Radiation Therapy in Treating Young Patients With Gliomas

DISEASE CHARACTERISTICS: * Histologically confirmed\* low-grade glioma, including any of the following: * Juvenile pilocytic astrocytoma (JPA) or pleomorphic JPA * Diffuse astrocytoma, including any of the following subtypes: * Fibrillary astrocytoma * Gemistocytic astrocytoma * Subependymal giant cell astrocytoma * Pleomorphic xanthoastrocytoma * Low-grade oligoastrocytoma * Low-grade oligodendroglioma * Low-grade glioma not otherwise specified NOTE: \*Patients with chiasmatic lesions who have contiguous extension of tumor into other regions of the visual pathway by CT scan or MRI are eligible without histopathologic confirmation; repeat biopsy required for patients with recurrent tumors that enhance but whose initial tumor did not * Measurable disease by radiography * Meets any of the following criteria: * Progressive nonresectable disease * Any location in the brain * Patients with a less than gross total resection are eligible provided they are symptomatic from their tumor OR the risk from neurologic impairment with progression is high enough to warrant immediate treatment * Has undergone biopsy only * Must have received ≥ 1 course of prior chemotherapy (for patients \< 10 years of age) * Prior chemotherapy optional (for patients ≥ 10 years of age) * No type-1 neurofibromatosis * No evidence of leptomeningeal dissemination PATIENT CHARACTERISTICS: Age * 3 to 20 Performance status * ECOG 0-2 OR * Karnofsky 50-100% (for patients \> 16 years of age) OR * Lansky 50-100% (for patients ≤ 16 years of age) Life expectancy * At least 1 year Hematopoietic * Absolute neutrophil count ≥ 1,000/mm\^3 * Platelet count ≥ 100,000/mm\^3 (transfusion independent) * Hemoglobin ≥ 10.0 g/dL (transfusions allowed) Hepatic * Not specified Renal * Not specified Other * Not pregnant * Negative pregnancy test * Fertile patients must use effective contraception * Not in status epilepticus, coma, or requiring assisted ventilation prior to study entry * Patients with a seizure disorder are eligible provided they are on anticonvulsants AND disease is well controlled PRIOR CONCURRENT THERAPY: Biologic therapy * Not specified Chemotherapy * See Disease Characteristics * Recovered from prior chemotherapy * No concurrent chemotherapy Endocrine therapy * Concurrent dexamethasone allowed provided patient has been on a stable dose for ≥ 2 weeks prior to study entry * Concurrent dexamethasone allowed for symptoms of increased intracranial pressure Radiotherapy * No prior radiotherapy Surgery * See Disease Characteristics Other * No other concurrent anticancer therapy