Cancer is a disease in which cells multiply wildly, destroy healthy tissue, and endanger life. About 100 kinds of cancer attack people. Cancer strikes people of all ages but is most common in the middle-aged and the elderly. The disease is a leading cause of death in many countries. Cancer occurs about equally in males and females. The disease can attack any part of the body and may spread to other parts. Cancer occurs in most other animals as well as in people.
The study of the development, treatment, and prevention of cancer is called oncology. It includes both research and clinical care. Physicians who specialize in oncology are known as oncologists.
Scientists have greatly increased their knowledge of how cancer develops. The cells of all living things contain genes that direct cell growth and reproduction. Cancer arises when these genes become damaged and cells multiply without control. The damaged genes may instruct the cell to divide in an abnormal fashion. Other genes may lose the ability to instruct the cell that it can no longer function and must die. Some damaged genes are changed in ways that enable cancer cells to invade healthy tissue, grow new blood vessels, or spread to other parts of the body.
Some types of abnormalities in genes are inherited. Substances in the environment can also damage genes. A substance that damages genes in a way that can lead to cancer is called a carcinogen. Many substances found in nature, both natural and artificial, are carcinogens.
Without proper treatment, most kinds of cancer are fatal. However, methods of diagnosing and treating the disease have improved greatly. About half of all cancer patients now survive at least five years after treatment. People who remain free of cancer during that time have a good chance of remaining permanently free of the disease.
More research is needed to find even better methods of curing cancer. Because cancer affects so many people, many countries spend public money to fund cancer research.
Major types of cancer
Cancers that affect human beings are classified in two ways: (1) by primary body siteâ€”that is, the part of the body where cancer first develops; and (2) by the type of tissue in which the cancer originates.
Classification by primary body site. Cancer first appears most often in the skin, the female breasts, and the organs of the digestive, respiratory, reproductive, blood-forming, lymphatic, and urinary systems. The number of cases of cancer in these sites varies from country to country. Cancer of the stomach, for example, is much more common in Japan than in the United States or India. Rates of breast cancer are high in the United States and the United Kingdom, but much lower in India and many Asian countries. The following discussion deals with the kinds of cancer that occur most often in many countries.
Skin cancer is the most common type of cancer in the world. Most skin cancers grow slowly and do not spread to other parts of the body. As a result, these cancers are among the easiest to cure. Most people treated for skin cancer recover completely. However, one form of skin cancer called malignant melanoma is particularly dangerous. This type of cancer begins in the skin's pigment cells. If not detected and treated early, it can spread rapidly to other parts of the body.
Cancers of the digestive organs most commonly affect the parts of the large intestine called the colon and therectum. About half of all people treated for cancer of the colon or rectum survive five years or longer after treatment with no return of the disease. Other digestive organs commonly affected by cancer include the esophagus, liver, pancreas, and stomach.
Cancers of the respiratory system involve the larynx and lungs. In most industrial countries, lung cancer kills more people each year than any other kind of cancer. The death rate is high because many lung cancers have spread to other body sites before they are detected.
Breast cancer occurs in both sexes, but it attacks women about 100 times more often than it strikes men. Most of these cancers occur in women over 40 years of age. Female breast cancer patients whose disease is found and treated before it has spread beyond the breast have good long-term survival rates.
Cancers of the reproductive system are relatively common among both men and women. The male organ most often affected is a small gland called the prostate. Most cases occur in men over 50 years old. Proper treatment can cure the disease in its early stages and control more advanced prostate cancer for many years.
The most common cancers of the female reproductive system affect the uterus, the organ in which babies develop. Some cancers arise in the main part of the uterus. Cancer may also affect the lower, necklike part of the organ called the cervix. Cancer of the cervix strikes more younger women than do other cancers of the uterus.
Cancers of the blood-forming and lymphatic systems. Cancer of the bone marrow and other blood-forming organs is called leukemia. In leukemia, immature white blood cells multiply wildly and interfere with production of other vital blood elements.
Cancer may also arise in tissues of the lymphatic system. This system is a network of vessels that returns fluids to the bloodstream and helps fight disease. Lymphatic cancer is called lymphoma. A well-known form of lymphoma is Hodgkin's disease, named after Thomas Hodgkin, the English physician who first described the disorder. Hodgkins disease affects both young adults and older individuals. The most common lymphatic cancer is non-Hodgkin's lymphoma. Cases of non-Hodgkin's lymphoma have risen steadily since the mid-1900's. Many people with AIDS develop this type of cancer.
