A 38-year-old woman from Mumbai presents with a 2-week history of rapidly enlarging left cervical lymphadenopathy, fever, night sweats, and a 4 kg weight loss. Excisional lymph node biopsy shows diffuse effacement of nodal architecture by large cells with abundant cytoplasm, prominent nucleoli, and frequent mitotic figures. Immunohistochemistry reveals CD20+, CD10+, BCL6+ B cells with a high Ki-67 proliferation index (>80%). Flow cytometry shows surface immunoglobulin light-chain restriction. A staging CT scan shows stage III disease with involvement of mediastinal, abdominal, and pelvic nodes. Serum LDH is 1,200 U/L (normal <400). What is the most appropriate next step in management?