Scientists do not fully understand the causes of cancer, but studies show that some people are more likely to develop the disease than others. Scientists called epidemiologists study particular populations to identify why cancer rates vary. One method they use is to compare cancer patients with healthy people in terms of behavior such as diet, exercise, and smoking and traits such as gender, age, and race. Population studies provide useful information about risk factors that increase the likelihood of developing cancer.
One of the greatest risk factors for cancer is prolonged or repeated exposure to carcinogens—chemical, biological, or physical agents that cause the cellular damage that leads to cancer. The details of how carcinogens cause cancer remain unclear. One theory is that exposure to carcinogens, when combined with the effects of aging, causes an increase in chemicals in the body called free radicals. An excessive number of free radicals causes damage by taking negatively charged particles called electrons from key cellular components of the body, such as DNA. This may make genes more vulnerable to the mutating effects of carcinogens.
2. Hereditary Factors
Evidence suggests that heredity plays a role in developing cancer. Some gene mutations associated with cancer are inherited. For example, inheriting mutated tumor suppressor genes BRCA1 or BRCA2 greatly increases a woman’s chances of developing breast cancer. About 50 to 60 percent of women with inherited BRCA1 or BRCA2 mutations will develop breast cancer by the age of 70. Inherited mutations in the genes MSH2, MLH1, PMS1, and PMS2, all of which repair DNA, are especially prevalent in a rare form of hereditary colon cancer.
Scientists suspect that many other hereditary factors contribute to cancer. In addition to inherited mutations, other genetic variations, particularly those influencing how the body responds to carcinogens, may create a greater susceptibility to cancer. The identities of the majority of these genetic variations are not yet known.
3. Steroid Hormones
Medical research suggests that cancers of the reproductive organs may be affected by naturally occurring steroid hormones produced by the endocrine system. These hormones stimulate reproductive organ cells to divide and grow. In women, relatively high or long exposure to the female sex hormone estrogen seems to increase the risk of breast and uterine cancers.
Thus, early age at first menstruation, late age at menopause, having a first child after age 30, and never having children, all of which affect the duration of estrogen exposure in the body, increase the risk for these cancers. Studies also indicate that hormone replacement therapy (HRT), in which women take estrogen to offset the unpleasant effects of menopause, increases the risk of breast cancer. Male sex hormones, particularly testosterone, appear to play a role in cancers of the male reproductive organs, but this role is not yet well understood.
4. Population Demographics
Population studies show that a person’s age, race, and gender affect the probability that he or she will develop cancer. Most cancers occur in adults middle-aged or older. The risk of cancer increases as individuals age because genetic mutations accumulate slowly over many years, and the older a person is, the more likely that he or she will have accumulated the collection of mutations necessary to turn an otherwise healthy cell into a cancerous cell.
More than three-fourths of all cancers in
Stomach cancer is nearly twice as common in men as in women, as are certain types of kidney cancer. However, the reasons for the discrepancy between the sexes are unknown. Some cancers are more prevalent in particular races than others. In the
Asian, Hispanic, and Native American women have the lowest breast cancer risk. On the whole, African Americans, especially men, are more likely to develop cancer—and more likely to die from it—than members of any other group in the