O6-Benzylguanine and Temozolomide in Treating Young Patients With Recurrent or Progressive Gliomas or Brain Stem Tumors

INCLUSION CRITERIA * Tumor: Participants must have a high-grade glioma (including e.g. histologically confirmed anaplastic astrocytoma, glioblastoma multiforme, anaplastic oligodendroglioma, anaplastic ganglioma, gliosarcoma) or a brainstem tumor (histologic confirmation waived) with documentation of disease recurrence or progression after treatment with standard therapy. Participants must have bi-dimensionally measurable disease, defined as at least 1 lesion that can be measured in ≥ 2 dimensions * Age: 21 years of age or less * Performance status: Karnofsky 60-100% (for patients \> 16 years of age) or Lansky 60-100% (for patients ≤ 16 years of age) * Life expectancy: Not specified * Hematopoietic: Must have adequate bone marrow function defined as absolute neutrophil count \> 1,500/mm\^3, platelet count \> 100,000/mm\^3 (unsupported), hemoglobin \> 8 g/dL (may be supported), and absolute lymphocyte count ≥ 500/mm\^3 * Hepatic: Must have SGOT and SGPT ≤ 2.5 times upper limit of normal (ULN), bilirubin ≤ 1.5 times ULN, and no overt hepatic disease * Renal: Participants must have creatinine clearance ≥ 60 mL/min or creatinine based on age as follows: no greater than 0.8 mg/dL (for patients ≤ 5 years of age), no greater than 1.0 mg/dL (for patients 6 to 10 years of age), no greater than 1.2 mg/dL (for patients 11 to 15 years of age), or no greater than 1.5 mg/dL (for patients \> 15 years of age). There must be no overt renal disease * Cardiovascular: Must have no overt cardiac…