DISEASE CHARACTERISTICS: Histologically proven oligodendroglioma or oligoastrocytoma (with at least 25% oligodendroglial elements) Recurrent or progressive disease following both radiotherapy and procarbazine/lomustine/vincristine chemotherapy (or other nitrosoureas-based chemotherapy) Contrast enhancing, measurable disease (at least one lesion measuring at least 1 cm) by CT or MRI required Within 2 weeks prior to study treatment Within 3 days following concurrent surgery for the recurrence Steroid doses stable or decreasing for at least 2 weeks prior to scan No extracranial disease