Simply put, cancer is the result of unregulated cell division. Cancer cells divide when they are not supposed to, don't stop dividing when they are supposed to and don't die when they should. In the worst cases, the cancer cells leave the area in which they arose and travel to other parts of the body.
Cancer cells do not look or act like the normal cells from which they originate. It is reasonable, then, to ask 'Why do cancer cells behave so badly?'. It turns out the the answers lie in the genes of the affected cells. In cancer cells, changes to key genes cause the cells to act abnormally. The changes are often the result of changes to the DNA (mutations) in the cells. Because there are many different things that are capable of causing mutation, there are an equally large number of causes of cancer.
The development of cancer takes place in a multi-step process. As the cells become more abnormal, they gain new capabilities, such as the ability to release growth factors and digestive enzymes. The cells continue to divide, impacting nearby normal cells, often reducing the function of the affected organ. Even abnormal cancer cells die sometime and a tumor that is large enough to feel can take years to reach that size. Although not all cancers share exactly the same steps, there are some general features that are shared in the development of many types of cancer. Another critical step in the growth of a tumor is the development of a blood supply (angiogenesis). Blood provides nutrients, carries away waste and the blood vessels provide a way for cancer cells to move around the body. The stages in the development of a solid tumor (i.e. breast cancer, or skin cancer) and the process of angiogenesis are described in the following pages.
The following sections describe some chemicals and other agents that are known to cause cancer and discuss how they are thought to cause changes in normal cells.
Below is a list of topics covered in this section.
Also see Viruses and Cancer