DISEASE CHARACTERISTICS: Histologically proven recurrent glioblastoma or anaplastic astrocytoma Stereotactically accessible, enhancing mass no greater than 50 mL total volume by MRI No significant mass effect Recent craniotomy allowed No anaplastic oligodendroglioma No tumors of the brainstem, cerebellum, or both hemispheres No diffuse subependymal or CSF disease If on stable or increasing dose of steroid, must have evidence of increasing contrast enhancement by MRI or CT scan Prior external beam radiotherapy required