Cancer Inducing Stimuli - Radiant Energy And Cancer
( Originally Published Early 1960's )
Among the cancer inducing stimuli of our environment is the very energy source upon which life itself depends, the light of the sun.
The ultraviolet radiation story begins with the clinical observation of Paul Unna of Germany, who in 1894 related exposure to sunlight with chronic skin changes and skin cancer. In 1928, G. M. Findlay of England succeeded in producing skin cancer in experimental animals exposed to intense sunlight. More refined studies by later investigators, particularly Harold F. Blum of the National Cancer Institute, showed that this effect was not due to heat and that it could be localized to a relatively narrow band of the wave spectrum. The cancer producing range is around 3000 Angstrom units (a measure of wave length) . It is interesting, and may be significant, that many of the carcinogenic hydrocarbons show a peak of absorption in the same wave range.
The cancer producing effects of ultraviolet radiation appear to be limited to the skin, probably because of its low penetration. In thin skinned animals such as albino mice, sarcomas are initiated as well as other cancers. Observations on human populations are convincing that at least one type of skin cancer, the squamous epitheliomas, are induced by ultraviolet radiation. The incidence of skin cancer is highest in the white population of the southern and western parts of the United States and lowest in the north, and related to the amount of sunshine in the area. Furthermore, skin cancer occurs more frequently among people who work outdoors, such as sailors and farmers, than among people who can guard themselves against exposure to the sun. The color of the skin is another factor. Cancer of the skin is most common among fair complexioned people, and much less frequent among Negroes than among the so called white races.
Some time ago, a claim was made that people with skin cancer developed less internal cancers than anticipated, and it was even suggested that skin cancers could be produced and then removed from people as a possible immunization procedure. Unfortunately, further careful studies failed to support the reports upon which the theory was based.
The discovery of the cancer producing effects of ionizing radiation from radium and X-rays was made on human beings. The pioneer workers with the new emanations developed dryness, ulcers and, eventually, cancers of their hands. Jean Clunet of France in 1910 produced cancers in several rats following application of radium to the skin. Thus, this experimental induction of cancer was known several years before the discovery that tar produced skin cancer in rabbits, but failed to arouse interest of laboratory investigators.
Ionizing radiation can cause several forms of cancer in man and in animals. Radiologists and others exposed to increased doses of radiation are more liable to develop leukemia than people who are not so exposed. Radium salts, which are deposited in bone, give rise to cancers of bone. A historical tragedy was the death of women from bone cancer following the ingestion of radium by licking brushes they used in painting watch dials with radium. In animals, exposure to high doses of ionizing radiation leads to the occurrence of leukemia, and in mice of certain strains tumors of the ovary, the pituitary and of the lung are increased in incidence.
The people of Hiroshima and Nagasaki who survived exposure to the two prototype atomic bombs have been carefully studied during the past 18 years by the Atomic Bomb Casualty Commission. The information leaves no doubt that a single exposure at high doses produces leukemia in man.
The man-made sources of additional ionizing radiation, not only in some uses of nuclear weapons, but in the increasingly greater dependence upon this energy for industrial purposes, undoubtedly represents one of the more serious potential cancer producing hazards of the future. The importance of the problem has made it a topic of emotional concern and international discussion. One of the key issues involved is the question of threshold. The carcinogenic effects of radiation are easily demonstrable when rather large doses are used. Does the dose have to reach some critical level before radiation becomes dangerous, or does any amount of radiation involve increased risk to cancer and other undesirable effects? This species of problem, of course, also exists in considering chemicals with carcinogenic activity.
We really have no answer to this question, nor any reasonable experimental design through which a definitive answer can be anticipated. For example, if 1 milligram of a chemical produces cancers in all of too mice, and 1/100th of a milligram produces only one cancer in another group of 100 mice, should it be assumed that 1/10,000th of a milligram is entirely safe, or that it will produce one cancer in 10,000 mice? It is feasible to set up an experiment with 10,000 or even 100,000 mice, if necessary, but the trouble would lie in the interpretation. Such a low order effect would be well into the range of occurrence of "spontaneous" tumors, and its relationship to the chemical would be open to doubt. And there is always the question of how far the findings on one species of animals apply to other species, including man.
There are several excellent investigations that show an increase in leukemia in children who are exposed to X-rays while still in the uterus, because the mother had required a diagnostic X-ray of her pelvis during pregnancy. At that low dose of X-ray, the risk to leukemia is increased by a factor of less than twofold. Whether this means that an addition of 2 rads (a unit of absorbed radiation energy) to the lifetime exposure of the earth's population will result in thousands of additional deaths from leukemia and other cancers remains in the realm of theory.
Life is a hazard, and man must always balance one risk against another. In relation to carcinogenic effects of chemicals that may be added to food, a functional decision has been made in the United States and placed on the law books of the land. Provision of the Food and Drug Act prohibits the addition of any amount of cancer producing chemicals to food. However, what is or is not cancer producing is decided on the basis of animal tests and the results have to be interpreted by a technically qualified committee, which is faced with the problems alluded to in this discussion.
In relation to radiation, to which all people are exposed constantly from cosmic sources, it would seem reasonable not to add to the total dose without sound and sufficient reason. The additional risk of exposure to radiation incurred by taking an X-ray film of the chest is far less than the risk of not detecting a curable tuberculous or cancerous lesion.