Breast cancer is cancer that begins with the formation of a malignant tumor in the breast.

A malignant tumor is a group of cancer cells that can grow into nearby tissues or spread (metastasize) to other areas of the body.

Prevalence

After skin cancer, breast cancer is the most-commonly diagnosed cancer in women in the United States.

One in eight women will develop invasive breast cancer during their lifetime, according to the American Cancer Society (ACS).

The ACS also reports that breast cancer is the second leading cause of cancer death in women, with lung cancer being the first.

While breast cancer almost entirely affects women, it can also occur in men.

In fact, the National Cancer Institute reports that there about 2,300 new cases of breast cancer in men each year in the United States, compared with about 230,000 new cases in women.

Breast Cancer Risk

While several lifestyle-related, environmental, and hormonal factors may increase your risk of developing breast cancer, none of these factors guarantee you’ll develop the disease.

Moreover, many women who develop the disease have no known risk factors.

Certain traits, however, are known to increase the risk of breast cancer, such as  obesity , personal or family history of breast cancer, and genetic mutations such as BRCA1 and BRCA2 gene mutations.

Breast cancer is most likely caused by a complex interaction of your genetic makeup, your behavior, and your environment.

Breast Cancer in Men

While male breast cancer makes up less than 1 percent of all cases of breast cancer, men of any age can develop the disease. However, it is usually found in men between 60 and 70 years old.

Breast cancer in men is sometimes caused by inherited genetic mutations. Other risk factors include radiation exposure, high levels of estrogen, and a family history of the disease.

Lumps are usually present, and can be felt, in men with breast cancer. Further tests that examine the breasts are used to diagnose the disease.

Survival rates for men with breast cancer are similar to those for women with the disease.

Breast Cancer Survival

Breast cancer rates in women were on the rise for more than two decades (possibly due to the increased use of mammograms) but in 2000 the rates began decreasing, with a 7 percent drop from 2002 to 2003, according to the ACS.

The decline in use of  hormone replacement therapy  to treat symptoms of menopause  is thought to be behind this decrease.

A Women’s Health Initiative study published in 2003 linked the use of hormone therapy to an increased risk of breast cancer.

In addition, death rates from breast cancer have been on the decline since 1989, especially among women under 50.

Earlier detection through screening and increased awareness, as well as a better understanding of the disease and improved treatment, are believed to be responsible for this decline.

The ACS states that there are currently more than 2.8 million breast cancer survivors in the United States, including women still receiving treatment, and those who are done with treatment.