Radiation and Cancer
(Letter sent to the editors of
21st Century Science & Technology magazine
by James Welsh, M.S., M.D.,
University of Wisconsin-Madison Medical School, Madison, Wisc.,
published in the Spring issue, p. 2)
To the Editor:
As a physician specializing in the treatment of cancer using radiation, I read with great interest Jerry Cuttler's article "The Significant Health Benefits of Nuclear Radiation" in the Fall issue [p. 48].
In clinical external beam radiotherapy we tipically use large dosis of carefully directed ionizing radiation to eradicate tumors while sparing normal tissues. In order to achieve this goal we exploit physics to accurately target the tumors (through the use of modern techniques such as 3-Dimensional Conformal Radiotherapy and Intensity-Modulated Radiation Therapy), and radiobiology to minimize the deleterious effect on the normal tissue that is inevitably exposed.
A large enough acute dose of radiation will destroy practically any human tumor, but will also destroy any adjacent normal tissue. By fractionating the large dose into smaller, daily doses, normal tissues can be largely spared while the tumor is still eliminated. Thus, although we are dealing with daily doses that are still quite large (on the order of 2 gray per day), normal tissues are still able to repair themselves with surprising efficiency in most cases. As Cuttler points out, the public and scientific community are well aware of the deleterious effects of large acute doses of radiation but the biological effects of low doses and dose-rates are far more obscure.
While the LNT [Linear No-Threshold] hypothesis is attractive for its simplicity, there are little human data at low exposures. As the article mentions, there are numerous epidemiological studies that refute the LNT hypothesis, and some which actually support the opposite view; namely, radiation hormesis, the concept that small doses of radiation are actually beneficial. While the cohort studies cited in the article are intriguing, hard-core skeptics will remain unconvinced by any observational study and may remain unconvinced by any data short of a prospective randomized trial, which will never happen.
Part of the skepticism may stem from the question of why there would be no negative effect (or perhaps a positive effect) at low doses, when it clear that large doses are so harmful. As Cuttler mentions, cells respond to radiation by increasing their. levels of protective enzymes such as superoxide dismutase (SOD) and DNA repair mechanisms. (In fact, we occasionally exploit a similar phenomenon by administering pharmacological doses of synthetic radioprotectants such as amifostine, a thiol-containing free radical scavenger, to further protect normal tissues during radiotherapy).
Perhaps a better understanding of why such natural protective mechanisms are in place in all living organisms can come from evolutionary biology and the geological sciences. The Earth was formed roughly 4.6 billion years ago, and the first direct evidence of life appears around 3.6 billion years ago in the form of stromatolites, mats laid down by cyanobacteria. (Indirect evidence of life can be found can be found even earlier in ratios of carbon-12 to carbon-13 in rocks, suggestive of photosynthesis.) At the time that life was evolving on this planet, ambient background radiation levels were much higher than they are today, because of the greater abundance of naturally occurring radioisotopes. Potassium-40, the greatest source of internal radiation dose (0.2 mGy/yr on average now), contributed estimated doses of 1.6 mGy/yr early in Earth's history.
External exposure from radioactive : elements in the uranium and thorium series has declined over the eons as well. The net result is much less radiation exposure today from natural background sources compared to the time when life first evolved (over five-fold on average). The highly efficient DNA repair mechanisms now in place in all organisms evolved in the distant past when radiation levels were much higher than at present. Accordingly, they are perhaps equipped to handle radiation levels much higher than currently exist. In light of these considerations, the concept of low doses of radiation not being of great harm seems far more plausible. In fact, in this light, the admittedly highly controversial concept of some human maladies being due to a "radiation-deficiency syndrome" may not be as far-fetched after all.
James Welsh, M.S., M.D.
University of Wisconsin
Madison Medical School, Madison, Wisconsin.
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