Cancers of the urinary system. The bladder is the urinary organ most commonly affected by cancer. The majority of bladder cancer patients are men, and most survive five years. Many such cancers are detected early because they arise in the inner layer of the bladder and cause bleeding in the urine.
Classification by tissue. Scientists group cancers based on the body tissue in which tumors begin. Carcinomasdevelop in epithelial tissue, which forms the outer layer of skin and lines internal body surfaces and organs. Many organs, including the breast, colon, and lung, also contain glands. Cancers that arise in gland tissue, calledadenocarcinomas, are among the most common cancers.
Cancers that develop in connective tissue are called sarcomas. Connective tissue forms the body's supporting structures, such as bones and cartilage. Sarcomas also form in the breast, digestive system, respiratory system, and reproductive system, but far less often than carcinomas.
Cancer in children, called pediatric cancer, differs from adult cancer in several ways. Cancers common in adults, such as those that occur in the lungs, breasts, prostate, or colon do not usually occur in children. The common types of cancer that strike children include neuroblastoma, which arises in nervous tissue;retinoblastoma, which develops in the eyes; nephroblastoma, also called Wilms tumor, which occurs in the kidneys; rhabdomyosarcoma, which arises from soft tissues; and osteogenic sarcoma and Ewing's sarcoma,which develop in the bones. These cancers are extremely rare in adults.
The emotional impact on the family and physical and emotional consequences to the child are important considerations in the treatment of pediatric cancer. Fortunately, many of these cancers and leukemias are highly curable, and many children survive to lead normal lives following treatment. However, the treatment of pediatric cancer may have long-term consequences for patients as they live out normal life spans. For example, radiation therapy and chemotherapy can actually cause secondary cancers, including leukemia, years after treatment has been completed. Other long-term consequences of cancer treatment may include stunted growth and infertility later in life.
Scientists have found that the cells of pediatric cancers often show specific genetic abnormalities. These abnormalities have given researchers a better understanding of the genetic difference between normal cells and cancer cells.
How damaged genes cause cancer
Every person begins life as a single fertilized egg cell. Through a complicated process of growth, division, and specialization, the egg multiplies into the trillions of cells in a healthy body. Every cell contains complex instructions that direct this process. The instructions are chemically coded in long coils of a substance called DNA (deoxyribo nucleic acid). Particular sections of DNA make up genes that control specific cell functions. DNA even contains genes that enable the molecule to repair itself. But as people grow older, damage can build up and destroy DNA's ability to repair itself.
Cancer often results from damage to the genes that control cell growth and division. Two important classes of these genes are called proto-oncogenes and suppressor genes. Proto-oncogenes promote cell growth or division. Damage to a proto-oncogene may transform it into an overactive form called an oncogene. Oncogenes can lead to cancer by directing a cell to multiply excessively. Scientists have identified dozens of oncogenes that contribute to cancers in many sites, including the bladder, breasts, liver, lungs, and colon.
Suppressor genes limit cell growth or division. Damage to a suppressor gene can lead to cancer by destroying that gene's ability to stop cell multiplication.
Scientists think most cancers involve transformation of many proto-oncogenes into oncogenes and inactivation of a number of suppressor genes. In most cases, genetic damage must accumulate for years before a cell becomes cancerous. Once cancer occurs, the disorganized, rapidly dividing cells gradually build up into a mass that compresses and destroys nearby tissue. As the cancer grows, cells can break away and travel through blood or lymph (fluid from body tissues) to invade other parts of the body. This spread of cancer to other sites is called metastasis. The likelihood of curing cancer drops sharply after the tumor has spread.
People acquire the genetic damage that can lead to cancer in two main ways: (1) by inheriting damaged genes; and (2) through exposure to substances in the environment that damage genes.
