BACKGROUND Increasing interest in the intraductal approach to the breast has necessitated revisiting the anatomy of the breast. METHODS Using six different complementary in vivo and in vitro approaches, the authors determined the number, distribution, and anatomic properties of the ductal systems of the breast, which extend from the nipple orifices to the terminal duct lobular units. RESULTS More than 90% of all nipples examined contained 5–9 ductal orifices, generally arranged as a central group and a peripheral group. Each nipple orifice communicated with a separate, nonanastomosing ductal system, which extended to the terminal duct lobular unit. CONCLUSIONS Increased knowledge of the ductal anatomy of the breast and the ability to access the nipple ductal orifices will provide a foundation for the intraductal approach to the breast. Cancer 2004. © 2004 American Cancer Society

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