- Regimen B (high-risk disease with or without CNS involvement): Patients receive R-CODOX-M chemotherapy with leucovorin calcium and G-CSF support as in regimen A for courses 1 and 3 and R-IVAC chemotherapy with leucovorin calcium and G-CSF support (as below) for courses 2 and 4. R-IVAC chemotherapy comprises high-dose ifosfamide IV over 3 hours and etoposide IV over 1 hour on days 1-5; cytarabine IV over 3 hours twice daily on days 2 and 3; and rituximab IV over 2-4 hours on day 0 and on day 6 or 7. Patients also receive leucovorin calcium orally every 6 hours on day 6 and G-CSF SC once daily beginning on day 6 or 7 and continuing until blood counts recover. Patients without CNS involvement receive CNS prophylaxis comprising cytarabine IT on days 1 and 3 and MTX IT on day 15 in courses 1 and 3 and MTX IT alone on day 5 in courses 2 and 4. Patients with proven CNS involvement at diagnosis receive cytarabine IT on days 1, 3, and 5 in course 1, on days 7 and 9 in course 2, and on days 1 and 3 in course 3. These patients also receive MTX IT on days 15 and 17 in course 1, on day 5 in courses 2 and 4, and on day 15 in course 3. Treatment repeats every 28 days for 4 courses in the absence of disease progression or unacceptable toxicity.