Inheriting damaged genes. Scientists have long known that the risk of some cancers increases for people with close relatives who also have the disease. This increased risk occurs because some types of genetic damage involved in cancer can be passed from parents to children. But most cancers require multiple injuries to DNA. Inherited damage to one gene thus raises risk but does not guarantee that any particular individual will develop cancer. Scientists have identified the inheritable genetic damage involved in certain forms of breast, colon, and other cancers. Researchers are working to learn much more about inherited cancer genetics. They hope their knowledge will lead to new strategies for preventing and treating cancer.
Genes damaged by substances in the environment. Most people who develop cancer do not have inherited genetic abnormalities. Their genes are damaged after birth by carcinogens in their environment.
Scientists identify carcinogens by investigating unusually high cancer rates in groups of unrelated people. Forexample, scientists might notice that people in a particular job tend to get a certain type of cancer. Experts would then study the ability of chemicals or other substances encountered in that job to cause cancer in laboratory animals. If a high percentage of the animals develop cancer, researchers strongly suspect that the agent may also cause cancer in people.
Once a carcinogen damages a cell's DNA, the damage can be passed on to new cells that arise from division of the damaged cell. The changes are thus passed on to all the cell's descendants. Meanwhile, the descendant cells can acquire additional DNA damage that is also passed along. Experts think many cancers arise from such combined effects of several carcinogens.
Three classes of carcinogens in human beings are (1) chemicals, (2) certain forms of radiation, and (3) viruses.
Chemicals. Scientists have identified many chemicals that can cause cancer in animals. These chemicals may also pose a cancer hazard to human beings. For example, cigarette smoke contains more than 4,000 chemical substances, of which dozens have been identified by scientists as carcinogens.
Some industrial chemicals create a cancer hazard for people who work with them. Such chemicals include aniline dyes, arsenic, asbestos, benzene, chromium, nickel, vinyl chloride, and certain products of coal, lignite, oil shale, and petroleum. Scientists work continuously to find chemicals that pose occupational cancer risks. Such identification can help ensure a safe workplace and identify substances that may also pose a risk to the general population.
Some substances that are added or applied to foods are also suspected of causing cancer in human beings. These substances include some chemicals used to control weeds and some that are used to kill insects. Government agencies regulate many of these substances, and, in some cases, prohibit their use. Molds that sometimes develop on such food crops as corn and peanuts may also contain carcinogens. The mold can be controlled through proper storage and handling of crops.
Radiation. Certain kinds of radiation can disrupt DNA and lead to cancer. X rays are a cancer hazard in large doses. However, doctors do not believe that routine medical and dental X rays pose a significant danger. A form of nuclear radiation known as ionizing radiation can also cause cancer. For example, in 1986, when Ukraine was part of the Soviet Union, a nuclear reactor at a power plant in Chernobyl exploded. Children who were exposed to radioactive fallout from that explosion have experienced an increased rate of thyroid cancer.
Viruses. Experiments have shown that certain kinds of viruses cause cancer in animals. Some viruses can also cause cancer in people. For example, the human papillomavirus causes most cases of cervical cancer. Another virus has been linked to a rare form of leukemia, and some forms of liver cancer may be caused by viruses. However, most experts feel that viruses are not a major cause of human cancers.
Reducing the risk of cancer
Scientists know that many cancers have a genetic cause which limits their ability to prevent the disease. However, scientists have identified several factors that can increase a person's chances of developing cancer. They have also discovered substances that may protect DNA and keep cancer from developing. Many scientists believe that people can decrease their overall chances of developing cancer by adopting a healthy lifestyle and avoiding contact with known carcinogens.
Avoiding carcinogens. Smoking is one of the most widespread and avoidable environmental causes of cancer. Scientists think that smoking causes about one-third of all cases of cancer, including most lung cancers. Smoking also causes many cancers of the mouth, larynx, trachea, esophagus, pancreas, kidney, bladder, and cervix. Cigarette smoke can even cause cancer in nonsmokers who live or work closely with smokers. Smokers who quit can significantly reduce their chance of developing lung cancer.
People can also reduce their risk of developing skin cancer. The sun is one of the most important sources of cancer-causing radiation. Most cases of skin cancerâ€”including deadly melanomaâ€”are caused by an invisible portion of the sun's radiation called ultraviolet rays. Physicians advise against sunbathing and recommend that people who work outdoors wear protective clothing or apply sunscreens that block ultraviolet rays.
