Infiltrating myoepithelioma of the breast.

This report describes the histologic and ultrastructural features of a spindle cell myoepithelioma arising in the breast of a 60-year-old woman. By light microscopy, the tumor consisted of interlacing bundles of elongated cells sometimes arranged in a storiform pattern. No intraductal or invasive carcinoma was identified. Ultrastructurally, the neoplasm was composed of activated, differentiated myoepithelial cells showing evidence of squamous metaplasia, i.e., spindle-shaped cells joined by mature desmosomes and encompassed by remnants of basal lamina containing a well-developed rough endoplasmic reticulum, pinocytotic vesicles, prekeratin filaments, and longitudinally oriented bundles of microfilaments with fusiform densities. Numerous cisternae of rough endoplasmic reticulum suggested heightened metabolic activity, whereas the presence of cytoplasmic prekeratin filaments indicated squamous metaplasia. The cells comprising most previously reported "myoepitheliomas" examined by electron microscopy lacked many of the distinctive fine structural features of the normal myoepithelial cell. At present, the diagnosis of myoepithelioma should be based on strict ultrastructural criteria. The clinical behavior of this particular lesion in the breast is unknown. If the tumor follows the course of comparable salivary gland neoplasms, the likelihood of systemic spread is low. Primary therapy for a tumor proven to be an infiltrating myoepithelioma of the breast should be directed at complete excision of the lesion.