 All body organs are composed of cells with specialized functions which must work in harmony in order to function correctly. After cellular division, each healthy cell differentiates to fulfill a designated function. After a certain time, a cell dies and is replaced by a new cell. This renewal process is constant. A defect may occur during cellular division causing cells to become malignant (cancerous). They then proliferate, since malignant cells grow much faster than healthy ones and spread into surrounding tissue. When the body's defense mechanisms (the immune system) can no longer destroy these malignant cells, a mass of tissue, known as a tumor, forms. First located in one organ, a tumor may then spread to surrounding ones. When tumor cells are transported through the blood and lymph systems to reach remote organs, we use the term metastases (from the Greek word meaning "displacement") or metastatic invasion. | 
