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