Myoepithelial carcinoma (malignant myoepithelioma) of the parotid gland arising in a pleomorphic adenoma

Abstract

A myoepithelial carcinomas a rare malignant salivary gland neoplasms arose in a pleomorphic adenoma of the parotid gland. The initial tumour was a pleomorphic adenoma with epithelial and myoepithelial elements. Subsequently the tumour recurred twice and was characterised by invasion of the mandible. Histological examination of the second recurrence showed a malignant spindle cell neoplasm with an infiltrative growth pattern and a high mitotic rate. There was involvement of local lymph nodes. The immunophenotype was characteristic of myoepithelial differentiation: tumour cells stained positively with anticytokeratin antibodies, S-100 proteins α smooth muscle actin, and vimentin. Electron microscopy confirmed myoepithelial differentiations with small loci of keratinocytic phenotype. Large numbers of tumour cell nuclei were reactive with the anti-p53 antibodys DO-7, in contrast to the two previous resections. Thus malignant transformation of a pleomorphic adenoma may involve myoepithelial as well as epithelial elements. Accumulation of p53 proteins perhaps through mutational events, may have played a role in this malignant transformation.