Answer: Extramedullary plasmacytoma

Histology: The submucosa in diffusely infiltrated by sheets of densely packed small round cells. At high power, the cells have basophilic cytoplasm and eccentrically placed nuclei with clumped chromatin (in a clock face appearance) and prominent nucleoli. Mitotic figures are abundantly present. Flow cytometry indicates that the cells are kappa light chain restricted.

Discussion: The region of the head and neck is a common site of extramedullary plasmacytoma. Indeed, 80% of extramedullary plasmacytomas arise in this region, with the sinonasal tract being the most commonly involved head and neck site. Like plasmacytomas involving non-head and neck sites, patients should be evaluated for systemic disease; however plasmacytomas involving the sinonasal tract are much more likely to represent localized disease. Although the disease is localized in the majority of patients at presentation, extended patient follow-up is required. By some estimates, dissemination ultimately occurs in 35-50% of patients with plasmacytomas of the head and neck.

The differential of plasmacytoma is lengthy and includes both reactive and neoplastic processes. Plasma cells are a prominent component of reactive inflammatory lesions of the upper aerodigestive tract. Unlike plasmacytoma, these reactive processes (including mucous membrane plasmacytosis) are characterized by a polyclonal proliferation of plasma cells. Thus, immunohistochemical staining can play a useful role in documenting light chain restriction in a plasma cell infiltrate. Immunohistochemistry can also be useful in excluding other small round cell neoplasms including esthesioneuroblastoma and mucosal melanoma.