Temozolomide Followed by Radiation Therapy in Treating Children With Newly Diagnosed Malignant CNS Tumors

DISEASE CHARACTERISTICS: * Histologically confirmed newly diagnosed malignant central nervous system tumor not requiring immediate radiotherapy * Patients with diffuse pontine tumors do not require histological confirmation * Eligible types include the following: * Ependymoma * Malignant glioma * Anaplastic astrocytoma * Glioblastoma multiforme * Anaplastic oligodendroglioma * Gliosarcoma * Anaplastic mixed oligoastrocytoma * Brainstem glioma * Primitive neuroectodermal tumor * Nongerminoma germ cell tumor * At least one bidimensionally measurable lesion * At least 1.5 cm2 within 72 hours of surgical resection or greater than 14 days after surgery * Diffuse pontine tumors are not required to be measurable * Neurologically stable PATIENT CHARACTERISTICS: Age: * 4 to 21 Performance status: * Karnofsky or Lansky 70-100% Life expectancy: * Greater than 12 weeks Hematopoietic: * Absolute neutrophil count at least 1,500/mm\^3 * Platelet count at least 100,000/mm\^3 * Hemoglobin at least 10 g/dL Hepatic: * Bilirubin less than 1.5 times upper limit of normal (ULN) * Alkaline phosphatase less than 2 times ULN * SGOT and SGPT less than 2.5 times ULN Renal: * BUN and creatinine less than 1.5 times ULN Other: * Must be able to swallow capsules * No acute infection treated with intravenous antibiotics * No nonmalignant systemic disease that makes patient a poor medical risk * No frequent vomiting or medical condition that may interfere with oral medication intake (e.g., partial bowel obstruction) * No other prior or concurrent malignancies except surgically cured carcinoma in situ of the cervix or basal or squamous cell carcinoma of the skin * HIV negative * Not pregnant or nursing * Negative pregnancy test * Fertile patients must use effective contraception PRIOR CONCURRENT THERAPY: Biologic therapy: * No more than one prior biologic therapy regimen * No concurrent biologic therapy * No concurrent growth factors or epoetin alfa Chemotherapy: * No more than one prior chemotherapy regimen * No other concurrent chemotherapy Endocrine therapy: * No increasing doses of steroids within one week of study Radiotherapy: * See Disease Characteristics * No concurrent radiotherapy Surgery: * At least 2 weeks, but no greater than 4 weeks, since prior surgical resection and recovered Other: * No other concurrent investigational drugs