Combination Chemotherapy in Treating Children With Progressive Brain Tumors

DISEASE CHARACTERISTICS: * Pathologically confirmed low grade residual astrocytomas or other eligible residual tumors of the brain interpreted as low grade (WHO grades I and II) such as the following: * Glial Tumors * Astrocytic tumors * Low grade astrocytoma (variants: fibrillary, protoplasmic, gemistocytic) * Pilocytic astrocytoma * Pleomorphic xanthoastrocytomas * Subependymal giant cell astrocytoma * Infantile desmoplastic astrocytoma * Low grade oligodendroglial tumors * Low grade oligodendroglioma * Low grade mixed gliomas * Oligo-astrocytoma * Neuronal Tumors * Ganglioglioma (excluding tumors with anaplastic astrocytic components) * Infantile desmoplastic ganglioglioma * Chiasmatic-hypothalamic tumor without histologic confirmation * All of the following diagnostic tests (radiological or clinical evidence of progression, surgery, or confirmatory MRI) must be carried out within 6 weeks of enrollment into this study * Progressive disease following surgical excision based on clear radiological or clinical evidence of progression, or an incomplete excision (less than 95% or greater than 1.5 cm2) with necessity to begin treatment because of a risk of neurologic impairment with progression * Chiasmatic lesions that have contiguous extensions of tumor into other regions of the visual pathways demonstrated on contrast MRI will be eligible for study without histopathological confirmation * Patients with neurofibromatosis who have radiographic diagnosis of chiasmatic-hypothalamic tumor are eligible for the study, without requiring a biopsy confirmation of tumor histology, but not unless tumor progression is documented radiographically * No intrinsic brain stem tumors of the pons or isolated optic nerve tumors without definitive involvement of the optic chiasm PATIENT CHARACTERISTICS: Age: * Under 10 Performance status: * Not specified Life expectancy: * Not specified Hematopoietic: * Absolute neutrophil count greater than 1,000/mm\^3 (arm II) * Platelet count greater than 100,000/mm\^3 (arm II) Hepatic: * Not specified Renal: * Creatinine less than 1.5 times upper limit of normal for age OR * Creatinine clearance or radioisotope GFR greater than 70 mL/min or equivalent GFR as determined by the institutional normal range PRIOR CONCURRENT THERAPY: Biologic therapy: * Not specified Chemotherapy: * No prior chemotherapy for the tumor Endocrine therapy: * Prior corticosteroid therapy allowed Radiotherapy: * No prior radiotherapy for the tumor Surgery: * See Disease characteristics Other: * Prior diuretic therapy allowed