Nutrition. Some chemicals naturally present in food may become a cancer threat if consumed in large quantities. Diets high in fats, for example, have been associated with cancers of the breast, colon, and prostate gland. Some studies have linked eating large amounts of salt-cured, salt-pickled, and smoked foods to cancers of the digestive system. Excessive alcohol consumption has been linked to cancer of the mouth, pharynx, larynx, esophagus, and liver.
Many scientists believe that certain foods contain substances that may help prevent cancers in people. Such foods include broccoli, cauliflower, cabbage, spinach, tomatoes, carrots, fruits, and whole-grain breads or cereals.
Cancer detection and diagnosis
Only a doctor can diagnose cancer. But in many cases, people consult a doctor only after the disease is far advanced. A person should therefore be alert to any physical change that may be a symptom of cancer. Detecting cancer while a tumor is small and confined to one location greatly increases the chances of a cure.
Cancer checkups. Many cancers cause no symptoms in their early stages. To detect early cancers, the American Cancer Society recommends that people aged 20 to 40 have a cancer-related checkup by a physician every 3 years. People aged 40 or older should have a checkup every year. A cancer-related checkup usually includes a physical examination. Physicians watch carefully for any visible signs of cancer when they perform physical examinations.
Depending on the patient, screening tests for specific types of cancer may also be performed. Screening tests may include a breast X ray called a mammogram to detect breast cancer in women. A mammogram can detect some breast cancers before any lumps can be felt.
Physicians recommend that beginning at age 50, patients have routine tests for colon cancer. In one such test, called a fecal occult blood test (FOBT), the patient uses special kit to collect tiny samples of stool (solid body wastes). Laboratories test the samples for microscopic amounts of blood, which may indicate cancer.
The Pap test, another recommended yearly test for women, has greatly reduced the death rate from cancer of the uterus. In the Pap test, experts examine cells scraped from the cervix under a microscope. Microscopic signs of cancer can be seen 5 to 10 years before symptoms appear.
A blood test for a protein called prostate-specific antigen (PSA) may detect early prostate cancer. PSA is made only in the prostate gland. Elevated levels of the protein may mean that the gland has become cancerous.
Preliminary diagnosis identifies a suspicious mass or other change in the body that requires further investigation. Some symptoms must be evaluated with special X rays and other advanced imaging techniques. One X-ray technique, called computed tomography or CT scan, can reveal suspicious masses in such internal organs as the brain and the lungs. A technique called magnetic resonance imaging (MRI) uses magnetic fields and radio waves to produce images of internal organs. These imaging techniques allow physicians to determine the exact location, size, and shape of cancer tumors and determine whether the cancer has spread.
Conclusive diagnosis. The various methods of preliminary diagnosis may reveal the presence of a tumor. But not all tumors are cancerous. Most lumps in the breast, for example, are benign (noncancerous). Doctors need to perform a test called a biopsy to make a definite diagnosis of cancer. In a biopsy, doctors remove a small piece of tissue from the tumor. In suspected cases of leukemia, they take a blood sample or remove tissue from a blood-forming organ. Experts then examine the tissue under a microscope to check for cancer cells.
Staging. Physicians use a process called staging to describe how a particular cancer in a patient has advanced. Staging is important in helping physicians decide which treatments are most likely to be successful in treating the cancer. The most common staging method is known as the TNM system. This system describes the tumor (T), whether the cancer has invaded lymph nodes (N), and whether there are any metastases (M). Physicians determine the staging level through physical examination, X rays, CT scans, and other specialized tests.
Since each type of cancer is different, there are different staging categories for each cancer. If cancer is discovered in the early stages, the cancer treatment may be successful. Advanced stage cancers are more difficult to treat. If tests show that the cancer has spread, patients may need to undergo additional treatments.
Cancer has a highly emotional impact on the patient who is diagnosed with the disease, and on his or her family. Most health care providers in the United States try to provide substantial medical and nonmedical support for both the patient and family. Many health care providers have developed comprehensive cancer centers to provide expert management of patients with cancer. The centers provide resources and information for both patients and the physicians treating them.
Methods for treating cancer include surgery; radiation therapy; and drug therapy. Most cancer treatment plans combine several or all of these methods in a technique called multimodality therapy. Physicians may usebiological response modifiers to enhance a patient's ability to fight cancer. However, some cancer patients seek nontraditional or alternative treatments.
