Patients receive rituximab IV on day 1; doxorubicin IV continuously, etoposide IV continuously, and vincristine IV continuously on days 1-4; oral prednisone twice daily on days 1-5; and cyclophosphamide IV on day 5. Patients also receive filgrastim (G-CSF) subcutaneously beginning on day 6 and continuing until blood counts recover. Treatment repeats every 21 days for up to 8 courses in the absence of disease progression or unacceptable toxicity. After 4 courses, patients with complete or partial response receive 2 additional courses.