Cells in the body normally divide (reproduce) only when new cells are needed. Sometimes, cells in a part of the body grow and divide out of control, which creates a mass of tissue called a tumor. If the cells that are growing out of control are normal cells, the tumor is called benign (not cancerous.) If however, the cells that are growing out of control are abnormal and don't function like the body's normal cells, the tumor is called malignant (cancerous).

Cancers are named after the part of the body from which they originate. Breast cancer originates in the breast tissue. Like other cancers, breast cancer can invade and grow into the tissue surrounding the breast. It can also travel to other parts of the body and form new tumors, a process called metastasis.

What Causes Breast Cancer?

We do not know what causes breast cancer, although we do know that certain risk factors may put you at higher risk of developing it. A person's age, genetic factors, personal health history and diet all contribute to breast cancer risk.

Who Gets Breast Cancer?

Breast cancer is the most common cancer among American women, after nonmelanoma skin cancer. Over the past 50 years, the number of women diagnosed with the disease has increased each year. Today, approximately 1 in almost every 8 women (13.4%) will develop breast cancer in her lifetime. Breast cancer is the second-leading cause of cancer death in women after lung cancer -- and is the leading cause of cancer death among women ages 35 to 54. The American Cancer Society estimates that in 2005, approximately 211,240 women will be diagnosed with invasive breast cancer and approximately 40,410 will die. Although these numbers may sound frightening, research reveals that the mortality rate could decrease by 30% if all women age 50 and older who need a mammogram had one.

Only 5%-10% of breast cancers occur in women with a clearly defined genetic predisposition for the disease. The majority of breast cancer cases are "sporadic," meaning there is no direct family history of the disease. The risk for developing breast cancer increases as a woman ages.

VitaDocs Medical Reference provided in collaboration with the Cleveland Clinic