Cancer is the name given to a collection of related diseases. In all types of cancer, some of the cells in the body begin to divide non-stop and spread to the surrounding tissues. Cancer can start almost anywhere in the human body, which is made up of billions of cells. Normally, human cells grow and divide to form new cells when the body needs them. When cells get old or damaged, they die and new cells take their place.
When the cancer develops, however, this orderly process breaks down. As cells become more and more abnormal, old or damaged cells survive when they should die, and new cells form when they are not needed. These additional cells can divide without stopping and can form tumors called tumors. Many cancers form solid tumors, which are masses of tissue. Blood cancers, such as leukemias, usually do not form solid tumors.
Cancerous tumors are malignant, which means they can spread or invade nearby tissues. In addition, as these tumors grow, some cancer cells may detach and travel to distant locations in the body through the blood or lymphatic system and form new tumors away from the original tumor.
Unlike malignant tumors, benign tumors do not spread or invade nearby tissues. Benign tumors can sometimes be quite large, however. When they are removed, they usually do not grow back, while malignant tumors sometimes do so. Unlike most benign tumors in other parts of the body, benign brain tumors can be life-threatening.