Rituximab, Temozolomide, and Methylprednisolone in Treating Patients With Recurrent Primary CNS Non-Hodgkin's Lymphoma

DISEASE CHARACTERISTICS: * Histologically confirmed primary CNS non-Hodgkin's lymphoma by brain biopsy, positive cerebrospinal fluid cytology, or vitrectomy * Recurrent disease * Measurable disease, define as bi-dimensionally measurable lesions with clearly defined margins by brain MRI or CT scan * Radiographical evidence of tumor progression by MRI or CT scan * Steroid therapy must be stable for 5 days prior to scan PATIENT CHARACTERISTICS: Age * 18 and over Performance status * Karnofsky 60-100% Life expectancy * More than 8 weeks Hematopoietic * WBC ≥ 3,000/mm\^3 * Absolute neutrophil count ≥ 1,500/mm\^3 * Platelet count ≥ 100,000/mm\^3 * Hemoglobin ≥ 10 g/dL (transfusion allowed) Hepatic * SGOT \< 2 times upper limit of normal (ULN) * Bilirubin \< 2 times ULN * No active or latent hepatitis B infection Renal * Creatinine \< 1.5 mg/dL OR * Creatinine clearance ≥ 60 mL/min Other * Not pregnant or nursing * Negative pregnancy test * Fertile patients must use effective contraception * No other malignancy within the past 3 years except nonmelanoma skin cancer or carcinoma in situ of the cervix * No uncontrolled significant medical illness that would preclude study treatment * No active infection * No active HIV infection * No concurrent disease that would dangerously alter drug metabolism or obscure toxicity PRIOR CONCURRENT THERAPY: Biologic therapy * At least 7 days since prior interferon or thalidomide * No concurrent prophylactic filgrastim (G-CSF) * No concurrent immunotherapy Chemotherapy * No prior temozolomide * At least 14 days since prior methotrexate * At least 21 days since prior procarbazine * At least 42 days since prior nitrosoureas * No other concurrent chemotherapy Endocrine therapy * See Disease Characteristics * At least 7 days since prior tamoxifen * No concurrent hormonal therapy Radiotherapy * No concurrent radiotherapy Surgery * Not specified Other * Recovered from all prior therapy * At least 28 days since prior investigational agents * At least 28 days since other prior cytotoxic therapy * At least 7 days since other prior non-cytotoxic agents (e.g., tretinoin) (radiosenitizers allowed) * No other concurrent investigational drugs