Extramedullary plasmacytoma. A form of marginal zone cell lymphoma?

Extramedullary plasmacytoma (EMP), solitary plasmacytoma of bone, and multiple myeloma are related neoplasms, but EMP is clearly a distinct entity. Moreover, there are histologic and clinical similarities between EMP and marginal zone B-cell lymphomas (MZLs) displaying extensive plasma cell differentiation, suggesting a possible histogenetic relationship. The histologic and clinical features of 5 EMPs with extensive plasma cell differentiation were histologically reviewed for features of MZL. The previously diagnosed MZLs, mucosa-associated lymphoid tissue (MALT) type, of 2 patients also were reviewed. All patients were women aged 48 to 79 years. The EMPs originated in the parotid gland, lymph nodes, dura, or small bowel. The initial tumors diagnosed as MALT-type MZL were located in the lung and small bowel. All patients were treated with resection, with or without irradiation therapy. One patient also received systemic chemotherapy. All patients are alive with no evidence of disease. All tumors contained large numbers of plasma cells, constituting between 55% and 90% of the lymphoid cells. Centrocyte-like cells and monocytoid B cells each represented 0% to 25% of the infiltrate. Lymphoepithelial lesions were observed in all of the tumors in sites where epithelium was present. Reactive follicles were found in all of the tumors. EMPs may represent MZLs that have undergone an extensive degree of plasmacytic differentiation.