Cancer is a group of more than 100 different diseases. Cancer occurs when cells become abnormal and keep dividing and forming more cells without control or order. All organs of the body are made up of cells. Normally, cells divide to produce more cells only when the body needs them. If cells divide when new ones are not needed, they form a mass of excess tissue, called a tumor or growth. Tumors can be benign (not cancer) or malignant (cancer). Benign (not harmful) tumors are not cancer. They can usually be removed by surgery and are not likely to come back. More important, cells from benign tumors do not spread to other parts of the body. Benign tumors are rarely a threat to life.

Brain tumor study and comparison of nuclear medicine and MR Upper row: fused image of nuclear medicine acquisition and MR acquisition Middle row: MR study yields excellent anatomic detail (spatial resolution) Bottom row: nuclear medicine (SPECT) images yield excellent functional information

Malignant tumors are cancer. The cells in malignant tumors can invade and damage nearby tissues and organs. Cancer cells can also break away from a malignant tumor and travel through the bloodstream or the lymphatic system to form new tumors in other parts of the body. The spread of cancer is called metastasis.

Most cancers are named for the type of cell or 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 (metastasizes) to the liver, the cancer cells in the liver are lung cancer cells. The disease is called metastatic lung cancer, it is not liver cancer.

Updated: April 2007