If cells keep dividing when new cells are not needed, a mass of tissue forms. This mass of extra tissue, called a growth or tumor, can be benign or malignant. Most important, cells from benign tumors do not spread to other parts of the body. Benign tumors are rarely a threat to life. Malignant tumors are cancer.
Cancer cells can invade and damage nearby tissues and organs.
Also, cancer cells can break away from a malignant tumor and enter the bloodstream or the lymphatic system. The spread of cancer is called metastasis. Most cancers are named for the type of cell or the organ in which they begin. When cancer spreads, the new tumor has the same kind of abnormal cells and the same name as the primary tumor. For example, if lung cancer spreads to the liver, the cancer cells in the liver are lung cancer cells.