Plasmablastic lymphoma of the retroperitoneum in an HIV- and HCV-positive patient: hard to diagnose and harder to treat.

Plasmablastic lymphoma (PBL) is an aggressive diffuse large B-cell lymphoma (DLBCL) with plasmablastic features that was initially described in the oral cavity of HIV-infected individuals. PBL remains a diagnostic challenge given its close morphologic resemblance and overlapping immunophenotypic patterns to other B-cell lymphoid malignancies and plasmablastic plasma cell myeloma (PCM) with extramedullary involvement. The presence of serum monoclonal protein and radiographic evidence of lytic bone lesions favors the diagnosis of plasma cell myeloma over PBL. Distinguishing PBL from PCM is important as PBL is treated with a completely different chemotherapy regimen compared to PCM. PBL carries a guarded prognostic profile among DLBCLs with high relapse rate and poor median survival. We present a case of a 44-year-old HIV-positive man who presented with a large retroperitoneal mass associated with obstructive uropathy, sacral radiculopathy, and inferior vena caval compression. The mass was initially mistaken to be a PCM on histopathology; however, subsequent investigations revealed an extra-oral PBL with plasmacytic differentiation. To our knowledge, this will be the first case of PBL of the retroperitoneum in an HIV- and HCV-positive patient and the second one at this location in the English-language literature. In this report, key differentiating points between PBL versus PCM and newer therapeutic agents such as proteasome inhibitors have been discussed along with related review of literature.