Surgery is the main method of treating most types of cancer. Cancer surgery chiefly involves taking out the tumor while minimizing the removal of healthy surrounding tissue. Surgeons work closely with a pathologist(expert on tissue changes) who can examine tissue during the surgery to determine if the tumor has been completely removed.
Surgeons may also cut out additional tissue that appears healthy. For example, breast cancer may be treated with a lumpectomy, an operation that removes the cancerous lump and a margin of the normal tissue surrounding it. Neighboring lymph nodes may also be removed and examined for signs of cancer. Although these nodes may appear normal, they might contain cancer cells that could travel to other parts of the body.
Radiation therapy, also called radiotherapy, attacks cancer cells with X-rays or other high energy particles from radioactive substances. Radiotherapy is often used to treat cancers of the bladder, prostate, head, and neck. Radiation kills cancer cells, but it also kills normal cells.
One type of radiotherapy, called external beam radiotherapy, sends X rays into the target tissue to destroy cancer cells. Machines called linear accelerators produce the X rays at increasingly greater energy. The higher the energy of the X rays, the deeper the beam can penetrate into the body to reach cancer tumors.
Physicians use CT scans and MRI to determine the exact size and shape of a cancerous tumor. In a technique called three-dimensional conformal radiation therapy, the radiation beam is shaped to exactly match the cancerous tumor. Physicians can then deliver a high dose of radiation to the tumor with little radiation exposure to healthy tissue.
A technique called stereotactic radiosurgery is often used to treat cancer tumors located in the brain that cannot be removed through conventional surgery. This technique uses a concentrated dose of gamma raysâ€”a form of high energy radiationâ€”from a radioactive source. With stereotactic radiosurgery, a total of 201 radiation beams intersect on a target area of cancer cells within the body. This precise technique destroys the cancer cells while sparing adjacent healthy tissue.
Brachytherapy is often used to treat cancers of the prostate and brain. In this type of radiotherapy, radiation comes from small capsules, called seeds, of radioactive material implanted by a surgeon close to, or within, the cancer tumor. The radiation only penetrates a short distance, so nearby healthy tissue is not harmed while the cancerous cells are killed.
Drug therapy, also called chemotherapy, is used against a wide variety of cancers. Chemotherapy has proved especially effective in treating leukemia, lymphoma, and testicular cancer. Cancer cells divide much more rapidly than normal cells. Therefore, many cancer drugs are designed to interfere with cell division.
Chemotherapy causes side effects by injuring the normal body cells, especially those that divide most rapidly. Rapidly dividing normal cells include blood-forming cells, cells that line the intestines, and hair-forming cells. Damage to blood-forming cells may increase a patient's risk of developing anemia or an infection. Injury to intestinal cells may cause nausea and vomiting. Disruption of hair-forming cells can cause hair loss. Researchers work constantly to develop drugs that reduce harm to normal cells.
Effective chemotherapy usually involves combinations of drugs. Doctors combine drugs that have different methods of acting on cancer cells and that produce different side effects. Combination therapy reduces the chance that cancer cells will develop resistance to the drugs. It also helps avoid serious side effects from large doses of a single drug.
Multimodality therapy involves some combination of surgery, radiotherapy, and drug therapy. In the most common multimodality therapy, doctors prescribe drug therapy to follow surgery, radiotherapy, or both of those treatments. Such follow-up treatment is called adjuvant drug therapy. Because the drugs reach all parts of the body, they may destroy cancer cells that have spread undetected to distant organs. Adjuvant drug therapy is used to treat some colon and breast cancers as well as some bone cancers.
Biological response modifiers increase a person's ability to fight cancer by strengthening natural body processes. Some biological modifiers are immunotherapiesâ€”that is, they stimulate the body's immune system to attack cancer cells. Other biological modifiers improve the body's ability to withstand aggressive drug therapy. The body makes small quantities of many biological response modifiers. Scientists can now produce some of them in laboratories in large quantities using techniques of molecular biology.
In 2010, the United States Food and Drug Administration (FDA) approved the first vaccine to treat cancer. The vaccine uses a patient's own immune system to fight advanced prostate cancer that has become resistant to other kinds of therapy.
