Carcinoma in situ involving sclerosing adenosis: a mimic of invasive breast carcinoma.

The distinction between invasive and in situ carcinoma of the breast is important with regard to the treatment and prognosis of the patient. When carcinoma in situ involves breast tissue in which the normal architecture is altered by pre-existing sclerosing adenosis, the resulting histological picture may closely mimic an invasive carcinoma. We record the histopathological features in 13 cases where there was difficulty in identifying the presence or extent of invasive carcinoma. The most useful clue was attention to the low power appearances of distorted lobular units in the areas of malignancy and comparison with surrounding breast tissue which usually showed recognizable sclerosing adenosis. The use of immunohistochemical stains for myoepithelium (alpha-actin and S-100 protein) and for basement membrane (collagen type IV and laminin) proved to be of considerable value in identifying the preservation of these features around glandular structures in areas of sclerosing adenosis containing in situ carcinoma.