THE IMMUNOHISTOCHEMICAL AND ULTRASTRUCTURAL-STUDY OF MALIGNANT PLASMACYTOID MYOEPITHELIOMA OF THE MAXILLARY SINUS

Myoepithelioma is a rare salivary gland tumor which is composed exclus ively of myoepithelial cells. Histologically, it can be divided into t hree cell types: spindle, plasmacytoid and mixed type. Malignant myoep ithelioma is characterized by invasive growth. In March 1995, a 60-yea r-old man presented with a left cheek tumor which he had first noted 2 years previously. Computed tomography revealed a large expansile tumo r in the maxillary sinus with invasion into the surrounding soft tissu e. Partial resection of the tumor was performed because of extensive i nvolvement of the surrounding tissue. The patient died due to spread t o the brain 5 months after surgery. Histologically, the tumor was comp osed exclusively of plasmacytoid cells, with bone destruction. Immunoh istochemically, these cells were negative for immunoglobulin light cha ins (kappa and lambda) and heavy chains (Ig G, A, M) but positive for S-100, cytokeratin, actin and vimentin. Ultrastructurally, the tumor c ells contained numerous randomly oriented actin-like microfilaments in the cytoplasm, and had desmosomes on the cell membrane. Malignant pla smacytoid myoepithelioma of the maxillary sinus was diagnosed. In addi tion 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 ce ll myoepithelioma. Morphologically, it is very difficult to differenti ate plasmacytoid myoepithelioma from plasmacytoma and immunohistochemi cal staining is necessary to make a correct diagnosis.