Breast cancer is the second most common type of cancer in women. It occurs when abnormal cells grow out of control in one or both breasts. These cancer cells can spread (metastasize) to nearby tissues and form a mass, called a tumor. The cells can spread within the breast, to nearby lymph nodes and other tissues, and to other parts of the body.
The most common type of breast cancer begins in the ducts of the breast. It is called ductal carcinoma.