Microglandular adenosis of the breast: a deceptive and still mysterious benign lesion

Microglandular adenosis (MA) of the breast, a benign glandular proliferation, was originally described approximately 35 years ago. The lesion is constituted by small glands, all of the same size. Glands are lined by one layer of cuboidal epithelial cells encircled by basal lamina without any evidence of interposed myoepithelial elements. Cells are positive for low-weight keratins and S-100 protein and negative for estrogen receptor, progesterone receptor, and HER-2. Since then, in the years that followed, several malignant lesions all showing microglandular architecture have been regarded either as a precursor or as an equivalent manifestation of MA. The latter has been associated with a large number of malignancies that include ductal carcinoma in situ, lobular carcinoma in situ, ademyoepithelioma, high-grade basal-like carcinoma, adenoid cystic carcinoma, matrix-producing carcinoma, invasive duct carcinoma not otherwise specified, and spindle cell carcinoma, not to mention acinic cell carcinoma. None of the above tumors were identical to MA. Differences mainly rested not only on the specific structure of the small glands but also on the cytological composition and immunohistochemical features of different lesions. Here, a review of the features of MA together with the differential diagnosis with lesions showing microglandular structure is discussed. MA shows similarities to a lesion named microglandular hamartoma/adenosis of the nasal cavity. The relation of the 2 similar lesions is discussed.