Illustration showing how breast cancer develops

Ductal breast cancer is thought to begin with abnormal tissue growth in a breast duct. A. Normal breast duct is shown. B. An overgrowth of normal cells may develop in the breast duct (hyperplasia). C. Over time, the cells develop abnormalitities and continue accumulating (atypical hyperplasia). D. The abnormal cells may continue to change in appearance and multiply, evolving into ductal carcinoma in situ. E. Eventually the cancer cells grow beyond the breast duct (invasive ductal carcinoma) and can spread to other areas of the body.