<qa>  What is the maximum VPA concentration in patients who require anti-convulsant prior to4. Patients must not have received any prior chemotherapy, radiation therapy, biologic therapy, or bone marrow transplant. Surgery and dexamethasone are permitted prior to study entry. In patients who require anti-convulsant prior to study entry, it is permissible to start VPA, but trough VPA concentration must be repeated within 48 hours of study entry. 