Patients receive induction chemotherapy comprising cyclophosphamide IV, doxorubicin IV over 15 minutes, and vincristine IV over 1-2 minutes on day 1; oral prednisone once daily on days 1-5; and filgrastim (G-CSF) subcutaneously (SC) once daily on days 7-11 or PEG-filgrastim once at least 24 hours after infusion. Patients also receive rituximab IV 2-3 days apart for a total of 2 doses during the week prior to the first course of chemotherapy and on day 1 of courses 2-4 of chemotherapy. Treatment repeats every 14 days for a total of 4 courses in the absence of disease progression or unacceptable toxicity.