Treatment Block 1: Patients receive cytarabine IT on day 1 and then methotrexate, cytarabine and hydrocortisone IT (triple intrathecal therapy; TIT) on days 8, 15, 22, and 29. Vincristine, prednisone, pegaspargase, doxorubicin, and imatinib mesylate are administered as in arm I.