Patients enrolled in the study will begin treatment with chemotherapy designed to reduce disease to a minimum that is, to achieve remission or shrink the tumor as much as possible. Chemotherapy will be administered on an outpatient basis over a period of around 12 to 18 weeks in 3-week cycles as follows: prednisone by mouth on days 1 through 5; etoposide, doxorubicin and vincristine intravenously through (a vein) on days 1 through 5; and cyclophosphamide intravenously on day 5. Starting day 6, patients receive no chemotherapy for 16 days. In addition, an antibody called rituximab, which attaches to lymphoma cells and may increase the effectiveness of the chemotherapy, will be given on day 1 of the cycle. Patients will also receive a protein called G-CSF starting day 6 of the cycle and continuing until the white blood cell count recovers or until day 19. G-CSF is naturally produced by bone marrow and may boost the immune system. The chemotherapy drugs and rituximab are infused through a vein by means of a lightweight portable pump, which patients are taught how to use. Patients are also how taught how to give themselves G-CSF injections under the skin, similar to insulin injections.