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