Methotrexate and Thiotepa in Treating Patients With Newly Diagnosed Primary CNS Lymphoma

DISEASE CHARACTERISTICS: Histologically confirmed primary CNS lymphoma Confirmed by 1 of the following: Brain biopsy or resection CSF cytology Positive cytology or immunohistochemical diagnosis of monoclonality and measurable intracranial tumor Vitreal biopsy Measurable and contrast-enhancing disease on the postoperative, pretreatment MRI or CT scan No radiographic evidence of ascites or pleural effusions PATIENT CHARACTERISTICS: Age 18 and over Performance status Karnofsky 60-100% Life expectancy Not specified Hematopoietic Absolute neutrophil count at least 1,500/mm^3 Platelet count at least 100,000/mm^3 Hepatic Bilirubin no greater than 2.0 mg/dL SGOT no greater than 4 times upper limit of normal Renal Creatinine no greater than 2 mg/dL Creatinine clearance at least 50 mL/min Other Mini mental score of at least 15 HIV negative Able to achieve hydration No other malignancy within the past 5 years except curatively treated basal cell or squamous cell skin cancer or carcinoma in situ No allergy to methotrexate No serious infection No medical illness that would preclude study participation Not pregnant or nursing Negative pregnancy test Fertile patients must use effective contraception PRIOR CONCURRENT THERAPY: Biologic therapy No prior immunotherapy or biologic therapy for this disease Chemotherapy No prior chemotherapy for this disease No other concurrent chemotherapeutic agents Endocrine therapy No prior hormonal therapy for this disease Prior glucocorticoid therapy allowed Radiotherapy No prior radiotherapy for this disease No prior cranial irradiation Surgery See Disease Characteristics Other At least 1 week since prior salicylates, non-steroidal anti-inflammatory drugs, probenecid, folic acid, or sulfonamide medications No other concurrent investigational agents