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 of cells. Normally, cells divide to produce more cells only when the body needs them. This orderly process helps keep us healthy. 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.
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. This is how cancer spreads from the original (primary) tumor 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 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 cancers cells in the liver are lung cancer cells. The disease is called metastatic lung cancer (it is not liver cancer).
Tumor grade refers to the degree of abnormality of cancer cells compared with normal cells. The term differentiated describes the extent to which cancer cells are similar in appearance and function to healthy cells of the same tissue type. The degree of differentiation often relates to the clinical behavior of the particular tumor. Grade is a classification system used by pathologists to describe the degree of differentiation of tumor cells.
