Combination Chemotherapy Followed by Bone Marrow and/or Peripheral Stem Cell Transplantation in Treating Patients With Recurrent Medulloblastoma or CNS Germ Cell Tumors

DISEASE CHARACTERISTICS: * Diagnosis of recurrent medulloblastoma or CNS germ cell tumor * Histologic review of the primary intracranial or spinal cord tumor required * Biopsy and reduction of tumor bulk prior to study encouraged but not required * No more than 1 prior primary therapy (radiotherapy or chemoradiotherapy) and/or 1 prior salvage therapy * Patients with progression on salvage therapy ineligible * Minimal residual disease (tumor bulk no more than 1.5 cm) or in second clinical complete remission (CR) * Bone marrow infiltration with or without mass lesions or isolated abnormal CSF cytology as the only manifestation of recurrent disease allowed if a clinical CR is first achieved with conventional therapy PATIENT CHARACTERISTICS: Age: * 2 to 25 Performance status: * Karnofsky 80-100% Life expectancy: * More than 8 weeks Hematopoietic: * Absolute neutrophil count greater than 1,000/mm\^3 * Platelet count greater than 100,000/mm\^3 Hepatic: * Bilirubin less than 1.5 mg/dL * SGPT less than 80 IU Renal: * Creatinine less than 1.2 mg/dL Cardiovascular: * LVEF normal Other: * No infection * Able to tolerate vigorous hydration * Not pregnant * Fertile patients must use effective contraception PRIOR CONCURRENT THERAPY: Biologic therapy: * Not specified Chemotherapy: * See Disease Characteristics * Prior cyclophosphamide or ifosfamide allowed Endocrine therapy: * No concurrent dexamethasone as an antiemetic * Other concurrent corticosteroids allowed Radiotherapy: * See Disease Characteristics * Pretransplantation radiotherapy boost allowed Surgery: * See Disease Characteristics * Pretransplantation surgery allowed Other: * At least 4 weeks since prior therapy except corticosteroids