- Regimen A (R-CODOX-M chemotherapy): Patients receive rituximab** IV and doxorubicin hydrochloride IV over 15 minutes on day 1, cyclophosphamide IV over 30-60 minutes on days 1 and 2, pegfilgrastim subcutaneously (SC) on day 3, vincristine IV on days 1 and 8, high-dose methotrexate IV over 2-4 hours on day 15, and leucovorin calcium IV beginning 24 hours after the start of methotrexate and continuing every 6 hours until the methotrexate level is less than 50 nmol/L. Patients receive CNS prophylaxis comprising methotrexate intrathecally (IT), cytarabine IT, and hydrocortisone IT on day 1. Patients with high-risk disease receive an additional dose of cytarabine IT on day 3. Patients also receive filgrastim (G-CSF) SC once daily on days 3-9. Once the methotrexate levels drops below 50 nmol/L, patients resume G-CSF SC once daily beginning on approximately day 18 and continuing until blood counts recover.