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Causes and Prevention of Cancer

The number of new cases of cancer in the United States is going up each year. People of all ages get cancer, but nearly all types are more common in middle-aged and elderly people than in young people. Skin cancer is the most common type of cancer for both men and women. The next most common type among men is prostate cancer; among women, it is breast cancer. Lung cancer, however, is the leading cause of death from cancer for both men and women in the United States. Brain cancer and leukemia are the most common cancers in children and young adults.

The more we can learn about what causes cancer, the more likely we are to find ways to prevent it. Scientists study patterns of cancer in the population to look for factors that affect the risk of developing this disease. In the laboratory, they explore possible causes of cancer and try to determine what actually happens when normal cells become cancerous.

Our current understanding of the causes of cancer is incomplete, but it is clear that cancer is not caused by an injury, such as a bump or bruise. And although being infected with certain viruses may increase the risk of some types of cancer, cancer is not contagious; no one can "catch" cancer from another person.

Cancer develops gradually as a result of a complex mix of factors related to environment, lifestyle, and heredity. Scientists have identified many risk factors that increase the chance of getting cancer. They estimate that about 80 percent of all cancers are related to the use of tobacco products, to what we eat and drink, or, to a lesser extent, to exposure to radiation or cancer-causing agents (carcinogens) in the environment and the workplace. Some people are more sensitive than others to factors that can cause cancer.

Many risk factors can be avoided. Others, such as inherited risk factors, are unavoidable. It is helpful to be aware of them, but it is also important to keep in mind that not everyone with a particular risk factor for cancer actually gets the disease; in fact, most do not. People at risk can help protect themselves by avoiding risk factors where possible and by getting regular checkups so that, if cancer develops, it is likely to be found early.

These are some of the factors that are known to increase the risk of cancer:

  • Tobacco. Tobacco causes cancer. In fact, smoking tobacco, using "smokeless" tobacco, and being regularly exposed to environmental tobacco smoke without smoking are responsible for one-third of all cancer deaths in the United States each year. Tobacco use is the most preventable cause of death in this country.

    Smoking accounts for more than 85 percent of all lung cancer deaths. If you smoke, your risk of getting lung cancer is affected by the number and type of cigarettes you smoke and how long you have been smoking. Overall, for those who smoke one pack a day, the chance of getting lung cancer is about 10 times greater than for nonsmokers. Smokers are also more likely than nonsmokers to develop several other types of cancer (such as oral cancer and cancers of the larynx, esophagus, pancreas, bladder, kidney, and cervix). The risk of cancer begins to decrease when a smoker quits, and the risk continues to decline gradually each year after quitting.

    The use of smokeless tobacco (chewing tobacco and snuff) causes cancer of the mouth and throat. Precancerous conditions, or tissue changes that may lead to cancer, begin to go away after a person stops using smokeless tobacco.

    Exposure to environmental tobacco smoke, also called involuntary smoking, increases the risk of lung cancer for nonsmokers. The risk goes up 30 percent or more for a nonsmoking spouse of a person who smokes. Involuntary smoking causes about 3,000 lung cancer deaths in this country each year.

    If you use tobacco in any form and you need help quitting, talk with your doctor or dentist, or join a smoking cessation group sponsored by a local hospital or voluntary organization. For information on such groups or other programs, call the Cancer Information Service or the American Cancer Society.

  • Diet. Your choice of foods may affect your chance of developing cancer. Evidence points to a link between a high-fat diet and certain cancers, such as cancer of the breast, colon, uterus, and prostate. Being seriously overweight appears to be linked to increased rates of cancer of the prostate, pancreas, uterus, colon, and ovary, and to breast cancer in older women. On the other hand, studies suggest that foods containing fiber and certain nutrients help protect us against some types of cancer.

    You may be able to reduce your cancer risk by making some simple food choices. Try to have a varied, well-balanced diet that includes generous amounts of foods that are high in fiber, vitamins, and minerals. At the same time, try to cut down on fatty foods. You should eat five servings of fruits and vegetables each day, choose more whole-grain breads and cereals, and cut down on eggs, high-fat meat, high-fat dairy products (such as whole milk, butter, and most cheeses), salad dressings, margarine, and cooking oils.

