Immunohistochemical localization of S100 protein in benign and malignant conditions of the breast.

A peroxidase--antiperoxidase technique for S100 protein has been applied to 122 breast lesions from 122 patients. These included 35 cases of fibrocystic disease, 16 cases of sclerosing adenosis, 24 cases of papilloma and papillomatosis, 43 intraduct carcinomas, and four intralobular carcinomas. In fibrocystic disease, S100 protein was demonstrable in large amounts in cells between the duct lining cells and the basement membrane of the ducts, being most pronounced in those exhibiting adenosis. Areas of epitheliosis showed scattered positive cells within the ducts with more strongly positive cells around these ducts. Apocrine metaplasia was moderately positive. No S100 protein was demonstrable in the epithelial lining cells of cysts or within the stroma. In sclerosing adenosis individual cells and groups of cells in the fibrous tissue were strongly positive. In papillomatosis and papilloma, the vascular core and epithelium failed to stain, but a discontinuous layer of cells between the epithelium and basement membrane was positive. In intraduct and intralobular carcinoma the tumor cells were uniformly negative, and wherever fibrocystic disease was also present, S100 protein was variably demonstrable. The study corroborated the view that fibrocystic disease and benign proliferative processes of the breast appear to contain cells that correspond to myoepithelial cells, and suggests that S100 protein may serve as a useful marker in the separation of benign proliferative breast lesions from in situ carcinoma.