Clearly the body in its normal processes regulates cell growth in an orderly manner. In cancer, there is no regulation and cell reproduction and growth is disorderly. The dangers of cancer are related to this chaotic reproduction of malignant cells. As the cancer cells continue to proliferate, the mass of abnormal tissue that they form enlarges, ulcerates, and begins to shed cells that spread the disease locally or to distant sites. This migration is called metastasis.
Some cells penetrate neighboring tissues, destroying normal cells and taking their place.
Others can enter the blood stream and lymphatic vessels and be carried along in the fluid to another part of the body. It is doubtful that one process is involved in the etiology of all cancers. The exact cause of the conversion of normal cells into cancerous ones is still not completely understood. An important factor is permanent alteration in the DNA of the cell, which is passed on to subsequent generations, but we do not know what triggers the change in DNA structure and why some people succumb to a cancer and others do not. Environmental causes are believed to account for at least 50 per cent and perhaps, in some types, as much as 80 per cent of all cancers.