Phenylacetate in Treating Children With Recurrent or Progressive Brain Tumors

DISEASE CHARACTERISTICS: Histologically confirmed recurrent or progressive brain tumors including the following: Anaplastic astrocytoma and glioblastoma multiforme Brain stem glioma Medulloblastoma or primitive neuroectodermal tumors present in supratentorial or posterior fossa locations Ependymoma Low grade gliomas Other Measurable or evaluable disease by CT or MRI OR Histologically confirmed previously untreated glial tumors including: Brain stem glioma Glioblastoma multiforme Measurable disease after surgery PATIENT CHARACTERISTICS: Age: 2 to 21 Performance status: Karnofsky 50-100% (over 10 years of age) Lansky 50-100% (under 10 years of age) Life expectancy: At least 8 weeks Hematopoietic: Absolute neutrophil count greater than 1,000/mm3 Platelet count greater than 50,000/mm3 Hemoglobin greater than 7.0 g/dL Transfusion support allowed after bone marrow transplantation or extensive radiation Hepatic: Bilirubin less than 1.5 mg/dL SGPT less than 2 times normal Renal: Creatinine no greater than 1.5 mg/dL OR Creatinine clearance greater than 60 mL/min Other: Not pregnant or nursing Fertile patients must use effective contraception during and for 6 months after study No significant systemic illness including infections No amino acidurias or organic acidemias PRIOR CONCURRENT THERAPY: Biologic therapy: Recovered from all prior immunotherapy No concurrent prophylactic hematopoietic growth factors Chemotherapy: At least 3 weeks since prior myelosuppressive chemotherapy (6 weeks for nitrosourea) and recovered No other concurrent cancer chemotherapy Endocrine therapy: Stable or decreasing dosage of dexamethasone for intracranial pressure within 2 weeks of study entry No concurrent dexamethasone used as an antiemetic Radiotherapy: At least 8 weeks since prior radiotherapy to evaluable lesions and recovered Surgery: At least 4 months since prior radiosurgery Other: No other concurrent investigational agents