Breast cancers are often described in terms of the tissues in which they develop and the degree to which the tumors have spread.
Almost all breast cancers are classified as adenocarcinomas, tumors that begin in glandular tissue. Breast adenocarcinomas generally originate either in ducts (ductal carcinomas) or in lobules (lobular carcinomas).
The most common noninvasive breast cancers are ductal carcinomas in situ. In situ is a term used to describe tumors that have not spread outside their original locations. Ductal carcinomas in situ are confined to the breast ducts. Almost all women diagnosed with this nonmalignant stage of breast cancer can be cured. Lobular carcinoma in situ is not usually technically classified as a breast cancer. This early grade of tumor involves abnormal cells that do not penetrate through the walls of the lobules. Although these growths are not life-threatening. women who have them are at a greater risk of developing breast cancer in another part of the same breast or in the other breast.
Approximately 80 percent of invasive breast cancers are invasive ductal carcinomas, tumors that have spread from the duct to the surrounding fatty tissue. Invasive lobular carcinomas, which start in the milk-producing glands and spread to other tissue, account for 10 to 15 percent of invasive breast cancers. These cancers are more difficult to detect using mammograms than ductal carcinomas.
Five percent of invasive breast cancers are medullary carcinomas. which are characterized by a distinct boundary marked by immune cells and by larger-sized cancer cells. The chance of survival associated with medullary carcinomas is higher than for other invasive breast cancers.
Other types of invasive breast cancer that occur less frequently include tubular carcinomas , inflammatory breast cancer , mucinous carcinoma (formed by mucus-producing cells), Paget's disease of the nipple, and phyllodes tumor (made of the breast's connective tissue).