The cause of breast cancer remains unknown, and, like other cancers, it is thought to develop in response to a number of interrelated factors. The most common factors that increase the risk of breast cancer include:
- Sex of the person. Breast cancer is overwhelmingly a disease of women; less than one percent of cases develop in men.
- Increasing age. Most breast cancer occurs following menopause; two thirds of all cases develop in women over the age of fifty. The disease is rare in women under the age of thirty; the incidence rises sharply in the early forties, leveling off at about age forty-five and then increasing again after age fifty-five.
- Family history. Women whose first-degree relatives—mothers or sisters—have had breast cancer are two to three times more likely to develop breast cancer; the risk is even greater if these relatives have had cancer in both breasts or developed it before menopause.
- Previous breast cancer. Women who have had cancer in one breast have an increased risk of its developing in the other one. An American Cancer Society analysis has found that 5 to 10 percent of women who have had cancer in one breast will eventually have it in both.
These are the major risk factors associated with breast cancer, but a number of other circumstances have been identified that appear to increase its incidence to varying degrees.
- Diet. A high-fat diet has been linked to an increased risk of breast cancer. Obesity also appears to increase the risk.
- Race or national origin. Breast cancer is more common among women of North American or northern European origin than among women in Asian and African countries. Among Americans, the disease is almost twice as common in women of the higher social and economic classes. Some experts now believe these differences may be related more to diet than to racial background. For example, there is a low incidence of breast cancer in Japan, where the diet is very low in animal fat. However, first-generation Japanese-American women in San Francisco have about twice the breast cancer incidence of their cousins in Japan.
- Menstrual history. A long menstrual history—early onset of menstruation plus late menopause—increases the risk, while early menopause, either natural or artificial, decreases it.
- Pregnancy. The risk of breast cancer is higher among women who have never had a baby or whose first full-term pregnancy occurred after the age of 30.
- Hormonal factors. The relationship of hormones to breast cancer is unclear. Some breast cancers are clearly estrogen-dependent while others are not, but there is no convincing evidence that either birth control pills or estrogen supplements cause breast cancer. The possible link between breast cancer and oral contraceptives has been investigated for years, with widely contradictory results. Some studies have found a higher incidence of breast cancer for long-term pill users (more than two to four years) who also have benign breast disease. Consequently the Food and Drug Administration now requires a notice of this information in each package of oral contraceptives.
Estrogen preparations used for replacement hormone therapy during menopause also carry an PDA-mandated warning regarding breast-cancer risk. As with oral contraceptives, a link between estrogen replacement therapy and breast cancer is suspected by some scientists, but has not been proved.
Another hormonal drug linked to an increased risk of breast cancer is diethylstilbestrol, or DES. This drug was used in the late 1940s and through the 1960s to prevent miscarriage. Some recent studies have found an increased incidence of breast cancer among the women who took it; consequently they are now advised to avoid other estrogen preparations and to take extra care in breast self-examination and checkups.
Many folk beliefs about the origin of breast cancer have no basis in fact. It is not true, for example, that an injury to the breast can cause cancer. Sexual stimulation is unrelated to breast cancer, and breast feeding does not seem to alter the risk. Certain viruses can cause breast cancer in some strains of mice, but there is no evidence of a viral cause for human breast cancer; in any event, it is not contagious.
Aside from avoiding a high-fat diet and obesity, at this time there do not seem to be any lifestyle modifications that markedly reduce the risk of breast cancer. Thus early detection and treatment remains the best approach to the disease.