Breast cancer develops when abnormal breast cells begin to grow out of control. When a group of these cells band together, they form a mass called a tumor. Benign tumors do not spread and are usually not harmful. Malignant tumors, however, spread from their sources and can grow into life-threatening cancers. When amalignant cells leave the breast and invade other parts of the body — a process called metastasis — the chance of successfully treating the disease is greatly diminished.

The majority of breast cancers occur in women- but men can also develop the disease. Still, breast cancer occurs almost entirely in women, and factors relating to the female body are known to affect breast cancer risk.

Recent scientific studies suggest that ovarian hormones play a role in the development of breast cancer. Estrogen, a hormone produced in the ovaries, induces the sex-related changes in women's bodies, including the growth of breast cells. Elevated estrogen levels have been linked to breast cancer risk. This association is partly based on evidence that women who develop breast cancer generally have higher levels of estrogen circulating in their bloodstream than do women without breast cancer. Future studies will assess the degree to which behaviors proven to decrease the body's production of estrogen protect against breast cancer, such as eating a low-fat diet and exercising on a regular basis.

Knowing about the type of cells found in breast tissue is important for understanding the differences among various types of breast cancer. The female breast is comprised of the following types of cells:

Lobules: milk-producing glands
Ducts: tube-like structures that connect lobules and the nipple
Stroma: fatty tissue and ligaments that cover the ducts, lobules, blood vessels and lymphatic vessels (passageways that carry fluid containing immune cells and waste products)

Most breast lumps are benign tumors caused by changes in cysts (fluid-filled sacs found in the breast). The formation of scar tissue around sacs can form lumps, which may result in swelling of the breast, but are often not cancerous.