Answer: Plasmablastic lymphoma

Histology: Microscopic examination shows a highly cellular sheet-like proliferation of malignant cells. The cells exhibit squared-off borders, eccentric nuclei, and a moderate amount of amphophilic cytoplasm. The nuclei contain a single, large, eosinophilic nucleolus. The mitotic rate is high. By immunohistochemistry, the malignant cells are immunoreactive for the plasmacytic markers CD38, CD138 and MUM-1. They are not immunoreactive for AE1:AE3, S100, CD45, CD3, CD5, CD20, CD79a, kappa or lamda. The Ki-67 proliferation rate is high at 90%. In situ hybridization for Epstein-Barr virus (EBER) is positive.

Discussion: Plasmablastic lymphoma (PBL) is a rare form of non-Hodgkin lymphoma. It is strongly associated with HIV infection, although it has been recognized in immunocompetent patients. Of the approximately 150 reported cases, PBL has most often been reported in the oral cavity of HIV-positive patients1, 3-6. Of the non-HIV-associated cases, one-third develops in patients with some other immunosuppressive condition such as steroid therapy or chemotherapy. Like other HIV-associated lymphomas, the Epstein-Barr virus (EBV) is believed to play a role in the development of PBL. The distribution of PBL is not entirely restricted to the oral cavity. Outside of the oral cavity, the nasal cavity is one of the more commonly involved sites. PBL is regarded as very aggressive neoplasm, and it usually presents at an advanced stage.

PBL is not always easily recognized by the unwary pathologist. Poorly differentiated carcinoma and melanoma can be quickly eliminated from the differential diagnosis based on the absence of epithelial and melanocytic differentiation by immunohistochemistry. At the same time, its lymphoid nature may go unrecognized when only a limited number of lymphoid markers are used during the initial immunohistochemical workup. Although it is lymphoid in nature, PBL does not express conventional B and T lymphocyte markers. The inclusion of additional lymphoid markers, in particular those for plasmacytic differentiation such as CD38, CD138 and MUM-1, help establish the diagnosis. The presence of EBV in an immunocompromised patient is helpful in distinguishing PBL from an extramedullary myeloma.