Cancer is a group of many related diseases that begin in cells, the body's basic unit of life. To understand cancer, it is helpful to know what happens when normal cells become cancerous. The body is made up of many types of cells. Normally, cells grow and divide to produce more cells only when the body needs them.
This orderly process helps keep the body healthy. Sometimes, however, cells keep dividing when new cells are not needed. These extra cells form a mass of tissue , called a growth or tumor . Tumors can be benign or malignant . Benign tumors are not cancer. They can often be removed and, in most cases, they do not come back.
Cells from benign tumors do not spread to other parts of the body. Most important, benign tumors are rarely a threat to life.
Malignant tumors are cancer. Cells in these tumors are abnormal and divide without control or order. They can invade and damage nearby tissues and organs. Also, cancer cells can break away from a malignant tumor and enter the bloodstream or the lymphatic system . That is how cancer spreads from the original cancer site to form new tumors in other organs. The spread of cancer is called metastasis .
Leukemia and lymphoma are cancers that arise in blood-forming cells. The abnormal cells circulate in the bloodstream and lymphatic system. They may also invade (infiltrate) body organs and form tumors. Most cancers are named for the organ or type of cell in which they begin.
For example, cancer that begins in the lung is lung cancer, and cancer that begins in cells in the skin known as melanocytes is called melanoma . When cancer spreads (metastasizes), cancer cells are often found in nearby or regional lymph nodes (sometimes called lymph glands). If the cancer has reached these nodes, it means that cancer cells may have spread to other organs, such as the liver, bones, or brain.
When cancer spreads from its original location to another part of the body, the new tumor has the same kind of abnormal cells and the same name as the primary tumor. For example, if lung cancer spreads to the brain, the cancer cells in the brain are actually lung cancer cells. The disease is called metastatic lung cancer (it is not brain cancer).
Possible Causes and Prevention of Cancer
The more we can learn about what causes cancer, the more likely we are to find ways to prevent it. In the laboratory, scientists explore possible causes of cancer and try to determine exactly what happens in cells when they become cancerous.
Researchers also study patterns of cancer in the population to look for risk factors , conditions that increase the chance that cancer might occur. They also look for protective factors, things that decrease the risk. Even though doctors can seldom explain why one person gets cancer and another does not, 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 over time. It is a result of a complex mix of factors related to lifestyle, heredity, and environment.
A number of factors that increase a person's chance of developing cancer have been identified. Many types of cancer are related to the use of tobacco, what people eat and drink, exposure to ultraviolet (UV) radiation from the sun, and, to a lesser extent, exposure to cancer-causing agents ( carcinogens ) in the environment and the workplace.
Some people are more sensitive than others to factors that can cause cancer. Still, most people who get cancer have none of the known risk factors. And most people who do have risk factors do not get the disease. Some cancer risk factors can be avoided. Others, such as inherited factors, are unavoidable, but it may be helpful to be aware of them.
People can help protect themselves by avoiding known risk factors whenever possible. They can also talk with their doctor about regular checkups and about whether cancer screening tests could be of benefit. These are some of the factors that increase the likelihood of cancer:
Smoking tobacco, using smokeless tobacco, and being regularly exposed to environmental tobacco smoke 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.
For smokers, the risk of getting lung cancer increases with the amount of tobacco smoked each day, the number of years they have smoked, the type of tobacco product, and how deeply they inhale. Overall, for those who smoke one pack a day, the chance of getting lung cancer is about 10 times greater than for nonsmokers.
Cigarette smokers are also more likely than nonsmokers to develop several other types of cancer, including oral cancer and cancers of the larynx, esophagus, pancreas, bladder, kidney, and cervix.
Smoking may also increase the likelihood of developing cancers of the stomach, liver, prostate, colon, and rectum. The risk of cancer begins to decrease soon after a smoker quits, and the risk continues to decline gradually each year after quitting. People who smoke cigars or pipes have a risk for cancers of the oral cavity that is similar to the risk for people who smoke cigarettes.
Cigar smokers also have an increased chance of developing cancers of the lung, larynx, esophagus, and pancreas. The use of smokeless tobacco (chewing tobacco and snuff) causes cancer of the mouth and throat. Precancerous conditions, tissue changes that may lead to cancer, often begin to go away after a person stops using smokeless tobacco.
Studies suggest that exposure to environmental tobacco smoke, also called secondhand smoke, increases the risk of lung cancer for nonsmokers. People who use tobacco in any form and need help quitting may want to talk with their doctor, dentist, or other health professional, or join a smoking cessation group sponsored by a local hospital or voluntary organization.
Information about finding such groups or programs is available from the Cancer Information Service (CIS) at 1-800-4-CANCER. CIS information specialists can send printed materials, and also can give suggestions about quitting that are tailored to a caller's needs.
