Regimen A: Patients receive low-dose vincristine IV and high-dose methotrexate IV continuously over 24 hours on day 1; leucovorin calcium IV or orally beginning 42 hours after start of high-dose methotrexate and continuing every 6 hours for at least 3 doses; oral mercaptopurine once daily on days 2-6; etoposide phosphate IV over ≥ 1 hour and cyclophosphamide IV over 1 hour on day 8; and methotrexate IT* on day 15. Treatment repeats every 21 days for 4 courses (with the exception of IT methotrexate which repeats for only 3 courses).