CONTINUATION I (WEEKS 20-41): Patients receive vincristine sulfate IV on day 1 in weeks 20 and 24; dexamethasone IV or PO BID on days 1-5 in weeks 20, and 24; triple IT chemotherapy comprising methotrexate, cytarabine, and hydrocortisone on day 1 in weeks 20 and 24; methotrexate IV on day 1 in weeks 21-24 and 25-27; etoposide IV over 2 hours on day 1-5 in week 28; cyclophosphamide IV over 30 minutes on days 1-5 in week 28; mercaptopurine PO on days 1-7 in weeks 21-23 and 25-27; and filgrastim SC or IV beginning on day 6 in week 28 and continuing until blood counts recover.