Researchers are exploring how dietary factors play a role in the development of cancer. Some evidence suggests a link between a high-fat diet and certain cancers, such as cancers of the colon, uterus, and prostate. Being seriously overweight may be linked to breast cancer among older women and to cancers of the prostate, pancreas, uterus, colon, and ovary. On the other hand, some studies suggest that foods containing fiber and certain nutrients may help protect against some types of cancer.
People may be able to reduce their cancer risk by making healthy food choices. A well-balanced diet includes generous amounts of foods that are high in fiber, vitamins, and minerals, and low in fat. This includes eating lots of fruits and vegetables and more whole-grain breads and cereals every day, fewer eggs, and not as much high-fat meat, high-fat dairy products (such as whole milk, butter, and most cheeses), salad dressing, margarine, and cooking oil.
Most scientists think that making healthy food choices is more beneficial than taking vitamin and mineral supplements.
Ultraviolet (UV) radiation .
UV radiation from the sun causes premature aging of the skin and skin damage that can lead to skin cancer. Artificial sources of UV radiation, such as sunlamps and tanning booths, also can cause skin damage and probably an increased risk of skin cancer.
To help reduce the risk of skin cancer caused by UV radiation, it is best to reduce exposure to the midday sun (from 10 a.m. to 3 p.m.). Another simple rule is to avoid the sun when your shadow is shorter than you are. Wearing a broad-brimmed hat, UV-absorbing sunglasses, long pants, and long sleeves offers protection.
Many doctors believe that in addition to avoiding the sun and wearing protective clothing, wearing a sunscreen (especially one that reflects, absorbs, and/or scatters both types of ultraviolet radiation) may help prevent some forms of skin cancer. Sunscreens are rated in strength according to a sun protection factor (SPF). The higher the SPF, the more sunburn protection is provided. Sunscreens with an SPF of 12 through 29 are adequate for most people, but sunscreens are not a substitute for avoiding the sun and wearing protective clothing.
Heavy drinkers have an increased risk of cancers of the mouth, throat, esophagus, larynx, and liver. (People who smoke cigarettes and drink heavily have an especially high risk of getting these cancers.) Some studies suggest that even moderate drinking may slightly increase the risk of breast cancer.
Cells may be damaged by ionizing radiation from x-ray procedures, radioactive substances, rays that enter the Earth's atmosphere from outer space, and other sources. In very high doses, ionizing radiation may cause cancer and other diseases. Studies of survivors of the atomic bomb in Japan show that ionizing radiation increases the risk of developing leukemia and cancers of the breast, thyroid, lung, stomach, and other organs.
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. Those who have received radiation therapy 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.
Radiation that patients receive as therapy for cancer can also damage normal cells. Patients may want to talk with their doctor about the effect of radiation treatment on their risk of a second cancer. This risk can depend on the patient's age at the time of treatment as well as on the part of the body that was treated. X-rays used for diagnosis expose people to lower levels of radiation than x-rays used for therapy.
The benefits nearly always outweigh the risks. However, repeated exposure could be harmful, so it is a good idea for people to talk with their doctor about the need for each x-ray and to ask about the use of shields to protect other parts of the body.
Chemicals and other substances.
Being exposed to substances such as certain chemicals, metals, or pesticides can increase the risk of cancer. Asbestos, nickel, cadmium, uranium, radon, vinyl chloride, benzidene, and benzene are examples of well-known carcinogens. These may act alone or along with another carcinogen, such as cigarette smoke, to increase the risk of cancer.
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 or minimize contact with dangerous materials.
Hormone replacement therapy (HRT).
Doctors may recommend HRT, using either estrogen alone or estrogen in combination with progesterone , to control symptoms (such as hot flashes and vaginal dryness) that may occur during menopause.
Studies have shown that the use of estrogen alone increases the risk of cancer of the uterus. Therefore, most doctors prescribe HRT that includes progesterone along with low doses of estrogen. Progesterone counteracts estrogen's harmful effect on the uterus by preventing overgrowth of the lining of the uterus; this overgrowth is associated with taking estrogen alone. (Estrogen alone may be prescribed for women who have had a hysterectomy , surgery to remove the uterus, and are, therefore, not at risk for cancer of the uterus.)
Other studies show an increased risk of breast cancer among women who have used estrogen for a long time; and some research suggests that the risk might be higher among those who have used estrogen and progesterone together. Researchers are still learning about the risks and benefits of taking HRT.
A woman considering HRT should discuss these issues with her doctor.
DES is a synthetic form of estrogen that was used between the early 1940s and 1971. Some women took DES during pregnancy to prevent certain complications. Their DES-exposed daughters have an increased chance of developing abnormal cells ( dysplasia ) in the cervix and vagina. In addition, a rare type of vaginal and cervical cancer can occur in DES-exposed daughters.
