Answer: Invasive ductal carcinoma

Histology: This case featured multiple small foci of tubular proliferations in the breast, which raises the differential diagnosis of invasive carcinoma versus adenosis. The diagnosis of invasive carcinoma is supported by the non-rounded, irregular architecture of the proliferation, the presence of cord-like patterns, the presence of desmoplastic stromal reaction, and the absence of myoepithelial cells as confirmed by immunohistochemical stains for p63 and smooth muscle myosin heavy chain.

Discussion: Sclerosing adenosis has a lobular, rounded low power appearance, and peripheral open tubules surround central sclerotic areas dominated by spindled, eosinophilic myoepithelial cells. Tubular or irregular adenosis consists of rounded, branching tubules which have a streaming, non-lobular pattern. This lesion is unassociated with stromal desmoplasia, and like sclerosing adenosis has intact myoepithelial cells. Microglandular adenosis consists of haphazardly arranged, rounded tubules composed of a single layer of cuboidal epithelium. The tubules have dense secretions in their lumens, and the cuboidal cells lack apical cytoplasmic snouts. This is an H&E diagnosis, since microglandular adenosis lacks myoepithelial cells in contrast to essentially all other benign proliferations in the breast.