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

Microglandular adenosis of the breast (MA), a benign glandular proliferation, was originally described about 35years 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 ER, PR and HER 2. Since then, in the years, several malignant lesions all showing microglandular architecture have been regarded either as a precursor or an equivalent manifestation of MA. The latter has been associated to a large number of malignancies that include DCIS, LCIS, ademyoepithelioma, high grade basal like carcinoma, adenoid cystic carcinoma, matrix producing carcinoma, invasive duct carcinoma NOS, 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 with a lesion named microglandular hamartoma/adenosis (MH) of the nasal cavity. The relation of the two similar lesions is discussed