Due to their infrequency and multiplicity of histopathology,
myoepitheliomas present difficulties in diagnosis and classification. Cellular varieties can be misdiagnosed as
malignancies. Improvements in and clarification of diagnostic criteria are, therefore, required. A key to determining diagnostic criteria for
myoepitheliomas is to study cellular morphology, cytoplasmic filament expression, and ultrastructural features of the nonluminal, i.e., neoplastic myoepithelial/basal,
tumor cells of
pleomorphic adenomas, and apply this information to defining
myoepitheliomas. Cytologic and growth patterns of nonluminal cells in
pleomorphic adenomas, including plasma-cytoid cells, are reflected in
myoepitheliomas. Results also indicate that muscle-specific actin and myofilaments are expressed only in a proportion of cases, and generally in not more than 60-70% of nonluminal cells in
pleomorphic adenoma; this also applies to benign and malignant
myoepitheliomas. The absence of these markers does not exclude a diagnosis of
myoepithelioma.
Vimentin and glial acidic fibrillary
protein, however, are strongly and diffusely expressed in the majority of
pleomorphic adenomas and
myoepitheliomas and are more reliable markers for these
tumors than muscle-specific actin. Like so many other salivary gland
tumors,
myoepitheliomas present an equally complex histomorphology and variable expression of antigenic markers, only some of which are associated with myoepithelial and basal cells of the acini and ducts of the normal salivary gland.