Ductal carcinoma in situ: introduction of the concept of ductal intraepithelial neoplasia.

: The current definition, diagnostic criteria, grading, and approach to assessment of extent of ductal carcinoma in situ (DCIS) are presented. The problem areas, particularly the difficulties in separating low-grade DCIS from atypical intraductal hyperplasia (AIDH), accurate assessment of size and/or extent of DCIS, and their impact on patient management are critically reviewed. On the basis of the route of progression of DCIS, recently confirmed by three-dimensional reconstructed models, an optimal and simple approach to uniform excision, orientation, and processing of biopsy samples is presented. Emphasis is placed on the role of intraductal proliferative lesions (IDH, AIDH, and DCIS) as risk factors of variable magnitude in subsequent development of invasive breast carcinoma. It is proposed that these proliferations should be classified as "mammary intraepithelial neoplasia, ductal type" or as "ductal intraepithelial neoplasia" (DIN); the rational for the application of this classification system is provided. This approach obviates the current separation of AIDH and low-grade DCIS into two very drastically different categories of cancer and non-cancer without interfering with appropriate management of the various lesions. The DIN classification is presented in a simple translational table, along with the current terminology for various lesions.