PRIOR CONCURRENT THERAPY: Biologic therapy: Not specified Chemotherapy: No prior chemotherapy for glioblastoma multiforme No more than 1 prior chemotherapy regimen for anaplastic astrocytoma, malignant oligodendroglioma, or malignant mixed glioma At least 4 weeks since chemotherapy (6 weeks since nitrosoureas or mitomycin) Endocrine therapy: Not specified Radiotherapy: Completion of 1 course of conventional external radiotherapy required At least 4 weeks since radiotherapy Surgery: Decompressive surgery for clinically suspected increased intracranial pressure performed prior to entry