PRIOR CONCURRENT THERAPY: Biologic therapy: More than 6 months since prior bone marrow transplantation More than 1 week since prior growth factor(s) Chemotherapy: At least 3 weeks since prior myelosuppressive chemotherapy (6 weeks for nitrosoureas) and recovered Endocrine therapy: Concurrent dexamethasone allowed if dose stable for at least 1 week prior to study Radiotherapy: More than 3 months since prior craniospinal irradiation greater than 24 Gy More than 3 months since prior total body irradiation More than 2 weeks since prior focal irradiation to symptomatic metastatic sites No prior stereotactic radiosurgery Concurrent total body irradiation allowed Surgery: See Radiotherapy Other: No other concurrent anticancer or experimental drug therapy Concurrent anticonvulsant drugs allowed