Soft tissue myoepithelioma: a case report

Myoepitheliomas are tumours composed predominantly or exclusively of myoepithelial cells. They are well described, especially within the salivary gland, but their occurrence in soft tissues is less well known and this often results in diagnostic problems. We report a case involving the deep soft tissues of the lower neck behind the clavicle. Grossly, the tumour was well circumscribed with solid and cystic areas. Histology showed a richly vascularised tumour composed of bland round, ovoid or spindle-shaped cells. Various growth patterns were present including solid, nested, microcystic and trabecular arrangements. In some areas there was an alveolar pattern with tumour cells lining fibrous septae. Immunohistochemistry showed diffuse strong positivity for S100 protein, calponin, vimentin and glial fibrillary acidic protein and focal positivity for epithelial membrane antigen, 34betaE12 and AE1/AE3, in keeping with myoepithelial differentiation. Electron microscopy revealed tumour cells surrounded by basal lamina with subplasmalemmal densities and containing cytoplasmic myofilaments. This case report highlights the rare occurrence of myoepitheliomas in deep soft tissues. Pathologists should be aware of this and should consider a myoepithelioma in the differential diagnosis of a soft tissue spindle cell neoplasm.