Answer: Microglandular adenosis

Histology: The lesion is essentially a haphazard collection of rounded tubules characterized by a single layer of bland, cuboidal epithelium with clear cytoplasm, and dense pink secretions within their lumens. These cells lack apical cytoplasmic snouts. A second myoepithelial cell layer would not be appreciated on actin stains, but a thick basement membrane can occasionally be appreciated around each tubule.

Discussion: Tubular carcinoma is characterized by angulated, teardrop-shaped glands with apical cytoplasmic snouts and a desmoplastic stroma. None of these describe the current lesion. Tubular adenosis is composed of elongated, branching tubules that have an intact myoepithelial layer. These tubules lack the dense pink secretion and perfect roundness of the tubules of microglandular adenosis. Sclerosing adenosis is a circumscribed, lobular proliferation. Sclerosing adenosis is typically centrally sclerotic with a prominent myoepithelial cell component, and rounded tubules with open lumens at the periphery. The dense pink secretion typical of microglandular adenosis is again not present in sclerosing adenosis.

Microglandular adenosis is usually an incidental finding within the breast. Characteristically, the tubules of microglandular adenosis are strongly S-100 protein positive, but do not label for epithelial membrane antigen. Microglandular adenosis is the one lesion that violates the rule that all benign breast lesions maintain a myoepithelial cell layer. This lesion does not have a myoepithelial cell layer, but it is benign.