All the organs are composed of cells with specialized functions. What is important for their normal functioning is that these functions harmonize themselves. At the time of cellular division, each normal cell differentiates itself to fulfill the function it has been assigned to. It dies after a certain time and is replaced by a new one. This renewal process is constant.

A defect can occur during cellular division which may cause cells to become malignant (cancerous). Malignant cells grow at a much higher rate than normal cells. These rapidly growing cells spread directly into surrounding tissue, and begin a cycle of duplication within our organs. When the immune system can no longer manage or destroy these malignant cells, a tumor which is an accumulation of cells appears. A tumor can spread to surrounding organs. The spread of these malignant tumors is usual by the lymphatics or the bloodstream. Once this movement begins metastatic invasion occurs.