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Image of the Week
Edited and moderated by Stacey Mills, MD, Pathology Network's Image of the Week Blog is a forum for the discussion of interesting and often diagnostically challenging pathology images.
Tuesday, April 01, 2014
A 46-year-old woman with a parotid gland mass.
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. 
Thank you for posting a great case. Do you find it useful to do a panel of antibodies to show myoepithelial differentiation, such as calponin, p63, etc in equivocal lesions (and if so, what would you choose) and is SMA your standard marker? [Dr. Mills' reply: I typically use SMA and p63 for myoepithelial markers in this setting. The stains are certainly helpful in some cases.]
About the Author

Stacey E. Mills, MD
Stacey E. Mills, MD, a graduate of University of Virginia (UVA) and the UVA Medical Center, has authored nearly 230 articles, 20+ books, atlases and monographs—including the renowned Sternberg's Diagnostic Surgical Pathology. He has been a practicing Professor and Staff Pathologist at UVA for 30+ years and is Director of Surgical Pathology and Cytopathology. His clinical specialty is general surgical pathology with emphasis on neoplasms and neoplasm-like lesions. Dr. Mills is also Editor-in-Chief of The American Journal of Surgical Pathology.