High-Dose Methotrexate in Treating Young Patients With Solid Tumors

DISEASE CHARACTERISTICS: * Histologically proven malignancy, including but not limited to, any of the following: * Patients with MRI findings in keeping with a diffuse intrinsic pontine glioma will be eligible without histological confirmation of tumor type * Patients with a diagnosis of diffuse intrinsic pontine glioma who are not eligible for the erlotinib hydrochloride phase I study (CCLG-NAG-2005-09) * Patients with relapsed ependymoma following the CCLG phase II study of intravenous etoposide (CCLG-CNS-2001-4) or prior to this are eligible at the discretion of the physician * Patients with relapsed osteogenic sarcoma, other soft tissue sarcomas, or other solid tumors may be suitable for this study at the discretion of the physician * Radiologically evaluable disease without bone marrow involvement PATIENT CHARACTERISTICS: Inclusion criteria: * Lansky performance status (PS) 30-100% (for patients ≤ 12 years of age) * ECOG PS ≤ 2 (for patients ≥ 13 years of age) * Life expectancy ≥ 9 weeks * ANC \> 1,000/mm³ * Platelet count \> 100,000/mm³ * Hemoglobin \> 9 g/dL * Serum creatinine ≤ 1.5 times upper limit of normal (ULN) for age * Serum total bilirubin normal * AST or ALT ≤ 2 times ULN * Glomerular filtration rate ≥ 60 mL/min * Negative pregnancy test * Fertile patients must use effective contraception Exclusion criteria: * Poor medical risk because of nonmalignant systemic disease or uncontrolled infection * Concurrent malignancies at other sites PRIOR CONCURRENT THERAPY: Inclusion criteria: * Prophylactic trimethoprim-sulfamethoxazole must be stopped 1 week prior to methotrexate administration Exclusion criteria: * Received chemotherapy or biologic therapy within the past 4 weeks * Received radiotherapy within the past 6 weeks