This report describes the light (LM) and electron microscopic (EM) features and the results of an indirect immunofluorescence study (IF), the latter using monoclonal and monospecific
antibodies to
cytoskeletal proteins, of a malignant, invasive and metastatic breast
myoepithelioma. A 53-year-old female underwent
mastectomy for a large necrotic mammary
tumor that had invaded the overlying skin. By LM, the
neoplasm was composed of interlacing bundles of large, elongated and interspersed stellate cells with acidophilic cytoplasm. The neoplastic cells displayed a moderate degree of
anaplasia, high mitotic activity, and strong tendency for
necrosis. Stromal desmoplasia was marked, especially toward the center of the
neoplasm. By IF, the
tumor cells revealed bright cytoplasmic fluorescence with
antibodies to actin,
prekeratin, and
cytokeratin. A few scattered spindle cells, which stained with the anti-
vimentin and anti-actin anti-bodies, most likely represented stromal myofibroblasts. The anti-
desmin reaction was negative. By EM, the
neoplasm was composed of variably differentiated, elongated and stellate myoepithelial cells connected by desmosomes, enveloped by remnants of basal lamina, and containing pinocytotic vesicles, a well-developed rough endoplasmic reticulum, large Golgi areas, aggregates of intermediate filaments that were often arranged in dense curvilinear bundles (tonofilaments), and bundles of microfilaments with fusiform, dense bodies. The combined LM, EM, and IF study of this mammary
tumor establishes its myoepithelial origin and, thus, identifies it as myoepithelial
carcinoma distinct from other spindle cell
breast tumors. This
neoplasms was locally invasive and cytologically malignant; moreover, its
malignancy was further confirmed by the development of lung and pleural
metastases.