Breast cancer in men is rare, accounting for less than 1% of all breast cancer cases. Although breast cancer in men occurs less frequently than breast cancer in women, the diseases are similar in many ways. Cancer begins when normal cells in the breast begin to 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).
The breast is mainly composed 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 to the nipple, located in the middle of the areola (darker area that surrounds the nipple of the breast). Blood and lymph vessels run throughout the breast; blood nourishes the cells, and the lymph system drains bodily waste products. The lymph vessels connect to lymph nodes, which are tiny, bean-shaped organs that normally help fight infection.
The main types of breast cancer are the same for men and women. Most breast cancer cases 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. Approximately 25% of male breast cancers are lobular carcinoma (cancer that begins in the lobules).
A type of breast cancer that has spread outside of the duct and into the surrounding tissue is called invasive or infiltrating carcinoma. The majority of male breast cancer cases are infiltrating ductal carcinomas (IDC).
Disease that has not spread is called in situ, meaning "in place." Ductal carcinoma in situ (DCIS) is the most common type of in situ breast cancer, but it is uncommon in men.
Inflammatory breast cancer  makes up about 1% to 5% of all breast cancers. Paget's disease of the nipple begins in the ducts, but spreads to the skin of the nipple. Paget's disease is more common in men than in women. Other, less common subtypes of breast cancer include medullary, mucinous, tubular, or papillary.
Cancer may begin as a single, genetically abnormal cell. As this one cell divides, it eventually becomes a tumor and develops a blood supply to nourish its continued growth. At some point, cells may break off from the primary mass and move to other parts of the body in a process called metastasis.
Breast cancer spreads when breast cancer cells move to other sites in the body through the blood vessels and/or lymph vessels. A common site of spread is the regional lymph nodes. The lymph nodes can be axillary (located under the arm), mediastinal (under the sternum or breast bone), or supraclavicular (located just above the collarbone). The most common sites of distant metastasis are 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.
Breast cancer in men is detected the same way as breast cancer in women isâthrough self-examination, clinical examination, or mammography (x-ray of the breast). Changes in the breast may be easier to detect because men have less breast tissue. However, the awareness of breast cancer in men is much lower than it is in women; therefore, men may not perform regular breast self-examinations or talk with their doctor about the disease.
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.
- 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.
Find out more about basic cancer terms used in this section .