Malignant myoepithelioma of the breast. An immunohistochemical study by light and electron microscopy.

Myoepitheliomas of the breast are extremely rare; only four cases have been reported to date. It is not clear whether these tumors behave in a benign or malignant fashion, and no metastatic spread has been documented. We report a myoepithelioma of the breast with metastatic spread to an axillary lymph node. Myoepithelial cell (MEC) differentiation in the tumor was characterized using electron microscopic (EM) criteria. Immunologic investigations at the LM and EM levels showed that the tumor cells were positive for S100 protein, actin, and epidermal cytokeratin; these findings are indicative of MEC differentiation. By immuno-EM, cytokeratin filaments were present in a perinuclear location, while actin filaments were concentrated along the cell periphery. To the authors' knowledge, this is the first report of a breast myoepithelioma that has metastasized and also the first report of the immunologic characterization of a myoepithelioma at the light and electron microscopic levels.