During Course 2, patients will be given methotrexate by infusion over 24 hours on the first day and cytarabine at a high dose over 2 hours every 12 hours for 4 doses (Days 2 and 3). Citrovorum factor (leucovorin), an antidote for side effects of methotrexate, will be given by vein or by mouth for 2-3 days (Day 2 and on). Solumedrol (a steroid) will be given by vein every 12 hours for 6 doses. Imatinib mesylate will be given by mouth with breakfast and a large glass of water on Days 1-14 or daily, depending on tolerance with Course 1. G-CSF will be given as in Course 1. The treatment to the brain inside the spinal fluid will be given as in Course 1 around Days 2 and 7.