Causes & Risks


What causes breast cancer? Unfortunately, there is no clear answer to this question — only theories.

One of the more widely accepted theories suggests that for a small percentage of woman with breast cancer there are certain genes within breast cells — inherited by some women — that can be linked to a higher risk for developing breast cancer. Most breast cancers are not genetically passed down in families and are called “sporadic.”

Current research is also looking at many environmental, dietary and drug factors as possible causes, but these have not yet been conclusively proven.

Risk Factors

Being a woman is the greatest risk factor for breast cancer. While men can and do develop breast cancer, there are only about 1,000 new cases of male breast cancer diagnosed annually in the United States – 1% of all breast cancers. Age, as well as family, personal and reproductive history, may also affect a woman's chance of developing breast cancer.


The risk of developing breast cancer increases with age. About 80 percent of all breast cancers occur in women over the age of 50. The disease is uncommon in women under the age of 35.

Family history

A woman whose mother, sister or daughter has had breast cancer is at higher risk. The risk is increased if the family member's cancer affected both breasts or was diagnosed before menopause. That's when a form of breast cancer that tends to run in families is more common. Women who carry certain inherited genes are at higher risk than other women of developing breast cancer in their lifetime. The most common genes identified so far are BRCA 1 and BRCA 2, but there are some less common genes and others are being studied. This "hereditary" type of breast cancer accounts for approximately 5 to 10 percent of all breast-cancer cases.

Personal history

Women who have had breast cancer in one breast, or who have had certain types of benign (non-cancerous) breast disease – such as atypical cells on a breast biopsy, are also at greater risk.

Reproductive history: The risk of breast cancer is slightly greater among women who started their periods before age 12, who went through menopause after the age of 50, or who did not have children before the age of 30.

Other factors

Recently, a major national study found that women taking hormone replacement therapy (HRT) — particularly those taking combined estrogen and progestin HRT for five years or longer — had an increased risk of developing breast cancer. Women who have been on estrogen replacement for five or more years should talk to their health-care provider about the risks and benefits of treatment. Women with a history of Hodgkin’s Disease treated with mantle radiation therapy also have an increased risk of developing breast cancer.

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