MYOEPITHELIOMA - DEFINITIONS AND DIAGNOSTIC-CRITERIA

Due to their infrequency and multiplicity of histopathology, myoepithe liomas present difficulties in diagnosis and classification. Cellular varieties can be misdiagnosed as malignancies. Improvements in and cla rification of diagnostic criteria are, therefore, required. A key to d etermining diagnostic criteria for myoepitheliomas is to study cellula r morphology, cytoplasmic filament expression, and ultrastructural fea tures of the nonluminal, i.e., neoplastic myoepithelial/basal, tumor c ells of pleomorphic adenomas, and apply this information to defining m yoepitheliomas. Cytologic and growth patterns of nonluminal cells in p leomorphic adenomas, including plasmacytoid cells, are reflected in my oepitheliomas. Results also indicate that muscle-specific actin and my ofilaments are expressed only in a proportion of cases, and generally in not more than 60-70% of nonluminal cells in pleomorphic adenoma; th is also applies to benign and malignant myoepitheliomas. The absence o f these markers does not exclude a diagnosis of myoepithelioma. Viment in and glial acidic fibrillary protein, however, are strongly and diff usely expressed in the majority of pleomorphic adenomas and myoepithel iomas and are more reliable markers for these tumors than muscle-speci fic actin. Like so many other salivary gland tumors, myoepitheliomas p resent an equally complex histomorphology and variable expression of a ntigenic markers, only some of which are associated with myoepithelial and basal cells of the acini and ducts of the normal salivary gland.