Arm I: Patients receive cyclophosphamide IV, doxorubicin IV, and vincristine IV on day 3 and oral prednisone on days 3-7. Patients receive rituximab on day 1. Treatment repeats every 3 weeks for a minimum of 4 courses or 2 courses beyond complete response in the absence of disease progression or unacceptable toxicity. Patients with stage I, stage IE (including bulky), or nonbulky stage II or IIE disease receive 3 courses of chemotherapy with rituximab followed by radiotherapy beginning 3 weeks after completion of the third course. Patients who achieve partial response for a minimum of 28 days or complete response receive maintenance rituximab IV beginning on day 28 of the final course of chemotherapy. Maintenance rituximab treatment repeats every 4 weeks for 3 courses.