Case Report: Sinonasal plasmablastic lymphoma in a human immunodeficiency virus-negative elderly woman diagnosed by immunohistochemistry.

BACKGROUND: Plasmablastic lymphoma is a rare, highly aggressive mature B-cell lymphoma with terminal B-cell/plasma-cell differentiation. It usually arises in the oral cavity in patients with human immunodeficiency virus infection or other forms of immunodeficiency, whereas primary sinonasal involvement in an apparently immunocompetent patient is uncommon. On hematoxylin-and-eosin examination, it may histomorphologically resemble poorly differentiated epithelial, melanocytic, and other hematolymphoid malignancies, necessitating a broad immunohistochemical panel for definitive diagnosis. CASE PRESENTATION: A 75-year-old woman presented with a 1-month history of progressive left-sided nasal obstruction and watery nasal discharge, one episode of epistaxis 12 days before presentation, and reduced olfaction. She had no history of human immunodeficiency virus infection, organ transplantation, or systemic immunosuppression. Anterior rhinoscopy showed a mass occupying the left nasal cavity. Contrast-enhanced computed tomography demonstrated a heterogeneously enhancing, locally destructive soft-tissue lesion occupying the left maxillary sinus with extension into the left ethmoid and frontal sinuses and left nasal cavity, with erosion of the lamina papyracea and maxillary sinus walls. Biopsy showed a high-grade malignant round-cell/plasmablastic neoplasm with extensive necrosis. The main morphologic differentials included sinonasal undifferentiated carcinoma, non-keratinizing squamous cell carcinoma, amelanotic melanoma, non-Hodgkin lymphoma, and a plasma-cell neoplasm. Immunohistochemistry showed strong, diffuse expression of CD38, CD138, and MUM1; strong but focal nuclear PAX5 expression; focal leukocyte common antigen and Epstein-Barr virus positivity; kappa light-chain restriction; and a Ki-67 proliferation index of 98%-99%. Tumor cells were negative for CD20, pancytokeratin, AE1/AE3, S100, human herpesvirus 8, and lambda light chain. A final diagnosis of plasmablastic lymphoma was rendered. The patient received CHOP chemotherapy (cyclophosphamide, doxorubicin, vincristine, and prednisone) and had completed three cycles at 21-day intervals by the 5-month follow-up, with reduced nasal obstruction and overall symptomatic improvement. Post-treatment imaging was unavailable; therefore, radiological and metabolic response could not be assessed. CONCLUSION: This case highlights the need to include plasmablastic lymphoma in the differential diagnosis of an aggressive unilateral sinonasal mass, even in a human immunodeficiency virus-negative elderly patient, and emphasizes the decisive role of a broad immunohistochemical panel with light-chain assessment in resolving morphologic overlap with other high-grade sinonasal malignancies.