INDUCTION: Patients receive cyclophosphamide intravenously (IV) and doxorubicin IV on day 1, etoposide IV on days 1-3, and prednisone orally (PO) on days 1-5. Patients also receive brentuximab vedotin IV over 30 minutes on day 1. Treatment repeats every 21 days for up to 6 courses in the absence of disease progression or unacceptable toxicity.