Combination Chemotherapy Following Radiation Therapy in Treating Patients With Malignant Glioma

DISEASE CHARACTERISTICS: Histologically confirmed malignant glioma Glioblastoma Gliosarcoma High-grade glioma Anaplastic astrocytoma Anaplastic mixed oligoastrocytoma Anaplastic oligodendroglioma Anaplastic ependymoma Must have completed radiotherapy PATIENT CHARACTERISTICS: Age: 18 and over Performance status: Karnofsky 60-100% Life expectancy: More than 3 months Hematopoietic: Absolute neutrophil count at least 1,500/mm^3 Platelet count at least 100,000/mm^3 Hemoglobin at least 10 g/dL Hepatic: Bilirubin less than 1.5 times upper limit of normal (ULN) SGOT/SGPT less than 3 times ULN Alkaline phosphatase less than 2 times ULN Renal: BUN less than 1.5 times ULN Creatinine less than 1.5 times ULN Pulmonary: Pulmonary function test with diffusion greater than 50 OR Clearance by the pulmonary service Other: Not pregnant or nursing Negative pregnancy test Fertile patients must use effective contraception HIV negative No AIDS-related illness No nonmalignant systemic disease that would preclude study No acute infection requiring IV antibiotics No psychiatric condition that would preclude study compliance No frequent vomiting or medical condition that would interfere with oral medication intake (e.g., partial bowel obstruction) No other prior or concurrent malignancy except surgically cured carcinoma in situ of the cervix or basal cell or squamous cell skin cancer PRIOR CONCURRENT THERAPY: Biologic therapy: No prior immunotherapy No prior biologic therapy Chemotherapy: No prior chemotherapy Endocrine therapy: Not specified Radiotherapy: See Disease Characteristics At least 2 weeks since prior radiotherapy Surgery: Not specified