Clear cell malignant myoepithelioma of the salivary glands.

Previous reports of monomorphic clear cell carcinoma of the salivary glands have shown inconsistent results with immunohistochemistry, especially for S-100 protein, and this has led to uncertainty about the nature of these tumours. We believe that much can be explained by considering this group as comprising not one but two separate neoplasms, one epithelial and the other myoepithelial. The former has been described as hyalinizing clear cell carcinoma--it generally occurs in the minor salivary glands, and strongly expresses cytokeratins but not S-100 protein or alpha smooth muscle actin. In contrast, this study presents five primary malignant tumours of the major salivary glands also composed largely of a single population of clear cells, but displaying histological and immunohistochemical features of myoepithelial differentiation, such as the formation of collagenous spherules and expression of S-100 protein and actin. A small number of similar tumours have been reported previously. We, therefore, believe that these neoplasms represent a clear cell variant of malignant myoepithelioma (myoepithelial carcinoma).