What Is It?

Breast cancer is a type of uncontrolled growth of abnormal cells that can develop in one of several areas of the breast, including the ducts that carry milk to the nipple, the lobules (small sacs that produce milk) and the nonglandular tissue.

The main forms of invasive breast cancer are:

• Invasive ductal carcinoma - This type of breast cancer develops in the milk ducts and accounts for about 75% of cases. It can break through the duct wall and invade the breast's fatty tissue, then spread (metastasize) to other parts of the body through the bloodstream or lymphatic system.

• Invasive lobular carcinoma - This type of breast cancer accounts for about 15% of cases and originates in the breast's milk-producing lobules. It also can spread to the breast's fatty tissue and other places in the body.

• Medullary, mucinous and tubular carcinomas - These are three slow-growing types of breast cancer that represent about 8% of all breast cancers.

• Paget's disease - This type represents about 1% of breast cancers. It starts in the milk ducts of the nipple and can spread to the areola (dark circle around the nipple). Women who get Paget's disease usually have a history of nipple crusting, scaling, itching or inflammation.

• Inflammatory carcinoma -This type accounts for about 1% of all cases. Of all breast cancers, inflammatory carcinoma is the most aggressive and difficult to treat, because it spreads so quickly.

As more women have regular mammograms, doctors are also detecting many more noninvasive or precancerous conditions before they become full-blown cancer. These conditions include:

• Ductal carcinoma in situ (DCIS) - DCIS occurs when cancer cells fill the ducts but haven't spread through the walls into fatty tissue. Nearly all women diagnosed at this early stage can be cured. Without treatment, about 25% of DCIS cases will lead to invasive breast cancer within 10 years.

• Lobular carcinoma in situ (LCIS) - LCIS is less of a threat than DCIS. It develops in the breast's milk-producing lobules. LCIS doesn't require treatment, but it does increase a woman's risk of developing cancer in other areas of both breasts.