Thiotepa and Radiation Therapy in Treating Young Patients With Newly Diagnosed Malignant Brain Tumors

DISEASE CHARACTERISTICS: * Histologically confirmed diagnosis of 1 of the following high-grade (malignant) astrocytic tumors: * Anaplastic astrocytoma * Glioblastoma * Giant cell glioblastoma * Gliosarcoma * Any anatomical site except brain stem * Newly diagnosed disease * Has undergone tumor biopsy or surgical resection within the past 2 weeks * Patients with post-operative residual disease (grade III or IV) are eligible * Post-operative imaging of tumor within 72 hours of surgery * Patients with no imageable post-operative disease are not eligible * No neurological deterioration within 3 days of study treatment * Increasing requirement for steroids to control symptoms of intracranial pressure is considered evidence of neurological deterioration PATIENT CHARACTERISTICS: * Lansky play score 40-100% * Absolute neutrophil count \> 1,000/mm\^3 * Platelet count \> 100,000/mm\^3 * Creatinine ≥ 1.5 times upper limit of normal PRIOR CONCURRENT THERAPY: * See Disease Characteristics * No prior chemotherapy or radiotherapy