IV. Measure concentrations of 5-methyltetrahydrofolate, homocysteine, Ado, S-adenosylmethionine, S-adenosylhomocysteine, and other potentially relevant compounds in serum and cerebrospinal fluid during interim maintenance therapy with low- or high-dose methotrexate regimens, respectively, and correlate these endpoints with the occurrence of acute neurologic toxicity and long-term neurocognitive dysfunction in these patients.