A 46-year-old woman underwent a superficial parotidectomy for a 2 cm. mass. Representative images from the resection specimen are shown below.
The resultant specimen consists of a monomorphic population of discohesive epithelioid cells with prominent "glassy" eosinophilic cytoplasm and eccentrically placed nuclei. The resultant cytologic appearance is quite signet-ring like, but a true cytoplasmic vacuole or mucin inclusion is not present. In other foci the neoplastic cells somewhat resemble plasma cells. There is no evidence of glandular differentiation. The neoplastic cells are quite homogeneous in appearance with uniform nuclei, only rare normal-appearing mitotic figures and no necrosis. The margin of the lesion is sharply demarcated and separated from the adjacent normal parotid gland by a thin fibrous capsule.
What is your diagnosis?
A cytokeratin stain was diffusely positive in the neoplastic cells. In addition, a smooth muscle actin stain was performed with the results shown below.
The strong SMA positivity supports a neoplasm with myoepithelial differentiation and this is an example of a salivary gland myoepithelioma. Many salivary gland tumors show partial myoepithelial differentiation, not to be confused with myoepithelial ORIGIN, about which we know nothing.
Benign neoplasms displaying diffuse myoepithelial differentiation may have a distinctly spindle cell configuration, have a "plasmacytic" appearance as seen in the current case, or show a mixture of histologic patterns. Ultrastructurally, the glassy eosinophilic cytoplasm in the plasmacytic variant of myoepithelioma is due to aggregates of both intermediate filaments (cytokeratin, vimentin), and slightly smaller actin filaments.
Myoepitheliomas can be viewed as one subtype of salivary gland monomorphic adenoma, with other subtypes including oncocytomas, basal cell adenomas, canalicular adenomas, etc. Salivary gland mixed tumors are often rich in myoepithelial cells, and it can be arbitrary where one draws the line between a myoepithelial-rich mixed tumor and a myoepithelioma. Fortunately, this distinction has no clinical implication.