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Understanding Breast Cancer -- the Basics

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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. On the other hand, 75% of women who develop breast cancer have no known risk factors. Among the most significant factors are advancing age and  family history. Risk increases slightly for a woman who has certain benign breast lumps and increases significantly for a woman who has previously had breast cancer or endometrial, ovarian, or colon cancer.

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. This is especially true if the cancer developed in the woman while she was premenopausal, or if the cancer developed in both breasts. Researchers have now identified two genes responsible for some instances of familial breast cancer -- BRCA1 and BRCA2. About one woman in 200 carries one of these genes. Having a BRCA 1 or BRCA 2 gene predisposes a woman to breast cancer and -- while it does not ensure that she will get breast cancer -- her lifetime risk is 56%-85%. These genes also predispose to ovarian cancer and are associated with pancreas cancer, melanoma, and male breast cancer (BRCA2).

Because of these risks, prevention strategies and screening guidelines for those with the BRCA genes are more aggressive. There are other genes that have been identified as increasing the risk of breast cancer, including the PTEN gene, the ATM gene, the TP53 gene, and the CHEK2 gene. However, these genes carry a lower risk for breast cancer development than the BRCA genes.

Generally, women over 50 are more likely to get breast cancer than younger women, and African-American women are more likely than Caucasians 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, potentially becoming cancerous.

A woman's exposure to estrogen and progesterone rises and falls during her lifetime. This is influenced by the age she starts menstruating (menarche) and stops menstruating (menopause), 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 (less than 2 times the risk), has her first child after 30, stops menstruating after 55, or does not breast feed.  Current information about the effect of birth control pills and breast cancer risk is mixed. Some studies have found that the hormones in birth control pills probably do not increase breast cancer risk or protect against breast cancer. However other studies suggest that the risk of breast cancer is increased in women who have taken birth control pills recently, regardless of how long she has taken them.

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