Inclusion Criteria:

          1. Written informed consent by the patient prior to screening

          2. Patients aged ≥ 18 years on the day of consenting to the study

          3. Pathologic diagnosis of PCNSL

          4. Relapse or refractory PCNSL with at least one prior HD MTX based therapy for PCNSL

          5. Measurable brain lesion with a minimum diameter > 1.0 cm in gadolinium enhanced
             magnetic resonance imaging (MRI) performed within 14 days before starting tirabrutinib
             treatment

          6. ECOG PS of 0, 1 or 2

          7. Life expectancy of at least 3 months

          8. Adequate bone marrow, renal, and hepatic function

        Exclusion Criteria:

          1. Intraocular PCNSL with no brain lesion

          2. Patient who is intolerant of contrast enhanced MRI due to allergic reactions to
             contrast agents

          3. Patient with non-B cell PCNSL

          4. Patient with systemic presence of lymphoma

          5. Prior chemotherapy within 21 days, nitrosourea within 42 days, an antibody drug with
             anticancer activity (e.g., rituximab) within 28 days, prior radiotherapy within 14
             days, prior major invasive surgery within 28 days, or allogeneic stem cell transplant
             within 6 months before starting tirabrutinib treatment

          6. Prior BTK inhibitor treatment

          7. Prior investigational drugs (including treatment in clinical research, unapproved
             combination products, and new dosage forms) within 28 days or 5 half-lives, whichever
             is shorter, before starting tirabrutinib treatment

          8. Concomitant systemic corticosteroid on an ongoing basis within 14 days before starting
             tirabrutinib treatment, with the exception of the following:

               -  Equivalent of up to 10 mg/day of prednisone for a disease other than PCNSL

               -  Equivalent of up to 50 mg/day of prednisone (equal to 8 mg/day of dexamethasone)
                  for patients with lesions of the brain or spinal cord or both

          9. Patient who has received a CYP3A4 inducer or P gp inducer within 14 days before
             starting tirabrutinib treatment

         10. Concomitant warfarin, any other warfarin derivative anticoagulant, vitamin K
             antagonists, novel oral anticoagulants, or antiplatelet therapy on an ongoing basis
             within 7 days before starting tirabrutinib treatment

         11. Active malignancy, other than PCNSL requiring systemic therapy

         12. Poorly controlled comorbidity, severe heart, severe lung disease, clinically
             significant liver diseases that could affect protocol compliance or safety or efficacy
             assessments

         13. Patient with bleeding diathesis

         14. Patients with a history of moderate or severe hepatic impairment

         15. QTcF > 480 milliseconds or requirement for ongoing treatment with concomitant
             medications that prolong the QT interval

         16. Active infection, including a HIV, cytomegalovirus infection or SARS-CoV-2, or has
             had, within 28 days before starting tirabrutinib treatment, an infection (other than
             nail trichophytosis) that requires hospitalization or an intravenous antibiotic

         17. Prior history of hypersensitivity or anaphylaxis to tirabrutinib

         18. Medical history of organ allografts

         19. Tests positive for HIV-1 antibody and HIV-2 antibody, human T-lymphotropic virus 1
             antibody, HBs antigen, or HCV antibody. Tests positive for HBs antibody or hepatitis B
             virus core protein antibody and has a result of at least detectable in a hepatitis B
             virus deoxyribonucleic acid assay despite testing negative for HBs antigen.

         20. Patient is unable to swallow tablets; has malabsorption, malabsorption syndrome, or a
             comorbidity that affects gastric function; has undergone complete resection of the
             stomach or small intestine; has ulcerative colitis or symptomatic inflammatory bowel
             disease; or has partial or complete intestinal obstruction.

         21. Women who are pregnant or lactating

         22. Patient is found incapable of giving consent due to dementia or another such condition

         23. Patient is found to be otherwise ineligible for the study by the Investigator or
             sub-Investigator.