Immunohistochemical and ultrastructural study of malignant plasmacytoid myoepithelioma of the maxillary sinus.

Myoepithelioma is a rare salivary gland tumor which is composed exclusively of myoepithelial cells. Histologically, it can be divided into three cell types: spindle, plasmacytoid and mixed type. Malignant myoepithelioma is characterized by invasive growth. In March 1995, a 60-year-old man presented with a left cheek tumor which he had first noted 2 years previously. Computed tomography revealed a large expansile tumor in the maxillary sinus with invasion into the surrounding soft tissue. Partial resection of the tumor was performed because of extensive involvement of the surrounding tissue. The patient died due to spread to the brain 5 months after surgery. Histologically, the tumor was composed exclusively of plasmacytoid cells, with bone destruction. Immunohistochemically, these cells were negative for immunoglobulin light chains (kappa and lambda) and heavy chains (Ig G, A, M) but positive for S-100, cytokeratin, actin and vimentin.Ultrastructurally, the tumor cells contained numerous randomly oriented actin-like microfilaments in the cytoplasm, and had desmosomes on the cell membrane. Malignant plasmacytoid myoepithelioma of the maxillary sinus was diagnosed. In addition to our case, only five cases of pure plasmacytoid myoepithelioma have been reported. Plasmacytoid myoepithelioma tends to occur in the minor salivary glands and has more aggressive behavior than spindle cell myoepithelioma. Morphologically, it is very difficult to differentiate plasmacytoid myoepithelioma from plasmacytoma and immunohistochemical staining is necessary to make a correct diagnosis.