or be taking specific medications that could interfere with the study.

If you meet these conditions and are interested, you will need to give written consent and work closely with the study doctors. Participation could help discover new treatments for PCNSL, though there are risks and side effects to consider. Always discuss with your doctor to make the best decision for your health.