What Is Breast Cancer?

Breast 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).

Breast cancer is the most common cancer in women (excluding skin cancer) and the second most common cause of death from cancer in women, after lung cancer, in the United States. Men can also develop breast cancer, but it's rare, accounting for less than one percent of all breast cancer cases.

How does breast cancer develop?

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 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.

About 90 percent of all breast cancer cases start in the ducts or lobes. Almost 75 percent 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. If the disease has spread outside of the duct and into the surrounding tissue, it is called invasive or infiltrating ductal carcinoma. If the disease has spread outside of the lobule, it is called invasive or infiltrating lobular carcinoma. Disease that has not spread is called in situ, meaning “in place.”

Other types of breast cancer

Other, less common cancers of the breast include medullary, mucinous, tubular, metaplastic and papillary breast cancer.

Inflammatory breast cancer is a faster-growing type of cancer that accounts for about one to five percent 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.

Paget disease is a type of in situ cancer that can begin in the ducts of the nipple. The skin often appears scaly and may be itchy.