Definition

 Myoepithelial carcinoma of the salivary glands is a neoplasm composed almost exclusively of tumour cells with myoepithelial differentiation, characterized by infiltrative growth and potential for metastasis. This tumour represents the malignant counterpart of benign myoepithelioma.

ICD-O code 8982/3

Synonym

 Malignant myoepithelioma

 Epidemiology

The mean age of patients at presentation is 55 years with a wide age distribution (range 14-86). Males and females are affected equally. In large series, myoepithelial carcinomas comprise less than two percent of all salivary gland carcinomas, but they may not be as rare as has been suggested before {2251,2304}. The very low historic incidence is probably due to their recent recognition as a separate tumour entity.

 Etiology

No etiological factors are known.

Localization

Most cases (75%) arise in the parotid, but they also occur in the submandibular and minor glands.

Clinical features

 The tumours are locally destructive. The majority of patients present with the complaint of a painless mass.

Macroscopy

Myoepithelial carcinomas are unencapsulated but may be well-defined with nodular surfaces. Tumour size varies considerably (2-10 cm). The cut surface is grey-white and can be glassy. Some tumours show areas of necrosis and cystic degeneration.

Tumour spread and staging

They can involve adjacent bone. Perineural and vascular invasion may occur. Regional and distant metastases are uncommon at presentation, but may occur late in the course of disease.

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, plasmacytoid (hyaline), or, occasionally, vacuolated 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 neoplasia. 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 myoepithelial carcinoma from benign myoepithelial tumours.

 Immunoprofile

 Reactivity for cytokeratin and at least one   of the other myoepithelial markers, including smooth muscle actin, GFAP, CD10, calponin and smooth muscle myosin heavy chain, is required for diagnosis {595,1827}.

Electron microscopy

Ultrastructural criteria for the diagnosis of myoepithelial carcinoma include longitudinally oriented 6-8 nm fine cytoplasmic microfilaments with focal dense bodies, pinocytic vesicles, desmosomes and hemidesmosomes, basal lamina and intermediate filaments {41,640}.

Precursor lesions

Myoepithelial carcinomas may arise de novo, but it is important to note that about half of cases develop in pre-existing pleomorphic adenomas, or from benign myoepitheliomas, particularly in recurrences {595,1827,2251}.

Genetics

Comparative genomic hybridization has revealed infrequent abnormalities in these lesions with only three of 12 myoepitheliomas manifesting various chromosomal losses. Of myoepithelial carcinomas, five have manifested chromosome 8 alterations {1154}.

Prognosis and predictive factors

Myoepithelial carcinomas are locally aggressive salivary gland neoplasms that exhibit diverse clinical outcomes. Approximately one third of patients die of disease, another third have recurrences, mostly multiple, and the remaining third are disease free. Marked cellular pleomorphism and high proliferative activity correlate with a poor clinical outcome {1827,2251}. There is no difference in clinical behaviour of ”de novo” myoepithelial carcinomas and of those arising in pleomorphic adenomas and benign myoepitheliomas {595,2251}.