DISEASE CHARACTERISTICS: Histologically confirmed supratentorial glioblastoma multiforme or gliosarcoma Must have evidence of tumor recurrence or progression by MRI scan after failing prior radiotherapy Bidimensionally measurable enhancing residual disease on MRI or CT scan Prior recent resection of recurrent or progressive tumor allowed if all of the following conditions apply: Recovered from surgery Residual evaluable disease present that is not artifactual postsurgical enhancement Baseline MRI or CT scan is performed within 14 days prior to study and while on a steroid dose that has been stable for at least 5-7 days