Ductal carcinoma in situ is a proliferation of malignant epithelial cells within the ductolobular system of the breast that show no light microscopic evidence of invasion through the basement membrane into the surrounding stroma (fig 1). Several forms of histological architecture are recognised, the most common of which are comedo, cribriform, solid, micropapillary, and papillary.

Until recently, ductal carcinoma in situ was a relatively uncommon disease, representing only about 1% of all newly diagnosed cases of breast cancer.1 It was usually regarded as a single disease with a single treatment, namely, mastectomy. Most patients presenting with ductal carcinoma in situ had symptoms—a palpable mass or discharge from the nipple. During the past decade, as mammography has become more widely used and technically better, the number of new cases has increased dramatically. Most patients now present with lesions that are not palpable and are clinically occult. Furthermore, the notion of ductal carcinoma in situ as a single disease has evaporated. It is now well recognised as a heterogeneous group of lesions with a diverse malignant potential. As our understanding of the disease has evolved and the range of treatment options has widened, the process of making decisions about management has become more complex and controversial. Ductal carcinoma in situ has become so common and so confusing that the first textbook devoted solely to the disorder was not published until 1997.2