A diffuse loose aggregation of plasma cells are seen, which can be well-differentiated (indistinguishable from normal plasma cells) to poorly differentiated with pleomorphic morphology. Binucleate morphology can be prominent, and if they comprise >5% of the neoplasm, it can indicate proliferation (Brandwein). The presence of distinct perinuclear hofs and clock-face chromatin easily characterize this lesion as plasma cell in type.
Loose aggregates of normal appearing plasma cells are appreciated in this well-differentiated plasmacytoma. CD138, a marker for plasma cells, would demonstrate strong membranous staining.
This particular case of extramedullary plasmacytoma arose in the parotid gland. The bottom right shows atrophic skeletal muscle, where the tumor has expanded outside the parotid into adjacent tissues.
Plasma cell dyscrasias are a group of disorders characterized by a malignant proliferation of plasma cells which may or may not secrete clonal immunoglobulin. The most common plasma cell dyscrasia is monoclonal gammopathy of unknown significance -- other disorders include multiple myeloma, Waldenstrom Macroglobulinemia, monoclonal light chain amyloidosis and solitary plasmacytoma, a tumorous aggregate of plasma cells.
Plasmacytomas can occur as a solitary lesion, usually seen in younger patients, or in conjunction with multiple myeloma seen in older individuals. Extramedullary plasmacytomas can occur in any anatomic site, but 80% of cases are found in the upper aerodigestive tract such as sinonasal passages (Fletcher, Brandwein).
More common in men and although it can occur at any age, most individuals are over 40. Extramedullary plasmacytomas have a predilection for head and neck (sinonasal tract, nasopharyx, larynx, oral cavity and salivary glands). Serum electrophoresis may be helpful in diagnosis as it may show a monoclonal gammopathy (Fletcher).
Approximately 40-50% of osseous plasmacytomas and 10-20% of extramedullary plasmacytomas will eventually progress to multiple myeloma. These neoplasms usually respond favorably to chemotherapy and may delay (but not prevent) progression to multiple myeloma. (Fletcher).
Brandwein-Gensler M. Head and Neck: Illustrated Surgical Pathology Series. New York, NY: Cambridge University Press; 2010: 78-81.
Fletcher CDM, ed. Diagnostic Histopathology of Tumors. 3rd Ed. Philadelphia, PA: Elsevier; 2007: 128-9, 1209.