Patients receive CD-expressing neural stem cells intracranially (IC) on days 1 and 15 and flucytosine orally (PO) every 6 hours on days 4-10 and 18-24. Depending on when a subject enters the study, s/he may also be given leucovorin orally every 6 hours on days 4-10 and 18-24. Courses repeat every 28 days in the absence of disease progression or unacceptable toxicity.