Age and Gender, As with most cancers, age is a significant factor. In fact, 77% of new cases and 84% of breast cancer deaths occur in women aged 50 and older. More than 80% of breast cancer cases occur in women over 50. Less than 1% of breast cancers occur in men. The risk of breast cancer is clearly related to hormonal influences, but how these affect the disease and particularly types of the disease is not yet clear.
Genetic Factors and Family History of Breast Cancer -- Some families appear to have a genetic tendency for breast cancer. Two variant genes have been found that appear to account for this: BRCA1 and BRCA2. The genes p53 and BARD1 also appear to be important.
Researchers have identified several other defective genes that may cause breast cancer, including BRCA3 and Noey2 (which is a disease inherited only from the father's side of the family). These discoveries suggest that breast cancer occurs when the body’s anti-cancer surveillance and control systems, which normally get rid of abnormal cells, fail to work. The body's reduced ability to get rid of abnormal cells leads to damage that gradually accumulates. Women carrying mutated BRCA1 and/or BRCA2 genes start with pre-existing dysfunction of this system and have a "head start" in this damaging process. Hormones are important because they encourage cell growth. High levels of hormones during a woman's reproductive years, especially when they are not interrupted by the hormonal changes of pregnancy, appear to increase the chances that genetically damaged cells will grow and cause cancer.
Early Menstruation and Late Menopause -- Women who get their periods early (before age 12) or went through menopause late (after age 55) are at higher risk. Also, women who have never had children or who had them only after the age of 30 have an increased risk. Oral Contraceptives (birth control pills) -- Birth control pills may slightly increase the risk for breast cancer, depending on age, length of use, and other factors. No one knows how long the effects of the pill last after stopping it.
Hormone Replacement Therapy(HRT) -- Use of HRT has been shown to increase the risk of breast cancer.
Obesity -- Obesity is controversial as a risk factor. Some studies report obesity as a risk of breast cancer, possibly associated with higher levels of estrogen production in obese women.
Alcohol Consumption -- Significant alcohol use (more than 1-2 drinks a day) has been associated with an increased risk of breast cancer.
Chemicals -- Some studies have pointed to exposure to estrogen-like chemicals that are found in pesticides and other industrial products as a possible increased risk of breast cancer.
DES -- Women who took diethylstilbestrol (DES) to prevent miscarriage may have an increased risk of breast cancer after age 40.
Radiation -- People exposed to radiation, particularly during childhood, may face an increased risk for breast cancer in adulthood. Especially at risk are those that received chest irradiation for prior cancers.
Additional Risk Factors -- Some studies show previous breast, uterine, ovarian, or colon cancer, and a strong history of cancer in the family may increase the risk for breast cancer. Such history may indicate genetic factors described above.
The Gail Model is a simple breast cancer risk assessment tool that is available online and takes into account the most important risk factors. A number of other models are also used.