COURSE III (CNS prophylaxis): Within 7 days after completing course II, patients receive methotrexate intrathecally (IT), methotrexate intravenously (IV) over 3 hours, and vincristine sulfate IV on days 1, 8, and 15; methotrexate PO every 6 hours on days 1-2, 8-9, and 15-16; leucovorin calcium IV on days 2, 9, and 16; and leucovorin calcium PO every 6 hours on days 3, 4, 10, 11, 17, and 18.