DISEASE CHARACTERISTICS:

• Histologically or cytologically confirmed CNS or solid tumors recurrent or refractory to standard therapy

  • Solid tumors:

    • Neuroblastoma
    • Ewing's Sarcoma/peripheral primitive neuroectodermal tumor (PNET)
    • Osteosarcoma
    • Rhabdomyosarcoma
    • Other extracranial solid tumors

  • CNS tumors:

    • Medulloblastoma/PNET
    • Ependymoma
    • Brain stem glioma
    • Other CNS tumor
    • Intrinsic brain stem tumor (biopsy required only if previously treated with radiosurgery)
    • Classic optic glioma (histologic requirement waived)

• Measurable disease by imaging studies

  • No lesions assessable only by radionuclide scan
• Previously irradiated lesions used to evaluate tumor response must show evidence of an interim increase in size

PATIENT CHARACTERISTICS:

Age:

• 1 to 21

Performance status:

• Karnofsky 50-100% if more than 10 years old OR
• Lansky 50-100% if 10 years or younger

Life expectancy:

• At least 8 weeks

Hematopoietic:

• Absolute neutrophil count greater than 1,000/mm^3
• Platelet count greater than 100,000/mm^3
• Hemoglobin greater than 8 mg/dL
• Inadequate peripheral blood counts due to bone marrow infiltration allowed

Hepatic:

• Bilirubin no greater than 1.5 mg/dL
• SGPT less than 5 times normal

Renal:

• Creatinine normal
• Glomerular filtration rate at least 70 mL/min

Other:

• No severe uncontrolled infection
• Not pregnant or nursing
• Negative pregnancy test
• Fertile patients must use effective contraception during and for 6 months after study

PRIOR CONCURRENT THERAPY:

Biologic therapy:

• At least 3 weeks since prior immunotherapy and recovered
• No concurrent biologic therapy

Chemotherapy:

• At least 3 weeks since prior chemotherapy (8 weeks since prior nitrosoureas) and recovered
• No more than 2 prior chemotherapy regimens
• No other concurrent chemotherapy
• Prior topotecan allowed
• No prior irinotecan

Endocrine therapy:

• Concurrent dexamethasone for brain tumor patients allowed if on a stable or decreasing dose for at least 2 weeks prior to study
• At least 3 weeks since prior endocrine therapy
• No other concurrent endocrine therapy

Radiotherapy:

• See Disease Characteristics
• At least 8 weeks since prior extended radiotherapy (including evaluable lesions) and recovered
• No prior total body radiotherapy
• No concurrent radiotherapy

Surgery:

• See Disease Characteristics

Other:

• At least 3 weeks since prior investigational agents
• No other concurrent investigational agents
• No concurrent anticonvulsants
• No concurrent medications that would interfere with the P-450 enzyme system function (e.g., erythromycin, cimetidine, fluconazole)