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.