Medical Journals Cancer:

Cancer is that the name given to a set of related diseases. In all sorts of cancer, a number of the body’s cells begin to divide no end and spread into surrounding tissues. Cancer can start almost anywhere within the physical body, which is made up of trillions of cells. Normally, human cells grow and divide to make new cells because the body needs them. When cells get older or become damaged, they die, and new cells take their place. When cancer develops, however, this orderly process breaks down. As cells become more and more abnormal, old or damaged cells survive once they should die, and new cells form once they aren't needed. These extra cells can divide no end and should form growths called tumors. Many cancers form solid tumors, which are masses of tissue. Cancers of the blood, like leukemias, generally don't form solid tumors. Cancerous tumors are malignant, which suggests they will spread into, or invade, nearby tissues. In addition, as these tumors grow, some cancer cells can break off and visit foreign places within the body through the blood or the lymph system and form new tumors far from the original tumor.