DISEASE CHARACTERISTICS: Histologically proven recurrent anaplastic oligodendroglioma or anaplastic mixed oligoastrocytoma following primary surgery and radiation therapy Tumors for anaplastic mixed oligoastrocytoma must contain at least 25% oligodendroglial elements Tumors must be clinically aggressive for patients with only 1 anaplastic feature Prior low grade oligodendrogliomas or oligoastrocytomas undergoing repeat biopsy following clinical or radiological progression are eligible Bidimensionally measurable and progressive lesions by CT or MRI