A name given to a collection of disease where cells begin to divide without stopping and spread to surrounding tissues. When cancers form solid masses, they are called tumors. Tumors can be either benign or malignant. Malignant tumors spread to and invade nearby tissue. Benign tumors do not spread. Also, pieces of tumors can break off and spread to other parts of the body. It should be noted that not all tumors are cancerous. Each cancer behaves very differently, depending on where it begins. Carcinomas begin in the skin or tissue surrounding internal organs. Sarcomas begin in bone, cartilage, fat, muscle, or other connective tissue. Leukemia begins in the blood and bone marrow. Lymphomas begin in the immune system. Central nervous system cancers begin in the brain and spinal cord.