  • Sunlight. Ultraviolet radiation from the sun and from other sources (such as sunlamps and tanning booths) damages the skin and can cause skin cancer. (Two types of ultraviolet radiation--UVA and UVB--are explained in the Medical Terms section.) Repeated exposure to ultraviolet radiation increases the risk of skin cancer, especially if you have fair skin or freckle easily. The sun's ultraviolet rays are strongest during the summer from about 11 a.m. to about 3 p.m. (daylight saving time). The risk is greatest at this time, when the sun is high overhead and shadows are short. As a rule, it is best to avoid the sun when your shadow is shorter than you are.

    Protective clothing, such as a hat and long sleeves, can help block the sun's harmful rays. You can also use sunscreens to help protect yourself. Sunscreens are rated in strength according to their SPF (sun protection factor), which ranges from 2 to 30 and higher. Those rated 15 to 30 block most of the sun's harmful rays.

  • Alcohol. Drinking large amounts of alcohol increases the risk of cancer of the mouth, throat, esophagus, and larynx. (People who smoke cigarettes and drink alcohol have an especially high risk of getting these cancers.) Alcohol can damage the liver and increase the risk of liver cancer. Some studies suggest that drinking alcohol also increases the risk of breast cancer. So if you drink at all, do so in moderation--not more than one or two drinks a day.

  • Radiation. X-rays used for diagnosis expose you to very little radiation and the benefits nearly always outweigh the risks. However, repeated exposure can be harmful, so it is a good idea to talk with your doctor or dentist about the need for each x-ray and ask about the use of shields to protect other parts of your body.

    Before 1950, x-rays were used to treat noncancerous conditions (such as an enlarged thymus, enlarged tonsils and adenoids, ringworm of the scalp, and acne) in children and young adults. People who have received radiation to the head and neck have a higher-than-average risk of developing thyroid cancer years later. People with a history of such treatments should report it to their doctor and should have a careful exam of the neck every 1 or 2 years.

    Also, radiation used in the treatment of some types of cancer can increase the risk of developing a second cancer. Patients having radiation therapy may want to discuss this issue with their doctor.

  • Chemicals and other substances in the workplace. Being exposed to substances such as metals, dust, chemicals, or pesticides at work can increase the risk of cancer. Asbestos, nickel, cadmium, uranium, radon, vinyl chloride, benzidene, and benzene are well-known examples of carcinogens in the workplace. These may act alone or along with another carcinogen, such as cigarette smoke. For example, inhaling asbestos fibers increases the risk of lung diseases, including cancer, and the cancer risk is especially high for asbestos workers who smoke. It is important to follow work and safety rules to avoid contact with dangerous materials.

  • Hormone replacement therapy. Many women use estrogen therapy to control the hot flashes, vaginal dryness, and osteoporosis (thinning of the bones) that may occur during menopause. However, studies show that estrogen use increases the risk of cancer of the uterus. Other studies suggest an increased risk of breast cancer among women who have used high doses of estrogen or have used estrogen for a long time. At the same time, taking estrogen may reduce the risk of heart disease and osteoporosis.

    The risk of uterine cancer appears to be less when progesterone is used with estrogen than when estrogen is used alone. But some scientists are concerned that the addition of progesterone may also increase the risk of breast cancer.

    Researchers are still studying and finding new information about the risks and benefits of taking replacement hormones. A woman considering hormone replacement therapy should discuss these issues with her doctor.

  • Diethylstilbestrol (DES). DES is a form of estrogen that doctors prescribed from the early 1940s until 1971 to try to prevent miscarriage. In some daughters of women who were given DES during pregnancy, the uterus, vagina, and cervix do not develop normally. DES-exposed daughters also have an increased chance of developing abnormal cells (dysplasia) in the cervix and vagina. In addition, a rare type of vaginal and cervical cancer has been found in a small number of DES-exposed daughters. Women who took DES during pregnancy may have a slightly increased risk of developing breast cancer. DES-exposed mothers and daughters should tell their doctor about this exposure. DES daughters should have regular special pelvic exams by a doctor familiar with conditions related to DES.

    Exposure to DES before birth does not appear to increase the risk of cancer in DES-exposed sons; however, reproductive and urinary system problems may occur. These men should tell the doctor and should have regular medical checkups.

  • Close relatives with certain types of cancer. A small number of cancers (including melanoma and cancers of the breast, ovary, and colon) tend to occur more often in some families than in the rest of the population. It is not always clear whether a pattern of cancer in a family is due to heredity, factors in the family's environment, or chance. Still, if close relatives have been affected by cancer, it is important to let your doctor know this and then follow the doctor's advice about cancer prevention and checkups to detect problems early.

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