DES daughters should tell their doctor about their exposure. They should also have pelvic exams by a doctor familiar with conditions related to DES. Women who took DES during pregnancy may have a slightly higher risk for developing breast cancer. These women should tell their doctor about their exposure.
At this time, there does not appear to be an increased risk of breast cancer for daughters who were exposed to DES before birth. However, more studies are needed as these daughters enter the age range when breast cancer is more common. There is evidence that DES-exposed sons may have testicular abnormalities, such as undescended or abnormally small testicles. The possible risk for testicular cancer in these men is under study.
Close relatives with certain types of cancer.
Some types of cancer (including melanoma and cancers of the breast, ovary, prostate, and colon) tend to occur more often in some families than in the rest of the population. It is often unclear whether a pattern of cancer in a family is primarily due to heredity, factors in the family's environment or lifestyle, or just a matter of chance.
Researchers have learned that cancer is caused by changes (called mutations or alterations) in genes that control normal cell growth and cell death. Most cancer-causing gene changes are the result of factors in lifestyle or the environment. However, some alterations that may lead to cancer are inherited; that is, they are passed from parent to child. But having such an inherited gene alteration does not mean that the person is certain to develop cancer; it means that the risk of cancer is increased.
Risk Factors Associated with Cancer
- Ultraviolet radiation
- onizing radiation
- Chemicals and other substances
- Hormone replacement therapy (HRT)
- Diethylstilbestrol (DES)
- Close relatives with certain types of cancer
People who have any of the cancer risk factors listed above should talk with their doctor. The doctor may be able to suggest ways to reduce the risk and can recommend an appropriate schedule of checkups.
Screening and Early Detection
Sometimes, cancer can be found before the disease causes symptoms. Checking for cancer (or for conditions that may lead to cancer) in a person who does not have any symptoms of the disease is called screening .
In routine physical exams, the doctor looks for anything unusual and feels for any lumps or growths. Specific screening tests, such as lab tests, x-rays, or other procedures, are used routinely for only a few types of cancer.
Breast. A screening mammogram is the best tool available to find breast cancer before symptoms appear. A mammogram is a special kind of x-ray image of the breasts. Breast cancer screening has been shown to reduce the risk of dying from this disease.
The National Cancer Institute recommends that women in their forties and older have mammograms on a regular basis, every 1 to 2 years.
Cervix. Doctors use the Pap test , or Pap smear, to screen for cancer of the cervix. For this test, cells are collected from the cervix. The cells are examined under a microscope to detect cancer or changes that may lead to cancer.
Colon and rectum. A number of screening tests are used to find colon and rectal (colorectal) cancer. If a person is over the age of 50 years, has a family medical history of colorectal cancer, or has any other risk factors for colorectal cancer, a doctor may suggest one or more of these tests.
Sometimes tumors in the colon or rectum can bleed. The fecal occult blood test checks for small amounts of blood in the stool. The doctor sometimes uses a thin, lighted tube called a sigmoidoscope to examine the rectum and lower colon. Or, to examine the entire colon and rectum, a lighted instrument called a colonoscope is used.
If abnormal areas are seen, tissue can be removed and examined under a microscope. A barium enema is a series of x-rays of the colon and rectum. The patient is given an enema with a solution that contains barium, which outlines the colon and rectum on the x-rays. A digital rectal exam is an exam in which the doctor inserts a lubricated, gloved finger into the rectum to feel for abnormal areas.
Although it is not certain that screening for other cancers actually saves lives, doctors also may suggest screening for cancers of the skin, lung, and oral cavity. And doctors may offer to screen men for prostate or testicular cancer, and women for ovarian cancer. Doctors consider many factors before recommending a screening test. They weigh factors related to the individual, the test, and the cancer that the test is intended to detect.
For example, doctors take into account the person's age, medical history and general health, family history, and lifestyle. The doctor pays special attention to a person's risk for developing specific types of cancer. In addition, the doctor will assess the accuracy and the risks of the screening test and any followup tests that may be necessary.
Doctors also consider the effectiveness and side effects of the treatment that will be needed if cancer is found. People may want to discuss any concerns or questions they have about screening with their doctors, so they can weigh the pros and cons and make informed decisions about having screening tests.
Symptoms of Cancer
Cancer can cause a variety of symptoms. These are some of them:
- Thickening or lump in the breast or any other part of the body
- Obvious change in a wart or mole
- A sore that does not heal
- Nagging cough or hoarseness
- Changes in bowel or bladder habits
- Indigestion or difficulty swallowing
- Unexplained changes in weight
- Unusual bleeding or discharge
When these or other symptoms occur, they are not always caused by cancer. They may also be caused by infections, benign tumors, or other problems. It is important to see the doctor about any of these symptoms or about other physical changes.
Only a doctor can make a diagnosis. One should not wait to feel pain: Early cancer usually does not cause pain.