MYOEPITHELIAL CARCINOMA (MALIGNANT MYOEPITHELIOMA) OF THE PAROTID-GLAND ARISING IN A PLEOMORPHIC ADENOMA

A myoepithelial carcinoma, a rare malignant salivary gland neoplasm, a rose 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 invas ion 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 differe ntiation: tumour cells stained positively with anticytokeratin antibod ies, S-100 protein, a smooth muscle actin, and vimentin. Electron micr oscopy confirmed myoepithelial differentiation, with small foci of ker atinocytic phenotype. Large numbers of tumour cell nuclei were reactiv e with the anti-p53 antibody, DO-7, in contrast to the two previous re sections. Thus malignant transformation of a pleomorphic adenoma may i nvolve myoepithelial as well as epithelial elements. Accumulation of p 53 protein, perhaps through mutational events, may have played a role in this malignant transformation.