Maintenance Obinutuzumab in Treating Patients With Central Nervous System Lymphoma Who Have Achieved a Complete or Partial Response

This trial is testing whether using a drug called obinutuzumab as maintenance therapy — given regularly after completing first-line treatment — can prevent the return of primary central nervous system lymphoma (PCNSL), a rare blood cancer that occurs inside the brain or spinal cord. **You may be eligible if...** - You have been diagnosed with CD20-positive B-cell primary CNS lymphoma (a type of lymphoma in the brain/spinal cord) - You have completed first-line chemotherapy (high-dose methotrexate-based) and achieved a partial or complete response - You are within 75 days of finishing your first-line treatment - You have had a brain MRI confirming your response to treatment **You may NOT be eligible if...** - You did not respond to first-line treatment (your lymphoma didn't shrink) - You have significant organ dysfunction affecting your heart, kidneys, or lungs - You are pregnant or breastfeeding - You have active, uncontrolled infections Talk to your doctor to see if this trial is right for you.