Diffuse Large B-Cell Lymphoma — Mesenteric Mass

A 58-year-old man presents with B-symptoms, progressive abdominal pain, and early satiety. CT abdomen shows a bulky mesenteric mass (marked **A** in the diagram) measuring 10 cm, encasing mesenteric vessels with aneurysmal dilatation of an adjacent small bowel loop. Excisional biopsy reveals sheets of large atypical lymphoid cells with CD20+, CD79a+, BCL6+, MUM1+ immunophenotype and Ki-67 index of 80%. FISH testing is negative for MYC, BCL2, and BCL6 rearrangements. The mass marked **A** is consistent with diffuse large B-cell lymphoma (DLBCL). Given the patient's age, markedly elevated LDH, stage IV disease with extranodal involvement, and germinal center B-cell-like phenotype, which of the following represents the most appropriate first-line treatment regimen?