Cancer Prevention

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Cancer prevention

Definition

Preventing the incidence of cancer is complex and involves many factors which ultimately work by avoiding or limiting exposure to carcinogens. Known carcinogens in humans include physical, chemical, viral, and bacterial carcinogens. Physical carcinogens include the hydrocarbon byproducts of cigarette smoke, radiation, and asbestos. Benzene and vinyl chloride are examples of chemical carcinogens. The human papillomaviruses, which play a role in the development of cervical cancer , are viral carcinogens. A bacterial carcinogen is the bacteria, Helicobacter pylori, which has been linked to the cancer B-cell lymphoma , unique to the gastric mucosa. Familial (hereditary) carcinogenesis plays a role in as many as 15% of all human cancers and has been implicated as the cause of some cases of melanoma , breast, colon, and other cancers.

Some factors that place individuals at high risk for the development of cancer can be modified to decrease risk for development. For example, lifestyle and environmental changes can be made to decrease risk. Behavior modification such as dietary changes, exercise, and avoiding exposure to known carcinogens are primary prevention measures that everyone should adopt.

An evolving field, chemoprevention , is the use of vitamins or medicines to prevent cancer development. Chemopreventive agents have the ability to potentially delay and even reverse the sequence of events at the cellular level that change a normal cell to a cancer cell. An example of a chemopreventive agent is tamoxifen , a drug that is effective in preventing breast cancer in women who are at high risk for the development of breast cancer. Vaccines for Hepatitis B virus will not only prevent primary Hepatitis B and liver failure, but also liver cancer.

Preventive surgery may be an option for those individuals who are considered to be at very high risk of developing cancer because of a genetic or inherited predisposition. Examples of preventive surgery are prophylactic mastectomy to reduce risk for breast cancer, and colon polyp removal in individuals at high risk for the development of colon cancer .

In 1999 the American Cancer Society (ACS) estimated that 173, 000 cancer deaths were caused by cigarette smoking, and an additional 20, 000 cancer deaths were due to excessive alcohol consumption . All cancers caused by smoking cigarettes and by excessive use of alcohol can be completely prevented. According to the ACS, up to one-third of the more than 550, 000 cancer deaths expected to occur in the United States in 2001 are related to poor nutrition or insufficient physical exercise. Many of the more than one million skin cancers which develop annually could be prevented by adopting protective measures from ultraviolet radiation caused by the sun.

Different cancers are associated with different risk factors. While modification of risk factors plays an important role in the prevention of cancer, it is known that some individuals who have one or more risk factors never develop cancer. Others, however, who have no known risk factors, are eventually diagnosed with cancer. Research aimed at identifying additional risk factors for specific cancers continues.

Lifestyle and cancer

Nutrition

The relationship between food intake and cancer is complex and not well understood. Research does seem to indicate that certain foods may have either protective or promoting effects on the development of cancer. Foods that have a protective effect seem to play a role in the prevention of certain types of cancers. Foods that have promoting effects are associated with an increased risk of developing certain types of cancers. According to the American Cancer Society, the single most important dietary intervention to lower risk for cancer is eating five or more servings of fruits and vegetables daily. Adopting a diet rich in plant sources provides phytochemicals that are nonnutritive substances in plants that possess health protective benefits. Dietary recommendations related to reducing risk of cancer include the recommendation to choose most of the foods eaten from plant sources such as fruits and vegetables. Five or more servings of fruits and vegetables should be consumed every day. Fruits and vegetables should be eaten at every meal and as snacks. Other foods from plant sources that should be included in the diet several times a day include breads, cereals, grain products (preferably whole grain), rice, and pasta. Beans should be eaten as an alternative to meat. The foods and herbs with the highest anticancer activity include garlic, soybeans, cabbage, ginger, licorice root, and the umbelliferous vegetables such as carrots. Citrus foods also contain a host of active phytochemicals.

A diet rich in foods from plant sources reduces the risk for development of cancers of the gastrointestinal tract, respiratory tract, and colon. Vegetables that seem to play a strong role in protecting against colon cancer include green and dark yellow vegetables, vegetables in the cabbage family, soy products, and legumes. Increased consumption of fruits and vegetables reduce risk for lung cancer, even for those individuals who smoke. Forms of fruits and vegetables that appear to provide the greatest protection include foods in fresh, frozen, canned, dried, or juice forms. Extractions from fruits and vegetable do not provide protective effects.

Diets high in fat have been associated with increased risk for colon, rectal, prostate, and endometrial (uterine) cancers. The association between high-fat diets and the development of breast cancer is much weaker. Specific dietary recommendations are to replace high-fat foods with fruits and vegetables, eat smaller portions of high fat foods, and limit consumption of meats, especially those that are considered high-fat.

Foods from animal sources remain a staple in American diets. Consumption of meat, especially red meats such as beef, pork, and lamb, have been associated with increased risk of colon and prostate cancer . Cooking methods have also been linked to the development of cancer. Mutagenic compounds are produced when proteins such as meat protein are cooked at high temperatures. These compounds may be responsible for the association between meat consumption and increased risk for colon cancer.