Monoclonal antibodies are an important type of immunotherapy. They are designed to recognize certain proteins that are found on the surface of some cancer cells. The monoclonal antibody then binds onto the protein. This action then triggers the body's immune system to attack the cancer cells and can also cause the cells to destroy themselves. For example, about one-third of breast cancer patients have high levels of a protein called HER2 on the surface of the cancer cells. Scientists have developed a monoclonal antibody that binds onto this protein and stops the cancer cell from growing and dividing. The antibody also causes the body's immune system to attack the cancer cells.
Some experiments are investigating the ability of monoclonal antibodies to deliver microscopic doses of drugs or radiation directly to tumor cells. Scientists are working to make use of tumor-surface antigens to produce vaccines against certain kinds of cancer. Antigens are viruses or other foreign substances in the body that trigger the immune response.
Erythropoietin, another biological response modifier, increases a cancer patient's production of red blood cells. Many kinds of chemotherapy cause anemia by killing red blood cells. In an anemic person, the blood cannot provide the tissues with enough oxygen, causing the patient to feel weak or tired. Erythropoietin helps cancer patients withstand the stress of chemotherapy and maintain an active life.
Alternative medicine. Many people diagnosed with cancer use alternative or other forms of nontraditional medicine. These treatments may include acupuncture, herbal medicine, homeopathy, vitamins, or various dietary supplements. Most of these alternative treatments have not been tested scientifically. Most physicians doubt the validity of claims that alternative treatments are beneficial and many believe that they are harmful.
Cancer research includes a wide range of projects, from identifying carcinogens to developing improved anticancer drugs. Advances in some areas have raised hopes of finding better methods of treatment and prevention. Scientists have made especially rapid progress in the fields of genetics and molecular medicine. These fields involve the development of treatments to target abnormal genes or abnormal functions of genes associated with a particular type of cancer. New cancer treatments are tested in clinical trials.
Genetics. Scientists are working to better understand the role of oncogenes and suppressor genes in the development of cancer. This knowledge could lead to new ways of controlling cancer cells. Scientists have developed tests that show if individuals have certain defective genes. But experts disagree about when such tests should be offered. In most cases, doctors cannot yet use these tests to treat or prevent cancer. Some experts question the value of telling people that they have a damaged gene when this knowledge carries little benefit. But researchers hope that genetic tests may one day make it possible to prevent cancer or to detect the disease in its earliest stages.
Molecular medicine involves the development of specific treatments or medicines to interfere with abnormal genes or abnormal functions of genes. Highly targeted molecular medicine can be directed at killing cancer cells without harming healthy cells.
Biologists are investigating a process called apoptosis Â«ap uh TOH sihsÂ». Apoptosis is also known asprogrammed cell death or â€œcell suicide.â€ In apoptosis, various genes activate mechanisms of self-destruction when cells become damaged or are no longer needed. Techniques that bring on apoptosis in cancer cells may one day offer new treatments.
Another active area of research focuses on preventing blood vessels from growing to nourish cancers. In order to grow beyond a small, harmless size, every cancer must develop its own blood supply. Development of blood vessels is called angiogenesis Â«AN jee oh JEHN uh sihsÂ». Many experts feel that substances that prevent angiogenesis, called angiogenesis inhibitors, can be developed into cancer-fighting drugs.
Researcher are also working to develop drugs that interfere with signal transduction, the process by which growth signals are transmitted to cells. Several types of cancer secrete too much growth factor. These factors act on nearby cells but also drive proliferation of the cells that produced them. Drugs that interfere with signal transduction would stop the uncontrolled cell growth characteristic of cancer cells.
Clinical trials. Despite many advances in cancer treatment, many cancers are not fully curable. Scientists continue to develop new treatments that are tested through clinical trials. In a clinical trial, a group of cancer patients is treated with standard available treatments while another group with the same disease receives a new treatment that is being evaluated. Unfortunately, only a small number of patients who are eligible to participate in clinical trials actually enter these programs. Patients who are interested in participating in a clinical trial should discuss the possibility with their physician.
Marc B. Garnick, M.D., Clinical Professor of Medicine, Beth Israel Deaconess Medical Center, Harvard Medical School.
This article is from The World Book Encyclopedia.