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

OBJECTIVES: * To assess the maximum tolerated dose (MTD) of temozolomide (TMZ) in combination with methotrexate (MTX) and rituximab (RTX) when administered prior to twice daily fractionated whole brain radiation therapy (WBRT) in patients with primary central nervous system lymphoma. * To compare the two-year survival rate in patients receiving pre-irradiation chemotherapy, twice daily fractionated whole brain radiation therapy and post-irradiation temozolomide to the reported two-year survival rate of Radiation Therapy Oncology Group (RTOG) trial 93-10. RTOG 9310 does not fall within ClinicalTrials.gov registration/reporting requirements.) * To compare the pre-irradiation chemotherapy tumor response rates to the reported rate from RTOG 93-10. * To report progression-free survival. * To assess acute and long-term neurologic toxicity, and to collect quality of life data for this patient group. Patients are followed every 3 months for 2 years, every 6 months for 3 years, and then annually thereafter.