What Causes Cancer
Causes of Cancer
No specific cause of human cancer has been identified. All cells divide to produce other cells, and it is thought that the formation of a cancer cell from a normal one is due to a genetic mutation at the time of cell division. Such a mutation can be initiated by many factors.
Through studies of single cells and of various animals, including man, scientists have found a number of chemical and physical agents that can initiate a neoplastic change. These cancer promoting agents are known as carcinogens.
Chemicals and Cancer
A large number of chemical substances have been shown to produce tumors when applied to the skin of mice and rabbits. Although it is not necessarily correct to assume that the same reaction occurs in man, there are many instances in which a true cause and effect relationship appears to exist between exposure to a carcinogen and the development of human cancer.
For example, tars, pitch, and certain industrial oils have been associated with a high incidence of skin cancer. It has also been found that workers in the aniline dye industries who absorb these dyes and excrete them in their urine have a high incidence of cancer of the urinary bladder.
The evidence for a relationship between cigarette smoking and lung cancer has been well documented in the 1964 report Smoking and Health, by the Advisory Committee to the Surgeon General of the U. S. Public Health Service. Although the exact carcinogenic factor in smoking has not been identified, it has been shown that smokers have a higher rate of lung cancer than nonsmokers.
Some of the chemicals in hair dyes have also been associated with several cancers, such as bladder cancer, non-Hodgkin lymphoma (NHL), and leukemia.
The relatively high incidence of skin cancer among the scientists who first worked with X-rays, radium, and other radioactive materials first indicated the ability of ionizing radiation to induce cancer. Subsequently, the development of bone cancer in women who painted radium dials served to confirm this relationship. (The women had ingested the radium by moistening their brushes with their tongues.)
The most recent large-scale study of radioactivity as a cause of cancer has been in the survivors of the atomic bomb blasts at Hiroshima and Nagasaki. The incidence of leukemia in these individuals has been found to be directly related to their distance from the center of the blast. For people who were 2,000 meters from the explosion, the incidence of leukemia is three times the normal rate and for those who were only 1,000 meters away, the incidence is twenty times the normal rate. Other examples of radiation-induced cancer are seen among people who have received unusually large doses of radioisotopes or X-ray therapy as treatment for other types of cancer.
It is estimated that about 21,000 people die each year from lung cancer caused by radon gas which can accumulate in buildings.
The first evidence to suggest that viruses can cause cancer was demonstrated by the American physician Francis Peyton Rous in the early 1900's. The virus Rous found was in a tumor of a chicken and it is known as the Rous sarcoma virus. Since then, painstaking laboratory work has demonstrated that certain other cancers in laboratory animals are caused by viruses and there has been an enormous effort devoted to investigations of the possibility that human cancers may also be induced by viruses. However, at the present time there is no direct evidence to support the theory that there may be an association between viruses and human cancer.
Other Causes of Cancer
Under special circumstances many other factors may play a role in the production of cancer. Alcohol, for example, when taken in excess, appears to promote cancer of the larynx. Nutritional deficiencies are believed to promote cancer of the liver and esophagus, and syphilitic lesions of the tongue may lead to cancer of the tongue. In addition, a definite link has been found between skin cancer and prolonged exposure to the sun.
Some factors that were once thought to lead to certain types of cancers have, on closer examination, been found to be unrelated to cancer.
The alleged relationship between pregnancy and cancer of the uterine cervix has been disproved as has the relationship between the therapeutic use of hormones and cancers of the breast, uterus, and prostate.
It is obvious that not every individual who is exposed to a potentially carcinogenic stimulus actually develops cancer. It appears, therefore, that there must be certain intrinsic factors that influence a person's susceptibility to the extrinsic factors, the cancer-inducing agents. It is believed that the intrinsic factors are related to a person's heredity, immunity, and sex, but very little is known about how these factors are involved.