DISEASE CHARACTERISTICS: Histologically confirmed primary intracranial low-grade glioma (i.e., astrocytoma or oligodendroglioma) No more than 2 years since tissue diagnosis Biopsy not required for intrinsic chiasmatic mass or tumor infiltration along the posterior optic tracts Evidence of progressive disease by at least one of the following: Papilledema or other clinical sign of increased intracranial pressure Documented change in neuroimaging studies, e.g.: Hydrocephalus 25% increase in product of maximum perpendicular diameters of tumor The following are required in patients with optic pathway gliomas: Progressive loss of vision documented by an ophthalmologist, i.e.: Doubling of octaves (e.g., 20/20 to 20/40 or 20/40 to 20/80) on 2 successive visits Loss of visual acuity not explainable by other causes, e.g., media abnormalities or amblyopia Greater than 3 mm increase in proptosis At least 2 mm increase in diameter of optic nerve on neuroimaging Increase in distribution of tumor involving the optic tracts or optic radiations demonstrated by CT or MRI using T1 (with or without contrast) or T2 imaging