Cancer is a disease in which cells do not grow the way they are supposed to. Normally, the cells divide in a controlled manner. 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. It will not spread to other parts of the body. A malignant tumor is cancer. Cancer spreads and causes damage to the area around it. The cancer cells can also enter the lymph and blood streams. This can spread cancer to other parts of the body.
Breast cancer is the most common form of cancer found in women. However, lung cancer claims far more lives. The majority of breast cancer cases are in women, but breast cancer can happen in men. This fact sheet focuses on breast cancer in women.
Normal Anatomy and the Development of Breast Cancer
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, but the most common places are in the ducts and lobules. The cancer cells may eventually form a tumor. The tumor can invade nearby tissue such as the chest wall or lymph glands.
Lymph vessels deliver fluid from around the tissue and put it back into the bloodstream. The lymph is part of the body's immune system. The lymph nodes have certain blood cells that fight off infection. The lymph vessels around the breast lead to lymph nodes under the arm, above the collarbone, and in the chest. This means they can carry cancer cells away from the original tumor site, and spread it to other lymph tissue or other parts of the body.
Breast cancer can start in more than one way and may affect many parts of the breast. The type of cancer and where it is found impacts how fast it spreads and how it is treated.
Most breast cancers are carcinomas—These tumors grow out of the surface or lining of the glandular tissue of the breast.
Very rare types of breast cancer are formed in other types of tissue. These are called sarcomas, acinar tumors, or lymphomas.
Breast cancer can also be grouped by how fast or deep they grow.
In situ cancers are localized. This means the cancer is contained to the original site and has not spread. In situ cancers are treated where the tumor is and offer the best chance for a cure.
Invasive cancers start to spread away from the original site to nearby structures or to other parts of the body. How far the cancer has spread depends how long it has been growing.
Ductal carcinoma in situ
(DCIS)—Starts in milk ducts. This form is mainly seen on a
mammogram. Unusual calcium deposits or puckering of the breast tissue are common traits. This type of cancer has a high cure rate. However if left untreated, it will become more invasive.
Lobular carcinoma in situ
(LCIS)—LCIS is not cancer. It is found by accident when looking for something else. It is considered a marker for breast cancer risk. Women with LCIS have a higher risk of having breast cancer (usually invasive lobular carcinoma) over the next 20 years.
LCIS does not need to be treated. Your doctor will watch for changes in LCIS with regular check-ups and tests.
Ductal carcinoma —This is the most common form of breast cancer. Nearly 8 in 100 women will have this type. This cancer starts in the milk ducts.
Lobular carcinoma —This starts 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 how they look in a lab.
Inflammatory breast cancer —This type is not common, but it is fast growing and hard to treat. Cancer cells invade the lymphatic vessels of the skin and is very likely to spread to the local lymph nodes.
Paget disease is cancer of the areola and nipple. It is very rare. Paget does not start in glandular breast tissue, but it can be linked to the other breast cancer types. In most cases, women who have this type of cancer have a history of nipple crusting, scaling, itching, or inflammation.
Breast cancer. Merck Manual Professional Version website. Available at: https://www.merckmanuals.com/professional/gynecology-and-obstetrics/breast-disorders/breast-cancer. Updated January 2018. Accessed March 12, 2019.
General information about breast cancer. National Cancer Institute website. Available at: https://www.cancer.gov/types/breast/patient/breast-treatment-pdq#_125. Updated February 6, 2019. Accessed March 12, 2019.
What is breast cancer? American Cancer Society website. Available at: https://www.cancer.org/cancer/breast-cancer/about/what-is-breast-cancer.html. Updated September 21, 2017. Accessed March 12, 2019.
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