Histopathology

Myoepithelial carcinoma characteristically has a multilobulated architecture. The range of cell types in myoepithelial carcinoma reflects that seen in its benign counterpart. The tumour cells often are spindled, stellate, epithelioid, plasmacy-toid (hyaline), or, occasionally, vacuolat-ed with signet ring like appearance. Other tumours tend to be more cellular composed of spindle-shaped cells, and they can resemble sarcoma. Rarely, myoepithelial carcinoma is composed of a monomorphic population of clear cells with myoepithelial features {1719}. The tumour cells may form solid and sheet-like formations, trabecular or reticular patterns, but they can also be dissociated, often within plentiful myxoid or hyaline stroma. The neoplastic nodules frequently have necrotic centres. Pseudocystic or true cystic degeneration can occur. Sparse areas with squamous differentiation may be found. Rarely, myoepithelial carcinoma contains ductlike lumina usually with non-luminal cell differentiation of the lining cells. A tumour containing more than the occasional true luminal cell should not be included in the category of purely myoepithelial neopla-sia.

Different cell types and architectural patterns may be found within the same tumour. In fact, most myoepithelial carcinoma s are less monomorphic than benign myoepithelioma. They also may demonstrate high mitotic activity with considerable variation {595,1154,1827, 2251}. Cellular pleomorphism can be marked, and necrosis may occur {1827, 2251}. However, unequivocal evidence of infiltrative, destructive growth is the major requirement for diagnosis, and it is this property that distinguishes myoep-ithelial carcinoma from benign myoep-ithelial tumours.

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