Examples of external factors that contribute to accumulation of muted genes leading to cancer are: physical inactivity, tobacco and alcohol, nutrition, radiation and chemicals, bacteria and viruses, medications, and reproductive factors. Although the exact biological mechanism for some of these factors by which they contribute to cancer remain elusive, results from epidemiological studies provide clear evidence of their major role in cancer causation.
There is sufficient evidence of the role of physical activity on the prevention of breast (see Breast cancer) and colon cancer and probably cancers of the endometrium (lining of the uterus) and prostate.
Tobacco and Alcohol
More than 25% of all cancer deaths could be prevented in the United States if people did not smoke. There is clear evidence not only that smoking causes lung cancer but also that passive smoking is a major cause of developing lung cancer. Tobacco consumption is the main cause of cancers of the upper respiratory tract, esophagus, bladder, and pancreas as well. As a result, the tobacco industry and its practices of marketing addictive products to targeted populations have come increasingly under scrutiny in the United States in recent years. This has led to litigation and multi-billiondollar settlements by the tobacco industry. Communities have turned to policymakers as a strategy to control both youth’s access to tobacco products and the general population’s exposure to environmental tobacco smoke.
While antitobacco efforts have led to a decrease in the prevalence of smoking in some populations in the United States and many other developed countries, we are observing a rapid rise in rates of smoking in developing countries. Consequently, an increased risk of smoking-related cancers in these populations in one or two decades is well expected. Since alcohol and tobacco consumption are closely correlated habits, investigating their association with cancer has been difficult in small studies. However, this difficulty has been overcome by pooling the data from 53 studies, with findings indicating increased risk of liver and breast cancer with higher consumption of alcohol.
Overall, abundant and routine consumption of fruits and vegetables contributes significantly to the prevention of many cancers. In contrast, consumption of red meat and saturated fat is associated with various cancers. These associations are established for cancer of the colon and rectum. Overnutrition during childhood that leads to disproportionate growth may lead to increased risk of cancer through adulthood. This has clearly been shown in epidemiological studies of breast cancer. Women who spent their childhood in a food deprivation period (e.g., during World War II) had a decreased risk for breast cancer when compared with women growing up in periods of abundant food.
Radiation and Chemicals
Ultraviolet radiation (sunlight) can damage DNA and cause melanoma—the most lethal form of skin cancer. Individuals with fair skin are more susceptible to the carcinogenic effect of sunlight. Radon, another naturally occurring radiation from the earth, is shown to be related to lung cancer. The risk is accelerated among smokers. Exposure to high-dose ionizing radiation causes various types of cancer. Demonstrated carcinogenic effect of some chemicals used in industries has led to a remarkable decrease or banning of these chemicals in many industrialized countries, while they are still available and used extensively in developing countries. Examples of these chemicals and their effects are asbestos (e.g., used in brake lining of vehicles and insulation material) causing lung cancer and more drastically among exposed smokers than nonsmokers, benzene in painting material causing leukemia, use of hair dyes causing bladder cancer, arsenic used in pesticides and insecticides associated with both lung and skin cancer depending on the mode of exposure.
Viruses and Bacteria
The only known type of bacteria that has been linked to cancer is H. pylori, which is associated with stomach cancer. Several viruses are established as main risk factors for cancer. Human papillomaviruses are associated with cancers of the cervix (see Cervical cancer), anus, and other genital organs. Hepatitis B and C are causally related to liver cancer. Despite the high prevalence of infected individuals with these bacteria and viruses, for some yet unknown reason, only some eventually develop cancer.
Reproductive Factors and Medications
Late age at first pregnancy, nulliparity (never giving birth), early age at menarche, and late age at menopause are factors associated with breast, endometrial, and ovarian cancer (see Breast cancer and Endometrial cancer). Hormonal replacement therapy (HRT) used for menopausal symptoms increases risk for breast and endometrial cancer. Estrogen alone HRT is associated with increased risk for endometrial cancer and estrogen plus progesterone HRT increases risk for breast cancer. Use of combined oral contraceptives (estrogen plus progesterone) decreases risk for endometrial and ovarian cancer and may slightly increase risk for breast cancer among younger women (see Breast cancer). Similarly, fertility drugs may increase risk for ovarian cancer. Children using growth hormones may be at increased risk for leukemia, and some drugs administered for lowering cholesterol may increase risk for cancers of the colon and rectum.
Low-dose aspirin has been shown to moderately prevent adenomas (polyps) in the large bowel. Adenomas are possibly always prerequisites for development of colon cancer. A significant reduction in the remission of colorectal adenomas in patients with previous colorectal cancer has been observed among those who use aspirin on a daily basis.
Cancer stage is the most important predictor of prognosis; diagnosis at early stages of cancer implies increased chances of survival, and in the case of many cancers, cure. Similarly, adequate cancer treatment and follow-up care are important to enhance chances of survival, and even chances of cure. Many cancers, including some of the most common cancers, such as prostate, colon and rectum, breast, and cervix of the uterus, are amenable to screening. Despite the highvisibility media campaigns, however, screening rates have remained relatively low. Financial barriers and lack of knowledge, among others, have contributed to disparities in cancer-related outcomes. The uninsured and underinsured, for example, are at an increased risk for being diagnosed at later stages of cancer, and for receiving disparate cancer treatment and follow-up care, further contributing to poor outcomes.
The profound effects of lifestyle and behavior modifications on cancer risk imply a greater ability to prevent cancer. In addition, the availability of and access to effective screening, therapy, and follow-up care are likely to greatly influence prognosis through early cancer detection, increased survival, and even cure. Together, these elements constitute the foundation in our fight against cancer. With significant strides in the science of genetics and its epidemiology, further progress is to be expected through that venue as well.