The purpose of this study is to determine efficacy of intrathecal mafosfamide, a preactivated derivative of cyclophosphamide against meningeal malignancies refractory to conventional therapy (radiation therapy and chemotherapy). The maximally tolerated dose for intrathecal mafosfamide will be established in a limited dosage escalation schedule. The CSF pharmacokinetics of intrathecal mafosfamide will also be studied.