Cancer is a group of abnormal cells often—though not always—consolidated into a mass called a tumor. The common characteristic of all cancer cells is the absence of normal growth control mechanisms. Normal, healthy cells grow and reproduce only to replace cells that have died or, in young animals, to support ordinary growth and development. Cancer cells have no such restraints; they keep growing and reproducing even when there is no need for new cells. They often invade and damage or destroy nearby healthy cells.
Any tissue in a body can develop cancer cells; no area of a body is immune to cancer. More than 100 types of cancer are known. Cancers are named based on the type of cell or organ in which the cancer develops. Thus, as an example, hepatocellular carcinoma is a specific type of cancer involving the liver.
While many types of cancer cause the development of tumors, not all tumors are malignant (cancerous). Tumors can be benign (noncancerous). Benign tumors, while they may need to be treated or removed, are not usually as dangerous as malignant tumors. Malignant tumors invade and destroy nearby cells and organs. They can be difficult to remove because of the involvement of nearby organs. Further, malignant tumors often spread (metastasize) to other parts of the body through the lymphatic system or the bloodstream.
Last full review/revision July 2011