INDUCTION INTENSIFICATION (WEEKS 6-9): Patients receive high-dose methotrexate IV continuously over 24 hours on days 1 and 8; triple IT chemotherapy comprising methotrexate, cytarabine, and hydrocortisone on days 1 and 8; leucovorin calcium IV or PO every 6 hours beginning 42 hours after start of high-dose methotrexate and continuing until methotrexate level is < 0.1 uM; cyclophosphamide IV over 30 minutes on days 15-19; etoposide IV over 2 hours on days 15-19; and filgrastim IV or SC beginning on day 20 and continuing until blood counts recover. Patients in morphologic remission proceed to re-induction therapy.