DISEASE CHARACTERISTICS: Newly diagnosed oligodendroglioma or oligoastrocytoma (with at least 25% oligodendral elements) Low-grade oligodendroastrocytoma or oligodendroglioma that is recurrent after surgery without radiotherapy is allowed Prior partial or gross total resection of tumor (or biopsy only in case of no further surgical option) required At least 3 of the following histologic anaplastic features: High cellularity Endothelial abnormalities Nuclear abnormalities Necrosis Mitoses