PRIOR CONCURRENT THERAPY: Biologic therapy: At least 6 months since prior bone marrow transplantation No concurrent immunomodulating agents Chemotherapy: At least 2 weeks since prior chemotherapy (4 weeks for nitrosourea) and recovered No other concurrent chemotherapy Endocrine therapy: No concurrent corticosteroids except for increased intracranial pressure due to CNS tumors Radiotherapy: At least 6 months since prior radiotherapy to craniospinal axis, spine, and/or more than 50% of the bony pelvis Concurrent radiotherapy to localized painful lesions allowed if at least one measurable lesion is not in radiation field Surgery: Not specified Other: No concurrent participation in another CCG, POG, or COG therapeutic study