In the United States, breast cancer is the most common cancer in women (excluding skin cancer). Men can also develop breast cancer, but breast cancer in men is rare, accounting for less than 1% of all breast cancers.
About the breast
The breast is mostly made up of fatty tissue. Within this tissue is a network of lobes, which are made up of tiny, tube-like structures called lobules that contain milk glands. Tiny ducts connect the glands, lobules, and lobes, carrying the milk from the lobes to the nipple, located in the middle of the areola (darker area that surrounds the nipple). Blood and lymph vessels also run throughout the breast; blood nourishes the cells, and the lymph system drains bodily waste products. The lymph vessels connect to lymph nodes, the tiny, bean-shaped organs that help fight infection.
About breast cancer
Cancer begins when normal cells in the breast change and grow uncontrollably, forming a mass called a tumor. A tumor can be benign (noncancerous) or malignant (cancerous, meaning it can spread to other parts of the body).
Breast cancer spreads when the cancer grows into other parts of the body or when breast cancer cells move to other parts of the body through the blood vessels and/or lymph vessels. This is called metastasis. Breast cancer most commonly spreads to the regional lymph nodes. The lymph nodes can be axillary (located under the arm), cervical (located in the neck), internal mammary (located under the chest bone), or supraclavicular (located just above the collarbone). When it spreads further through the body, it most commonly spreads to the bones, lungs, and liver. Less commonly, breast cancer may spread to the brain. The cancer can also recur (come back after treatment) locally in the skin, in the same breast (if it was not removed as part of treatment), other tissues of the chest, or elsewhere in the body.
Types of breast cancer
Most breast cancers start in the ducts or lobes. Almost 75% of all breast cancers begin in the cells lining the milk ducts and are called ductal carcinomas. Cancer that begins in the lobules is called lobular carcinoma. The difference between ductal and lobular cancer is determined by a pathologist (a doctor who specializes in interpreting laboratory tests and evaluating cells, tissues, and organs to diagnose disease), who examines a tumor sample removed during a biopsy (see Diagnosis).
If the disease has spread outside of the duct or lobule and into the surrounding tissue, it is called invasive or infiltrating ductal or lobular carcinoma. Cancer that is located only in the duct or lobule is called in situ, meaning “in place.” How in situ disease grows and spreads, as well as how it is treated, depends on whether it is ductal carcinoma in situ (DCIS) or lobular carcinoma in situ (LCIS).
Most in situ breast cancers are DCIS. Currently, oncologists recommend surgery to remove DCIS to help prevent the cancer from becoming an invasive breast cancer and spreading to other parts of the breast or the body. Radiation therapy and hormonal therapy may also be recommended for DCIS (see Treatment for more information).
LCIS is not considered cancer and is usually monitored by the doctor. LCIS in one breast is a risk factor for developing invasive breast cancer in both breasts (see the Risk Factors section for more information).
Other, less common types of breast cancer include medullary, mucinous, tubular, metaplastic, and papillary breast cancer, as well as other even less common types. Inflammatory breast cancer is a faster-growing type of cancer that accounts for about 1% to 5% of all breast cancers. It may be misdiagnosed as a breast infection because there is often swelling of the breast and redness of the breast skin that starts suddenly. Paget’s disease is a type of cancer that begins in the ducts of the nipple. The skin often appears scaly and may be itchy. Although it is usually in situ, it can also be an invasive cancer.
Find out more about basic cancer terms used in this section.
Looking for More of an Overview?
If you would like additional introductory information, explore these related items on Cancer.Net:
- ASCO Answers Fact Sheet: Read a one-page fact sheet (available in PDF) that offers an easy-to-print introduction for this type of cancer.
- ASCO Answers Guide: This 52-page booklet (available as a PDF) helps newly diagnosed patients better understand their disease and treatment options, as well as keep track of the specifics of their individual cancer care plan.
- Cancer.Net Patient Education Video: View a short video led by an ASCO expert in this type of cancer that provides basic information and areas of research.
- Cancer.Net En Español: Read about breast cancer in Spanish. Infórmase sobre cáncer de mama en español.
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