Obesity is often the result of meat-based, high-fat diets. Obesity has been linked to cancers at several sites including colon and rectum, prostate, kidney, and endometrium and breast cancer in postmenopausal women.

Physical activity

Recommendations related to physical exercise include engaging in moderate levels of activity for at least 30 minutes on most days of the week. Studies have revealed an association between physical activity and a reduced risk of the development of certain types of cancers, including colon, breast, and prostate cancer. For example, physical activity is thought to stimulate the movement of stool through the bowel, resulting in less exposure of the bowel lining to mutagens in the stool.

Consumption of alcohol

Drinking alcohol increases the risk of developing cancers of the mouth, esophagus, pharynx, larynx, and liver in both men and women, and increases the risk of breast cancer in women. Cancer risk increases as the amount of alcohol consumed increases. An individual who both smokes and drinks alcohol greatly increases the risk of developing cancer when compared to either smoking or drinking alone. Risk increases significantly for cancers of the mouth, esophagus, and larynx when more than two drinks per day are consumed. A drink is defined as 5 ounces (141.75 grams) of wine, 12 ounces (340.20 grams) of regular beer, or 1.5 ounces (42.52 grams) of 80-proof distilled spirits. Women who drink are at increased risk for the development of breast cancer. Studies have shown that the risk of breast cancer increases with just a few drinks per week.

Consumption of tobacco

Smoking-related illnesses account for more than 400, 000 deaths each year in the United States. These deaths occur 12 years earlier than would be expected on average. Tobacco is known as one of the most potent human carcinogens. Tobacco causes more than 148, 000 deaths each year in the form of various cancers. Most of the cancers of the lung, trachea, bronchus, larynx, pharynx, oral cavity, and esophagus diagnosed each year are caused by tobacco. Smoking is also associated with cancers of the pancreas, kidney, bladder, and cervix. Smoking is known to affect the health of nonsmokers through environmental or secondhand smoke, which is implicated in causing lung cancer. Cigarette smoking is more common among men; however, because of the increase in the number of women who smoke, more women die from lung cancer each year than from breast cancer. Mortality from lung cancer for men appears to have peaked and has been declining since the 1980s. This decline in mortality is attributed to a decrease in tobacco product use among men.

Substantial health benefits occur once an individual stops smoking. If a smoker stops smoking before the age of 50 years, that individual's risk of dying in the next 15 years is half of what a continuing smoker's risk of dying is. Even if the smoker stops smoking after the age of 70 years, the risk of dying is still reduced substantially. After 10 years of not smoking, an ex-smoker's risk of lung cancer is reduced by 30-50%. After five years of not smoking, an ex-smoker's risk of oral and esophageal cancer is reduced by 50%. Risk for cervical and bladder cancer is also reduced once smoking is stopped.

The three treatment elements identified as particularly effective in smoking cessation treatment include pharmacotherapy, such as nicotine replacement patches and gums, social support from physicians and other clinicians, and skills training and problem solving, particularly in the areas of smoking cessation and abstinence techniques.

Radiation exposure

Only high-frequency radiation such as ionizing radiation (IR) and ultraviolet (UV) radiation has been proven to cause cancer in humans. A source of ultraviolet radiationis sunlight. Prolonged, unprotected exposure to UV radiation is the major cause of basal and squamous cell skin cancers. UV radiation is also a major cause of melanoma. Disruption of Earth's ozone layer by pollution is thought to result in increasing levels of UV radiation reaching the earth's surface which has been linked to the rise in the incidence of skin cancers and melanomas.

IR has cancer-causing capability as proven by studies on atomic bomb survivors and other groups. Virtually any part of the body can be affected by IR, but the areas most affected are the bone marrow and the thyroid gland. IR is released in very low levels from diagnostic equipment such as medical and dental X-ray equipment. Much higher levels of IR are released from machines delivering radiation therapy . Great precautions are taken during treatment not to expose patients or staff unnecessarily to the effects of IR. Another occupational group affected by IR includes uranium miners. Exposure to radon, a naturally occurring gas which is a form of IR, can increase risk for lung cancer, especially among smokers.

See Also Antioxidants; Familial cancer syndromes; Occupational exposures and cancer

Resources

BOOKS

National Research Council Carcinogens and Anticarcinogens in the Human Diet Washington, D.C.: National Academy Press, 1996.

U.S. Department of Health and Human Services Physical Activity and Health: A Report of the Surgeon General Atlanta, GA: Centers for Disease Control and Prevention, National Center for Chronic Disease Prevention and Health Promotion, President's Council on Physical Fitness and Sports, 1996.

Yarbro, J.W. "Carcinogenesis" Cancer Nursing Principles and Practice, edited by C.H. Yarbro, M.H. Frogge, M. Goodman, and S.L. Groenwald. Boston: Jones and Bartlett, 2000, pp.48-59.

PERIODICALS

Craig, W., and L. Beck. "Phytochemicals: Health Protective Effects." Canadian Journal of Dietetic Practice and Research 60 (1999): 78-84.

Glade, M.J. "Dietary fat and cancer: Genetic and molecular interactions annual research conference, American Institute for Cancer Research." Nutrition 13 (1997): 75-77.