- Determine the efficacy of rituximab, cyclophosphamide, vincristine, doxorubicin hydrochloride, and high-dose methotrexate (R-CODOX-M ) alone or alternating with rituximab and ifosfamide, etoposide phosphate, and high-dose cytarabine (IVAC) and intrathecal CNS prophylaxis in patients with newly diagnosed, previously untreated, HIV-associated Burkitt's lymphoma or atypical Burkitt's lymphoma.