Cancer is a disease in which cells grow in an abnormal way. Normally, the cells divide in a controlled manner to replace old or damaged cells. If cells keep dividing when new cells are not needed, a mass of tissue called a tumor forms.

A tumor can be benign or malignant. A benign tumor is not cancer and will not spread to other parts of the body. A malignant tumor is cancer. Cancer cells invade and damage tissue around them. The cancer cells can also enter the lymph and blood streams, spreading to other parts of the body. Breast cancer is the development of malignant cells in the breast tissue.

Cancer Cell Growth

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Breast cancer is the most common form of cancer found in women, though lung cancer claims far more lives. Although the majority of breast cancer cases occur in women, it occurs in men as well. This fact sheet focuses on breast cancer in women.

A breast consists of glandular tissue called lobes. These lobes are divided into lobules, which can produce milk. Milk is carried from the lobules to the nipple by small ducts. All of this is surrounded by fatty and connective tissue, as well as blood and lymph vessels.

Breast cancer can start anywhere in the breast tissue, but the most common places are in the glandular tissue like ducts and lobules. The cancer cells may eventually form a tumor, or invade nearby tissue, such as the chest wall or lymph glands.

Lymph vessels delivers fluid from around the tissue, back into the bloodstream. When it detects foreign matter like bacteria or viruses, lymph nodes will help create blood cells to fight the infection. The lymph can also carry cancer cells away from the original tumor site and allow cancer to travel to other areas of the lymph or body. The lymph vessels around the breast lead to lymph nodes under the arm, above the collarbone, and in the chest. Cancer that has spread to other areas of the body is called metastatic cancer. The most common sites for metastatic breast cancer are the bones, lung, brain, and liver.

Breast Cancer

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Breast cancer can develop in different ways and may affect different parts of the breast. The location of cancer will affect the progression of cancer and the treatment.

  • Most breast cancers are carcinomas—Malignant tumors that grow out of the surface or lining of the glandular tissue of the breast.
  • Other very rare types of breast cancer are formed in the surrounding and supporting tissues. These are referred to as sarcomas, acinar tumors, or lymphomas.

Breast cancer can also be classified by its invasiveness.

  • In situ cancers are localized. This means the cancer is contained to the tissue is originated from only, and has not spread. Treatment for in situ cancers are generally local and in most cases, curable.
  • However, invasive cancers have begun to spread beyond the primary site. Depending on how long the cancer has been growing, invasion can occur in adjacent tissue or distant sites in the body.
  • Ductal carcinoma in situ (DCIS)—Develops in milk ducts. More common diagnosis now because this form is generally seen on a mammogram. It is identified by unusual calcium deposits or puckering of the breast tissue (called stellate appearance). This type of cancer has a high cure rate. However, if left untreated, DCIS will progress to a more invasive breast cancer.
  • Lobular carcinoma in situ (LCIS)—Unlike DCIS, LCIS is not really cancer at all. Most doctors consider the finding of LCIS to be incidental, and it is thought to be a marker for breast cancer risk. Women with LCIS seem to have a 7-10 times increased risk of developing some form of breast cancer (usually invasive lobular carcinoma) over the next 20 years.

LCIS does not warrant treatment by surgery or radiation therapy. Your doctor will monitor the LCIS with regular check-ups and tests to see if there is any further development.

  • Ductal carcinoma—This is the most common form of breast cancer and accounts for 80% of breast cancer cases. This cancer develops in the milk ducts.
  • Lobular carcinoma—This originates in the milk-producing lobules of the breast. It can spread to the fatty tissue and other parts of the body.
  • Medullary, mucinous, and tubular carcinomas—These are 3 relatively slower-growing types of breast cancer. They are named based on their appearance under a microscope.
  • Inflammatory breast cancer—Though relatively uncommon, inflammatory breast cancer is fast growing and difficult to treat. Cancer cells invade the lymphatic vessels of the skin and can be very extensive. It is very likely to spread to the local lymph nodes.

Paget's disease is cancer of the areola and nipple. It is very rare, occurring in less than 1% of all breast cancers. Although Paget's does not arise from glandular tissue in the breast, it can be associated with both in situ and invasive breast cancers. Generally, women who develop this type of cancer have a history of nipple crusting, scaling, itching, or inflammation.