Combination Chemotherapy, Monoclonal Antibody, and Radiation Therapy in Treating Patients With Primary Central Nervous System Lymphoma

RATIONALE: Drugs used in chemotherapy such as methotrexate and temozolomide use different ways to stop cancer cells from dividing so they stop growing or die. Monoclonal antibodies such as rituximab can locate cancer cells and either kill them or deliver cancer-killing substances to them without harming normal cells. Radiation therapy uses high-energy x-rays to damage cancer cells. Combining methotrexate, temozolomide, and rituximab with radiation therapy may kill more cancer cells. PURPOSE: This phase I/II trial is studying the side effects and best dose of temozolomide when given together with methotrexate and rituximab followed by radiation therapy and to see how well they work in treating patients with primary central nervous system lymphoma.