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What is cancer?
About 1.4 million new cases of cancer are diagnosed each year in the United States, and more than 550,000 people will die each year of the disease. Cancer is the second leading cause of death in the USA. However, improvements in cancer detection, diagnosis, and treatment have increased the survival rate for many types of cancer. About 64 percent of all people diagnosed with cancer will be alive 5 years after diagnosis.
Cancer is a group of many related diseases that begin in cells, the body's basic building blocks. 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 as they are needed to keep the body healthy. Sometimes, this orderly process goes wrong. New cells form when the body does not need them, and old cells do not die when they should. The extra cells form a mass of tissue called a growth or tumor. Not all tumors are cancerous; 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 in 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 malignant tumors are abnormal and divide without control or order. Cancer cells invade and destroy the tissue around them. Cancer cells can also break away from a malignant tumor and enter the bloodstream or lymphatic system.
Blood vessels include a network of arteries, capillaries, and veins through which the blood circulates in the body. The lymphatic system carries lymph and white blood cells through lymphatic vessels (thin tubes) to all the tissues of the body. By moving through the bloodstream or lymphatic system, cancer can spread from the primary (original) cancer site to form new tumors in other organs. The spread of cancer is called metastasis.
What causes cancer?
Scientists have learned that cancer is caused by changes in genes that normally control the growth and death of cells. Certain lifestyle and environmental factors can change some normal genes into genes that allow the growth of cancer. Many gene changes that lead to cancer are the result of tobacco use, diet, exposure to ultraviolet (UV) radiation from the sun, or exposure to carcinogens (cancer-causing substances) in the workplace or in the environment. Some gene alterations are inherited (from one or both parents). However, having an inherited gene alteration does not always mean that the person will develop cancer; it only means that the chance of getting cancer is increased. Scientists continue to examine the factors that may increase or decrease a person's chance of developing cancer.
Although being infected with certain viruses, such as the human papillomavirus (HPV), hepatitis B and C (HepB and HepC), and human immunodeficiency virus (HIV), increases the risk of some types of cancer, cancer itself is not contagious. A person cannot catch cancer from someone who has this disease. Scientists also know that an injury or bruise does not cause cancer.
Can cancer be prevented?
Although there is no guaranteed way to prevent cancer, people can reduce their risk (chance) of developing cancer by:
- not using tobacco products
- choosing foods with less fat and eating more vegetables, fruits, and whole grains
- exercising regularly and maintaining a lean weight
- avoiding the harmful rays of the sun, using sunscreen, and wearing clothing that protects the skin
- talking with a doctor about the possible benefits of drugs proven to reduce the risk of certain cancers
Although many risk factors can be avoided, some, such as inherited conditions, are unavoidable. Still, it is helpful to be aware of them. 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 who have an increased likelihood of developing cancer can help protect themselves by avoiding risk factors whenever possible and by getting regular checkups so that, if cancer develops, it is likely to be found and treated early. Treatment is often more effective when cancer is detected early. Screening exams, such as sigmoidoscopy or the fecal occult blood test, mammography, and the Pap test, can detect precancerous conditions (which can be treated before they turn into cancer) and early-stage cancer.
The National Cancer Institute (NCI) is conducting many cancer prevention studies to explore ways to reduce the risk of developing cancer. These studies are evaluating dietary supplements, chemopreventive agents, nutrition, personal behaviors, and other factors that may prevent cancer. More information about cancer prevention trials is available in the following NCI resources:
- Breast Cancer Prevention Studies
- Selenium and Vitamin E Cancer Prevention Trial (SELECT): Questions and Answers
- The Study of Tamoxifen and Raloxifene (STAR): Questions and Answers