What Causes Breast Cancer?

Although the precise causes of breast cancer are unclear, we know what the main risk factors are. Still, most women considered at high risk for breast cancer do not get it, while many who do have no known risk factors.  Among the most significant factors are advancing age and a family history of breast cancer.  Risk increases slightly for a woman who has had a benign breast lump and increases significantly for a woman who has previously had cancer of the breast or the ovaries. 

A woman whose mother, sister, or daughter has had breast cancer is two to three times more likely to develop the disease, particularly if more than one first-degree relative has been affected.  Researchers have now identified two genes responsible for some instances of familial breast cancer -- called BRCA1 and BRCA2.  About one woman in 200 carries it.  Having the gene predisposes a woman to breast cancer but does not ensure that she will get it.

Generally, women over 50 are more likely to get breast cancer than younger women, and African-American women are more likely than Caucasions to get breast cancer before menopause.

A link between breast cancer and hormones is gradually becoming clearer. Researchers think that the greater a woman's exposure to the hormone estrogen, the more susceptible she is to breast cancer. Estrogen tells cells to divide; the more the cells divide, the more likely they are to be abnormal in some way, possibly becoming cancerous.

A woman's exposure to estrogen and progesterone rises and falls during her lifetime, influenced by the age she starts and stops menstruating, the average length of her menstrual cycle, and her age at first childbirth.  A woman's risk for breast cancer is increased if she starts menstruating before age 12, has her first child after 30, stops menstruating after 55, or has a menstrual cycle shorter or longer than the average 26-29 days.  Current information indicates that the hormones in birth control pills probably do not increase the risk.  Some studies suggest that taking hormone replacement therapy after menopause may increase risk, especially when taken for more than 5 years. The jury is still somewhat out on this matter though.  Heavy doses of radiation therapy may also be a factor, but low-dose mammograms pose almost no risk.

The link between diet and breast cancer is debated. Obesity is a noteworthy risk factor, and drinking alcohol regularly -- more than a couple of drinks a day -- may promote the disease. Many studies have shown that women whose diets are high in fat are more likely to get the disease. Researchers suspect that if a woman lowers her daily calories from fat -- to less than 20%-30% -- her diet may help protect her from developing breast cancer.