Busulfan in Treating Children and Adolescents With Refractory CNS Cancer

DISEASE CHARACTERISTICS: * Histologically confirmed CNS malignancy, including any of the following: * Primary malignant brain tumor refractory to standard therapy and metastatic to the cerebrospinal fluid (CSF) or leptomeningeal subarachnoid space * Recurrent or persistent leptomeningeal leukemia, lymphoma, or germ cell tumor refractory to conventional therapy * In second or greater relapse * CSF white blood count greater than 5 cells/mm3 with blasts on cytospin OR * Evidence of leptomeningeal tumor by MRI * No concurrent bone marrow disease * No obstruction or compartmentalization of CSF flow on CSF flow study PATIENT CHARACTERISTICS: Age: * 3 to 21 Performance status: * Lansky 50-100% (under 10 years) * Karnofsky 50-100% (10 to 21 years) Life expectancy: * Greater than 8 weeks Hematopoietic: * Absolute neutrophil count greater than 1,000/mm\^3 * Platelet count greater than 75,000/mm\^3 Hepatic: * Bilirubin normal for age * ALT and AST less than 5 times upper limit of normal (ULN) * No hepatic disease Renal: * Creatinine no greater than 1.5 times ULN OR * Glomerular filtration rate greater than 70 mL/min * No renal disease Cardiovascular: * No cardiac disease Pulmonary: * No pulmonary disease Other: * No uncontrolled infection * Not pregnant or nursing * Negative pregnancy test * Fertile patients must use effective contraception PRIOR CONCURRENT THERAPY: Biologic therapy: * Not specified Chemotherapy: * At least 3 weeks since prior myelosuppressive chemotherapy (6 weeks for nitrosoureas) * At least 1 week since prior intrathecal chemotherapy (2 weeks for cytarabine) and recovered * Evidence of subsequent disease progression * Concurrent systemic chemotherapy allowed for recurrent disease after first course of treatment except for the following: * Chemotherapy targeted at leptomeningeal disease * Other phase I agent * Any agent that significantly penetrates the CSF (e.g., high dose methotrexate greater than 1 g/m2, thiotepa, high dose cytarabine, fluorouracil, IV mercaptopurine, nitrosoureas, or topotecan) * Any agent that causes serious unpredictable CNS side effects Endocrine therapy: * Prior dexamethasone allowed with decreasing or stable dose at least one week before study * Concurrent dexamethasone or prednisone with chemotherapy regimen allowed Radiotherapy: * At least 1 week since prior focal irradiation to the brain or spine * At least 8 weeks since prior craniospinal irradiation * No concurrent cranial or craniospinal irradiation Surgery: * Not specified Other: * No other concurrent intrathecal or systemic therapy for leptomeningeal disease