Cancer is a group of diseases caused by the out-of-control growth of abnormal cells with the potential to spread to other parts of the body.
The disease called cancer has been around as long as human beings have existed. Evidence of cancerous growths, or tumors, has been found among fossilized bones and in human mummies dating from Ancient Egypt. The ancient Greek physician Hippocrates (hi-POK-ra-tees) was the first to use the word carcinoma (kar-si-NO-ma) to describe various kinds of tumors. Hippocrates noted that the outgrowths of some tumors looked like the limbs of a crab. The word cancer comes from the Latin word for crab. In 1913, only one in nine people had a chance of being alive five years after a diagnosis of cancer. A century later, depending on the site of the cancer, more than 50 percent of people survive the disease for five or more years. For many types of cancer, early detection and treatment result in a normal lifespan. Despite this progress, cancer was the second leading cause of death in the United States as of 2016.
Cancer can arise almost anywhere in the body, but no matter where it occurs, it is characterized by three features: first, the appearance of abnormal cells, meaning cells that do not function as they should; second, their uncontrolled growth into a nonstructured mass (tumor); and third, their spread (metastasis) (meh-TAS-ta-sis) to surrounding tissues and distant parts of the body. Because cancer cells can spread (metastasize), cancer is also called a malignancy or a malignant * tumor, as opposed to a benign * tumor, which remains in the part of the body in which it starts and does not endanger life.
The main types of cancers are as follows:
A cancer is generally named for the location in the body where it starts; no matter where it spreads to later, it retains the name of its initial site.
Life proceeds by cell growth and division, and this process is directed by a collection of genes whose proteins work like traffic police to encourage growth or to halt it. When these genes are mutated, the proteins they make may erroneously tell cells to continue growing, like a traffic light stuck on green. Such mutated genes are called oncogenes because they are capable of transforming normal cells into cancer cells. Normal genes that regulate cell growth and proliferation but that can cause cancer when they mutate are called proto-oncogenes. Dozens of proto-oncogenes have been identified in human cancer. Normal cells with damaged DNA die if the body cannot repair the DNA, but cancer cells produced by oncogenes do not die-they keep growing.
Many tumors need 30 to 40 years to develop, which explains why children rarely get cancer. It is possible for a person to inherit a mutant cancer-causing gene, in which case cancer will begin to develop at an earlier age.
Genes can undergo mutations as a result of cancer-causing substances called carcinogens * (kar-SIN-o-jens), which are found in the environment and in chemicals in human cells. Another source of mutations is mistakes in the information copying that occur when DNA is replicated during cell division. Cells normally have repair systems to correct such errors. When the repair system slips up, the damage becomes a permanent part of that cell and its descendants. If a person has a faulty repair system, mutations in the genes build up rapidly, making the cells more likely to become cancerous. Faulty repair plays a role in certain kinds of colon, skin, and breast cancers.
Metastasis (me-TAH-sta-sis) is the name of the process by which cancer cells spread to other parts of the body where they again begin to grow and replace normal tissue. During this process, cancer cells break away from the primary tumor and enter the bloodstream or lymphatic system * , which can carry them to almost any part of the body. Carcinomas metastasize primarily through lymphatic channels, whereas sarcomas metastasize primarily through the bloodstream. Cancer cells frequently first spread to the regional lymph nodes, meaning the nodes near the primary tumor, which is called regional disease (or involvement). Cancer that spreads to other organs or to lymph nodes distant from the primary tumor is called metastatic disease. New tumors resulting from metastasis are called metastatic (MET-uh-STAT-ik) tumors and their cells are the same as those of the primary tumor. The most common sites of metastases from solid tumors are the lungs, bones, liver, and brain.
New techniques show that abnormal cells from a tumor often are circulating even when doctors can find no evidence of spread, which is referred to as undetectable spread micrometastasis (MY-kro-meh-TASta-sis). Once a cancer cell has found a new site, it must reverse all the steps it took in freeing itself from the primary site. It has to attach to the inner lining of a blood or lymph vessel, cross through it, invade the tissue beyond, and multiply. Probably fewer than one in 1 million of these cancer cells are able to survive to take up residence elsewhere. Much of the way in which cancer spreads remains a mystery, however. Some tissues—for example, cartilage and brain tissue—seem more resistant to cancer. And some animals almost never develop cancer.
A risk factor is anything that increases a person's chance of getting a disease. But having a risk factor does not mean that a person will always get the disease. People usually get cancer as a result of a complex set of interactions between many risk factors, and this is why it is often called a multifactorial (MUL-tih-fak-TOR-ee-ul) disease. Many of the causes of cancer have been identified. Besides factors such as heredity, diet, and hormones, studies point to key external factors that promote cancer such as carcinogens, pollution, radiation, and viruses or bacteria.
Carcinogens are substances that directly cause or can help to cause cancer. Thousands of chemicals used in farming and industry can cause cancer in humans or animals after prolonged or excessive exposure. Notorious chemical carcinogens include asbestos (az-BES-tos), and several insecticides *
Some research is focused on trying to answer more questions about diet and cancer. One European study, called EPIC, involved 500,000 people in 10 countries and aimed to produce detailed and reliable information about diet and cancer. The first results confirmed that a high-fiber diet reduces the risk of colorectal cancer, and that eating red or processed meat increases colorectal cancer risk whereas the intake of fish lowers the risk.
