DISEASE CHARACTERISTICS: Histologically confirmed supratentorial * grade I/II glioma, including: Diffuse fibrillary astrocytoma No pilocytic astrocytoma No mixed tumor with ependymoma elements * Supratentorial sites include: Frontal, temporal, parietal, or occipital lobes Thalamus, basal ganglia, or midbrain Lateral or third ventricles Pons, medulla, or optic chiasm tumors allowed only if secondary to eligible tumor More than 1 separate tumor allowed Diagnosis based on surgical biopsy or subtotal resection Measurable or evaluable disease on T2-weighted MRI required