Cancer is a group of many related diseases. All forms of cancer involve out-of-control growth and the spread of abnormal cells.
Normal body cells grow, divide, and die in an orderly fashion. During the early years of life, normal cells divide more rapidly until the person becomes an adult. Thereafter, normal cells of most tissues divide only to replace worn-out or dying cells and to repair injuries.
Cancer cells, however, continue to grow and divide and can spread to other parts of the body (metastasize). These cells accumulate and form tumors that may compress, invade, and destroy normal tissue. When cancer spreads, however, it remains named after the part of the body where it originated. For example, if a prostate cancer spreads to the bones, it is still called prostate cancer, and if a breast cancer spreads to the lungs it is still called breast cancer.
Leukemia, a form of cancer, does not usually form a tumor. Instead, these cancer cells of the blood and blood-forming organs (bone marrow, lymphatic system, and spleen) circulate through other tissues where they can accumulate.
It is important to realize that not all tumors are cancerous. Benign (noncancerous) tumors do not metastasize and are generally not life-threatening. Cancer is classified by the part of the body in which it began and by its appearance under a microscope. Different types of cancer vary in their rates of growth, patterns of spread, and responses to different types of treatment. That's why people with cancer need treatment that is aimed at their specific form of the disease.
In the United States, about half of all men and one-third of all women will develop cancer during their lifetimes. Today, millions of people are living with cancer or have been cured of their disease. The risk of developing many types of cancer can be reduced by changes in a person's lifestyle, including quitting smoking, eating a better diet, and reducing sun exposure. The sooner a cancer is found and the sooner treatment begins, the better a patient's chances are of cure.
Today in the United States, over half of all cancer patients are cured, and the disease can be prevented in many others.