DISEASE CHARACTERISTICS: Histologically confirmed malignant glioma determined from prior stereotactic biopsy or cytoreductive surgery for removal of a supratentorial brain tumor Unilateral supratentorial tumor, measuring at least 1 cm, as determined by CT scan or MRI No more than 1 focus of tumor and no tumor crossing the midline Surgical treatment indicated at baseline evaluation Received prior definitive (greater than 5,000 cGy) external beam radiotherapy more than 3 months ago Evidence of progression At time of tumor resection and Gliadel wafers implantation: Intraoperative pathological diagnosis on frozen section or squash preparation of malignant glioma OR Glioblastoma multiforme or anaplastic astrocytoma on permanent sections from a prior surgery and an intraoperative pathological diagnosis on frozen section or squash preparation of tumor, glioma, or malignant glioma (not necrosis)

PATIENT CHARACTERISTICS: Age: 18 and over Performance status: Karnofsky 60-100% Life expectancy: At least 60 days Hematopoietic: Absolute neutrophil count at least 1,500/mm3 Platelet count at least 100,000/mm3 Hepatic: Bilirubin no greater than 1.5 mg/dL Transaminases no greater than 4 times upper limit of normal Renal: Creatinine no greater than 1.7 mg/dL Other: Not pregnant or nursing Negative pregnancy test Fertile patients must use effective contraception for 1 year after surgery No other concurrent significant life threatening disease No known hypersensitivity to nitrosoureas No other malignancy in past five years except curatively treated carcinoma in situ of the cervix or basal cell carcinoma of the skin

PRIOR CONCURRENT THERAPY: Biologic therapy: Not specified Chemotherapy: At least 4 weeks since prior chemotherapy (6 weeks for nitrosoureas) and recovered No other concurrent chemotherapy during first 56 days of study Endocrine therapy: No concurrent dexamethasone as an antiemetic Radiotherapy: See Disease Characteristics Surgery: See Disease Characteristics