Pharmacokinetic (PK) studies will be performed with a total of 2 doses of intrathecal mafosfamide. PK studies tell us how the patient's body handles the study drug, mafosfamide. Sampling times (times when we collect cerebrospinal fluid for the PK studies) will be prior to drug administration and at 10 minutes, 2 hours, and 4 hours following drug administration. These studies will be performed after one dose of drug given through the spinal tap and after one dose of drug is given through the Ommaya reservoir. The pharmacokinetic study is optional and you can choose not to allow these samples to be drawn. Refusing the pharmacokinetic study will not affect the patient's participation/treatment on this study.