The role of immunocytochemical markers in the differential diagnosis of proliferative and neoplastic lesions of the breast.

The differential expression of keratins in myoepithelial and epithelial cells of the breast makes immunohistochemical distinction of lesions an attractive possibility. High molecular weight keratin, 34BE12, is a monoclonal antibody that recognizes keratins 1, 5, 10, and 14. Because myoepithelial cells predominantly express keratins 5 and 14 and epithelial cells predominantly express keratins 8 and 18, it is natural to assume that 34BE12 may be a good marker of myoepithelial cells but not epithelial cells. However, recent studies of the breast have reported conflicting results. To determine the potential role of 34BE12 in the breast, we studied by immunohistochemistry 19 tubular carcinomas, 14 radial scars, two microglandular adenoses, and 9 sclerosing adenoses, using monoclonal antibodies to high molecular weight keratin, smooth muscle actin, type IV collagen, and antiserum to S100 protein. Actin was negative in all 19 (100%) tubular carcinomas, but it delineated the myoepithelial cells in 22 of 23 (95.6%) benign lesions of sclerosing adenosis and radial scars; it was also negative in microglandular adenosis. In comparison, epithelial cytoplasmic 34BE12 reactivity was seen in 3 of 19 (15.8%) tubular carcinomas, whereas myoepithelial cells failed to react in 4 of 23 (17.3%) benign conditions. Antiserum to S100 protein had a similar disadvantage of labeling both epithelial and myoepithelial cells with reactivity in 5 of 19 (26.3%) tubular carcinomas. In microglandular adenosis, the epithelial cells were strongly S100 protein positive and focally 34BE12 positive, but no staining was observed for actin. Type IV collagen staining outlined distinct basement membranes in microglandular adenosis and other benign conditions but not in tubular carcinomas. However, staining for type IV collagen requires enzymatic pretreatment and is difficult to perform, especially in sclerotic breast tissue. In conclusion, actin appears to be the most consistent and specific marker for distinguishing tubular carcinomas from other benign conditions, and type IV collagen has a contributory role, whereas 34BE12 is less valuable than in prostatic biopsies.