Causes Of Breast Cancer

Many women who develop breast cancer have no risk factors other than age and sex.
Gender is the biggest risk because breast cancer occurs mostly in women.
Age is another critical factor. Breast cancer may occur at any age, though the risk of breast cancer increases with age. The average woman at age 30 years has one chance in 280 of developing breast cancer in the next 10 years. This chance increases to one in 70 for a woman aged 40 years, and to one in 40 at age 50 years. A 60-year-old woman has a one in 30 chance of developing breast cancer in the next 10 years.
White women are slightly more likely to develop breast cancer than African American women in the U.S.
A woman with a personal history of cancer in one breast has a three- to fourfold greater risk of developing a new cancer in the other breast or in another part of the same breast. This refers to the risk for developing a new tumor and not a recurrence (return) of the first cancer.
Genetic Causes
Family history has long been known to be a risk factor for breast cancer. Both maternal and paternal relatives are important. The risk is highest if the affected relative developed breast cancer at a young age, had cancer in both breasts, or if she is a close relative. First-degree relatives, (mother, sister, daughter) are most important in estimating risk. Several second-degree relatives (grandmother, aunt) with breast cancer may also increase risk. Breast cancer in a male increases the risk for all his close female relatives. Having relatives with both breast and ovarian cancer also increases a woman's risk of developing breast cancer.
There is great interest in genes linked to breast cancer. About 5-10% of breast cancers are believed to be hereditary, as a result of mutations, or changes, in certain genes that are passed along in families.
BRCA1 and BRCA2 are abnormal genes that, when inherited, markedly increase the risk of breast cancer to a lifetime risk estimated between 40 and 85%. Women with these abnormal genes also have an increased likelihood of developing ovarian cancer. Women who have the BRCA1 gene tend to develop breast cancer at an early age.
Testing for these genes is expensive and may not be covered by insurance.
The issues around testing are complicated, and women who are interested in testing should discuss this with their health-care providers.
Hormonal Causes
Hormonal influences play a role in the development of breast cancer.
Women who start their periods at an early age (11 or younger) or experience a late menopause (55 or older) have a slightly higher risk of developing breast cancer. Conversely, being older at the time of the first menstrual period and early menopause tend to protect one from breast cancer.
Having a child before age 30 years may provide some protection, and having no children may increase the risk for developing breast cancer.
Oral contraceptives have not been shown to definitively increase or decrease a woman's lifetime risk of breast cancer.
A large study conducted by the Women's Health Initiative showed an increased risk of breast cancer in postmenopausal women who were on a combination of estrogen and progesterone for several years. Therefore, women who are considering hormone therapy for menopausal symptoms need to discuss the risk versus the benefit with their health-care providers.
Lifestyle and Dietary Causes
Breast cancer seems to occur more frequently in countries with high dietary intake of fat, and being overweight or obese is a known risk factor for breast cancer, particularly in postmenopausal women.
This link is thought to be an environmental influence rather than genetic. For example, Japanese women, at low risk for breast cancer while in Japan, increase their risk of developing breast cancer after coming to the United States.
Several studies comparing groups of women with high- and low-fat diets, however, have failed to show a difference in breast cancer rates.
The use of alcohol is also an established risk factor for the development of breast cancer. The risk increases with the amount of alcohol consumed. Women who consume two to five alcoholic beverages per day have a risk about one and a half times that of nondrinkers for the development of breast cancer. Consumption of one alcoholic drink per day results in a slightly elevated risk.
Studies are also showing that regular exercise may actually reduce a woman's risk of developing breast cancer. Studies have not definitively established how much activity is needed for a significant reduction in risk. One study from the Women's Health Initiative (WHI) showed that as little as one and a quarter to two and a half hours per week of brisk walking reduced a woman's breast cancer risk by 18%.
Benign Breast Disease
Fibrocystic breast changes are very common. Fibrocystic breasts are lumpy with some thickened tissue and are frequently associated with breast discomfort, especially right before the menstrual period. This condition does not lead to breast cancer.
However, certain other types of benign breast changes, such as those diagnosed on biopsy as proliferative or hyperplastic, do predispose women to the later development of breast cancer.
Environmental Causes
Radiation treatment increases the likelihood of developing breast cancer but only after a long delay. For example, women who received radiation therapy to the upper body for treatment of Hodgkin's disease before 30 years of age have a significantly higher rate of breast cancer than the general population.