There is mounting evidence that toxic pollution causes cancer, especially in children. Cancer-causing chemicals have been released into the air and water systems of the United States over the years. The Environmental Protection Agency is the government agency that monitors and reports the levels of these pollutants.
Studies have also been done to determine the relationship between air pollution or environmental tobacco smoke (ETS) and the incidence of cancers of the bladder, lung, oral cavity, pharynx, larynx, or blood (leukemia).
Some forms of radiation have been shown to cause cancer, especially ionizing radiation, which has been proven to cause genetic damage leading to cancer. X-rays, gamma rays, cosmic rays, and particles released by radioactive materials such as alpha particles and beta rays are examples of ionizing radiation. Leukemia was at one time thought to be the major cancer to arise from high-dose radiation exposure, based on the experience of people exposed to the atomic blasts in Japan in 1945. It was later determined that other cancers can result from radiation exposure, although they may take 10 to 15 years to develop.
Most cancer deaths are caused by non-ionizing radiation from natural sources such as the sun's ultraviolet * rays. For example, sunburns during childhood are a major factor in causing a kind of skin cancer called melanoma (MEL-a-NO-ma), and most skin cancers are a direct result of sunlight exposure. On the other hand, electric power lines, household appliances, and cellular telephones have not been proven to cause cancer.
Viruses have been shown to trigger some cancers. These findings do not mean that these cancers can be caught like a flu infection. What happens is that the virus can cause genetic changes in cells that make them more likely to become cancerous. For instance, links have been established between the following pairs:
Many symptoms of cancer, including weight loss, fever, fatigue, and various kinds of lumps, could be caused by other diseases. Some cancers may cause no symptoms until they have spread. Based on the most commonly occurring cancers, the American Cancer Society publishes a list of seven warning signs of cancer:
Various treatments for cancer are surgery, radiation therapy, and chemotherapy * (kee-mo-THER-a-pee), either prescribed alone or together. Other treatment methods such as immunotherapy and antiangiogenesis treatment may be used. Because cancers vary in how fast they grow, where they spread, and how they respond to treatment, treatment is tailored to the specific cancer that a person has.
Many cancers could be prevented by changes in a person's lifestyle. For example, cancers caused by cigarette smoking and drinking a lot of alcohol could be prevented completely. Limiting certain kinds of foods, such as red meats and animal fats, and eating lots of fruits and legumes (such as peas and lentils) may help reduce the risk of getting many cancers.
Physical activity helps to avoid obesity * and is believed to have other protective effects against cancer. Most of the 1 million skin cancers that are diagnosed each year could be avoided by staying out of the sun during the times of day when the sun's rays are most intense, and by using sunscreens and protective clothing.
Regular cancer checks, called screenings, for cancer of the breast, colon, rectum, cervix, prostate, testes, mouth, and skin are an effective way of detecting cancer early enough so that it can be treated successfully. In addition, self-examination for breast, testicular, and skin cancers may also help to detect tumors at earlier stages. The American Cancer Society estimates that if all Americans participated in regular cancer screenings, survival would be dramatically improved.
A cancer diagnosis is usually shocking and frightening. A person's life is suddenly disrupted by visits to the doctor, surgery, treatment, and changing personal relationships. Children with cancer may have to miss school for a time or to give up sports or other activities. Children especially may feel that something they did caused the cancer, especially if it is a brother or sister who is sick. Family, physicians, friends and organizations, religious leaders, and self-help groups may be important sources of support. Each person's way of dealing with cancer is unique. Even with life-threatening cancers, a person may live for many years, and more than 70 percent of children and adolescents with cancer are successfully treated.
Many patients seek alternative therapies during their treatment. These therapies generally are of two kinds:
See also Alcoholism • Bladder Cancer • Brain Cancer • Brain Tumor • Breast Cancer • Cervical Cancer • Cirrhosis of the Liver • Colorectal Cancer • Ewing's Sarcoma • Fibrocystic Breast Disorder • Hepatitis • Human Papillomavirus (HPV) • Immune System and Other Body Defenses: Overview • Kidney Cancer • Leukemia • Liver and Biliary Tract Cancers • Lung Cancer • Lymphoma • Malignant Melanoma • Oral Cancer • Ovarian Cancer • Pancreatic Cancer • Polyps • Prostate Problems: Overview • Skin Cancer • Stomach Cancer • Testicular Cancer • Thyroid Cancer • Tumor • Uterine Cancer • Viral Infections
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* malignant (ma-LIG-nant) refers to a condition that is severe and worsening.
* benign (be-NINE) refers to a condition that is not cancerous or serious and will probably improve, go away, or not get worse.
* lymph nodes (LIMF) are small bean-shaped masses of tissue containing immune system cells that fight harmful microorganisms. Lymph nodes may swell during infections.
* immune system (im-YOON SIStem) is the system of the body composed of specialized cells and the substances they produce that help protect the body against disease-causing germs.
* bone marrow is the soft tissue inside bones where blood cells are made.
* carcinogens (kar-SIN-o-jenz) are substances or agents that can cause cancer.
* lymphatic system (lim-FAH- tik) is a system that contains lymph nodes and a network of channels that carry fluid and cells of the immune system through the body.
* insecticides are chemicals used to kill insects and prevent infestation.
* chemotherapy (KEE-mo-THER-apee) is the treatment of cancer with powerful drugs that kill cancer cells.