Breast cancer is cancer that starts in the cells of the breast. It occurs most often in the cells that make up the ducts and glands of the breast whose job is to make and release milk. When these cells start to lose control over how fast they divide, they expand in numbers to form a solid mass called a tumor. However, at this point the tumor is not necessarily cancer: a tumor is only considered cancerous if it moves beyond the borders of the tissue in which it originated. In breast cancer, this usually involves invading the fatty tissues that surround and support the ducts and glands. Even when this occurs and the tumor is cancerous, the disease is still usually curable at this point. However, if cells from the tumor start to break free and spread around the body (called metastasis) the cancer becomes more difficult to treat.

Breast cancer is the most common cancer in American women (besides non-melanoma skin cancers) and is the second leading cause of female cancer deaths (behind lung cancer). The National Cancer Institute estimates that in 2008 more than 180,000 women will develop breast cancer in the United States and around 40,000 women will die of the disease. Men can also get breast cancer but the annual rate in the U.S. is less than 2,000 cases with around 500 deaths.

What are the different types of breast cancer?

In order to understand breast cancer, it helps to know a little about the anatomy of the breast. In women, each breast is made up of around 15 to 20 lobes - each of which are collections of small “lobules” which themselves contain the glands capable of producing milk. The lobules are each connected to ducts which carry the milk out to the surface of the skin at the nipple. Fatty connective tissues hold the duct and gland tissue together and also provide the framework for a network of blood vessels and a web of lymph vessels that help screen the breast tissue for foreign substances circulating in the lymph.

There are several different types of breast cancer. Each affects different cells in the breast. The main types are discussed below.

Breast cancer terminology

The various terms used to identify cancers can be a mouthful. The definitions below should help make them a little more understandable:

• Carcinoma — a cancer that starts in a type of tissue called epithelial tissue which includes all the cells that line the inside passageways and the cells that make up glands.
• Adenocarcinoma — a cancer that originates in glandular epithelial cells. This term is sometimes used to describe breast cancers of the ducts and glands.
• Carcinoma in situ — a type of “pre-cancer” in which epithelial cells divide abnormally but have not yet spread beyond the layer of cells in which it began.
• Invasive carcinoma — a cancer that originates in epithelial cells but has grown beyond the confines of the layer in which it started and invaded the surrounding tissue. Most breast cancers are invasive carcinomas.
• Sarcoma — a cancer that originates in connective tissues such as fat or blood vessels. Sarcomas of the breast are rare.

Benign breast conditions

There are conditions of the breast that are benign (i.e. not cancerous):

Atypical hyperplasia

Atypical hyperplasia is a condition in which cells in the breast have undergone mutations that caused them divide abnormally and to look different than they normally do, but they haven’t yet accumulated the mutations that allow the cells to become cancerous. However, after awhile atypical hyperplasia may progress to become carcinoma in situ.

Carcinoma in situ

In carcinoma in situ, cells have begun to proliferate rapidly, have undergone changes in their appearance, and have collected enough mutations to be capable of invading surrounding tissue. However, because the dividing cells have not spread beyond the tissue in which they started and invaded the surrounding fatty and connective tissue, carcinoma in situ is not cancer. This condition is really a “pre-cancer” because it may progress to become cancer in some cases.

In ductal carcinoma in situ (DCIS), cells in one of the ducts begin dividing rapidly and form a small mass but have not yet grown beyond the walls of the ducts (in situmeans “in tissue”, meaning the abnormal cells have stayed within the borders of the original tissue). If the cells do invade the surrounding tissue, this step marks the transition to cancer. This may happen if the DCIS is not treated. However, when caught before the transition to cancer occurs, nearly all cases of DCIS are curable.

Like ductal carcinoma in situ, lobular carcinoma in situ (LCIS) is a pre-cancer, only in this case it is the cells of the lobules that are abnormal. As long as they do not invade the surrounding tissue in the breast, it is not cancer. This condition rarely progresses to become cancer and when it’s caught before this occurs, it is very curable.

Invasive breast cancers

Invasive ductal carcinoma

Invasive ductal carcinoma is the most common type of breast cancer, accounting for about 80% of all invasive cancers. Unlike ductal carcinoma in situ, cells in invasive ductal carcinoma have not only proliferated rapidly, they have broken through the wall of the duct and invaded the surrounding breast tissue. Early in this disease, the tumor formed by the rapidly dividing cells may not be palpable, but it may be visible in a mammogram. If it has not yet spread to other parts of the body, this condition in still very treatable.

Invasive lobular carcinoma

Invasive lobular carcinoma starts in the glandular tissue of the lobules and is called invasive when it has spread beyond the layer of gland tissue in which it started. It may progress and spread to other parts of the body but is quite treatable if caught before that happens. Invasive lobular carcinoma accounts for around 10% of breast cancer cases.

Medullary carcinoma

Medullary carcinoma begins in the connective tissue (or “stromal”) cells in the breast and accounts for approximately 5% of all breast cancer. The prognosis for medullary carcinoma is slightly better than for other types of invasive carcinomas.

Inflammatory breast cancer (IBC)

Inflammatory breast cancer is a term used to describe carcinoma that also involves the dermal lymph vessels in the skin of the breast. In this condition the cancer cells may not form a tumor, but the cancerous cells always invade the lymph vessels in the skin of the breast. This causes the skin to thicken and turn red in places, making it appear inflamed (hence the name inflammatory breast cancer). However there is no actual inflammation. Nonetheless, the breast may feel warm, become tender or itchy, become larger or firmer, or even take on a pitted appearance like an orange peel. In its early stages, IBC may be mistaken for an infection of the breast or a reaction to blocked milk ducts. Even worse, since there is often no individual lump associated with IBC, it may not appear on a mammogram making it more difficult to catch in its early stages. It also spreads more quickly and easily than the other types of cancer. Not surprisingly, IBC has a poorer prognosis than other invasive carcinomas.