Breast cancer is a malignant tumor that starts from cells of the breast. A malignant tumor is a group of cancer cells that may invade surrounding tissues or spread (metastasize) to distant areas of the body. The disease occurs almost entirely in women, but men can get it, too.
The remainder of this document refers only to breast cancer in women. For information on breast cancer in men, see the American Cancer Society's document, Breast Cancer in Men.
Normal Breast Structure
In order to understand breast cancer, it is helpful to have some basic knowledge about the normal structure of the breasts.
The female breast is made up mainly of lobules (milk-producing glands), ducts (tiny tubes that carry the milk from the lobules to the nipple), and stroma (fatty tissue and connective tissue surrounding the ducts and lobules, blood vessels, and lymphatic vessels).
Most breast cancers begin in the cells that line the ducts (ductal cancers); some begin in the cells that line the lobules (lobular cancers), and the rest in other tissues.
The Lymph (Lymphatic) System
The lymph system is important to understand because it is one of the ways in which breast cancers can spread. This system has several parts.
Lymph nodes are small, bean-shaped collections of immune system cells that are connected by lymphatic vessels. Lymphatic vessels are like small veins, except that they carry a clear fluid called lymph (instead of blood) away from the breast. Lymph contains tissue fluid and waste products, as well as immune system cells (cells that are important in fighting infections). Breast cancer cells can enter lymphatic vessels and begin to grow in lymph nodes.
Most lymphatic vessels in the breast connect to lymph nodes under the arm (axillary nodes). Some lymphatic vessels connect to lymph nodes inside the chest (internal mammary nodes) and those either above or below the collarbone (supraclavicular or infraclavicular nodes).
Knowing if the cancer cells have spread to lymph nodes is important because if it has, there is a higher chance that the cells could have also gotten into the bloodstream and spread (metastasized) to other sites in the body. The more lymph nodes that are involved with the breast cancer, the more likely it is that the cancer may be found in other organs as well. This is important to know because it could affect your treatment plan. But not all women with lymph node involvement develop metastases, and it is not unusual for a woman to have negative lymph nodes and later develop metastases.
Benign Breast Lumps
Most breast lumps are not cancerous; that is, they are benign. Still, some need to be sampled and viewed under a microscope to prove they are not cancer.
Most lumps turn out to be fibrocystic changes. The term "fibrocystic" refers to fibrosis and cysts. Fibrosis is the formation of fibrous (or scar-like) tissue, and cysts are fluid-filled sacs. Fibrocystic changes can cause breast swelling and pain. This often happens just before a period is about to begin. Your breasts may feel lumpy and, sometimes, you may notice a clear or slightly cloudy nipple discharge.
Other Benign Breast Lumps
Benign breast tumors such as fibroadenomas or intraductal papillomas are abnormal growths, but they are not cancer and cannot spread outside of the breast to other organs. They are not life threatening. Still, some benign breast conditions are important because women with these conditions have a higher risk of developing breast cancer.
For more information see the section, "What Are the Risk Factors for Breast Cancer?" and the American Cancer Society document, Noncancerous Breast Conditions.
Breast Cancer General Terms
It is important to understand some of the key words used to describe breast cancer.
This is a term used to describe a cancer that begins in the lining layer (epithelial cells) of organs such as the breast. Nearly all breast cancers are carcinomas (either ductal carcinomas or lobular carcinomas).
An adenocarcinoma is a type of carcinoma that starts in glandular tissue (tissue that makes and secretes a substance). The ducts and lobules of the breast are glandular tissue (they make breast milk), so cancers starting in these areas are sometimes called adenocarcinomas.
Carcinoma In Situ
This term is used for the early stage of cancer, when it is confined to the layer of cells where it began. Specifically in breast cancer, in situ means that the cancer cells remain confined to ducts (ductal carcinoma in situ) or lobules (lobular carcinoma in situ). They have not invaded into deeper tissues in the breast or spread to other organs in the body, and are sometimes referred to as non-invasive breast cancers.
Invasive (Infiltrating) Carcinoma
An invasive cancer is one that has already invaded beyond the layer of cells where it started (as opposed to carcinoma in situ). Most breast cancers are invasive carcinomas -- either invasive ductal carcinoma or invasive lobular carcinoma.
Sarcomas are cancers that start from connective tissues such as fat tissue or blood vessels. Sarcomas of the breast are rare.
Types of Breast Cancers
There are several types of breast cancer, although some of them are quite rare. It is not unusual for a single breast tumor to be a combination of these types and to have a mixture of invasive and in situ cancer.
Ductal Carcinoma In Situ (DCIS)
Ductal carcinoma in situ (also known as intraductal carcinoma) is the most common type of non-invasive breast cancer. DCIS means that the cancer cells are inside the ducts but have not spread through the walls of the ducts into the surrounding breast tissue.
About 1 out of 5 new breast cancer cases will be DCIS. Nearly all women diagnosed at this early stage of breast cancer can be cured. A mammogram is often the best way to find DCIS early.
When DCIS is diagnosed, the pathologist (a doctor specializing in diagnosing disease from tissue samples) will look for an area of dead or dying cancer cells, called tumor necrosis, within the tissue sample. If necrosis is present, the tumor is likely to be more aggressive. The term comedocarcinoma is often used to describe DCIS with necrosis.