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UKRC 2004 debate |
Departments of Medical Physics, Radiology and Physics, University of Wisconsin, Madison WI, USA
Abstract
This paper presents little-known data to support the hypothesis that we need increased background radiation to improve our health. Attention will be drawn to results that demonstrate health benefits of ionizing radiation that have been largely ignored by the news media.
Science progresses by interpreting new data not by accepting unfounded assumptions. Arthur Conan Doyle cautioned against making assumptions until one has enough data on which to base the assumption. The International Commission on Radiological Protection failed to follow that advice when it adopted the linear assumption of radiation risk in 1977, primarily to simplify radiation protection regulations. Conan Doyle also pointed out that a premature assumption results in a tendency to interpret data to agree with the assumption. I believe this is one of the reasons the linear assumption has survived for so long.
Few radiologists, or other healthcare workers involved with radiation, are aware that billions of their cells are bombarded daily by natural ionizing radiation, much of it from 
9 kBq of natural radioactivity in their bodies. Nearly all the trillions of cells in our body are hit each year, many more than once. Despite this huge amount of radiation damage, cancer is primarily a disease of the elderly. It is reasonable to assume that our very early ancestors solved the problem of cellular repair billions of years ago and that we now have highly efficient repair mechanisms. Cells beyond repair undergo controlled destruction by lysis (apoptosis). According to Cohen [1] if all cancers were curable, longevity would only be increased by about 3 years. It is illogical to suggest that radiation damage to one cell may cause cancer. The probability of one damaged cell causing cancer is infinitesimal – less than ones chance of winning a World lottery if everyone had a ticket. Scientists should not base health effects on assumptions that cannot be proved or disproved. The linear assumption of radiation risk was made to simplify radiation protection regulations. It is unfortunate that many persons have accepted the assumption as a scientific truth despite the contradictory evidence of lower cancer mortality in high background areas.
Population studies
There is much evidence that radiation induction of cancer is not linear with dose and that a threshold dose rate of the order of 1 Gy year–1 must be exceeded to induce cancer. In addition, recent studies of radiation workers show that moderate dose rate radiation produces a very significant reduction in death rate from non-cancer. The increase in longevity in two studies of radiation workers was about 3 years, about equal to the increase in longevity if all cancer were curable [2].
If ionizing radiation is as dangerous as many now believe, it seems impossible for life to have evolved. I have suggested that ionizing radiation may be an "essential trace energy" analogous to the many essential trace elements we need for good health [3].
A report by the US Atomic Energy Commission in 1973 showed that the population in the six US States with the highest radiation background had 15% lower cancer death rates than the average for the 48 States [4]. In 1998 the results of this study were confirmed by a comparison of the cancer mortality and background radiation of three mountain States to those of three Gulf States [5]. The annual level of natural background radiation in the three Rocky Mountain States (Idaho, Colorado and New Mexico) is 3.2 times that in three Gulf Coast States (Louisiana, Mississippi and Alabama), but the overall age-adjusted cancer death rate in the Gulf States is 1.26 times higher. Thus the difference from proportionality is a factor of 4.0.
For lung and bronchus cancer mortality there is a strong negative correlation with natural radon levels (the main cause of the difference in background levels) – factors of 5.7 to 7.0.
In such studies, the possibility of confounding factors must always be considered. However, to quote from Jagger's paper "It is possible that confounding factors such as smoking, poverty or environmental pollution, contribute to the differences in cancer mortality between Rocky Mountain and Gulf Coast States. However, the factor of disproportion is so great (4.0–7.5) that it strains credulity that such confounding factors could reverse this negative correlation".
Although this finding was published in a well-read journal, it attracted no attention from the news media.
The 100-year study of British radiologists (1897–1997) is the most important study of health effects of moderate dose rate radiation ever published [6, 7]. It compared the death rates of British radiologists from cancer, non-cancer and all causes to those of all male non-radiologist physicians in England and Wales, hereafter referred to as controls. The study showed that radiologists who joined a radiological society between 1897–1920 had 75% greater cancer mortality than the controls. It is not possible to make close estimates of doses received by radiologists at that time. Braestrup [8] estimated average accumulated doses to US radiologists in the 1920s and 1930s using non-protective equipment at about 1 Gy year–1 (in modern units). 75% of the dose arose from fluoroscopy (assumed at 1 h day–1), the remainder was equally divided between diagnostic radiography and therapy. There is no doubt that the significant cancer increase (p<0.001) was due to high radiation doses in those early years of radiology.
The increased radiation had a significant beneficial effect that was not noticed at the time. The radiologists' death rate from non-cancer was 14% lower (p<0.05) than the controls. Their deaths from all causes were slightly less than the controls, that is to say the longevity of the earliest radiologists was not reduced despite their 75% increase in cancer death rate. British radiologists who joined a radiological society after 1920 have never shown a statistically significant excess of cancer mortality compared with the controls. This dramatic contradiction of the linear assumption has been largely ignored. The abrupt decrease in cancer deaths after 1920 suggests that X-ray induction of cancer has a threshold as suggested by two earlier studies [9, 10].
With the introduction of beam collimation and personal protection, and much later image intensifiers, doses to radiologists fell dramatically. By the late 1950s the average had fallen to about 0.01 Gy year–1 and by the 1990s to about 0.5 mGy year–1 [11]. The healthiest British radiologists were those who joined a radiological society between 1955 and 1979. Their death rate from cancer was 29% lower (not significant); from non-cancer was 36% lower (p<0.001) and from all causes was 32% lower (p<0.001) than the controls. Their increase in longevity over the controls is estimated to be about 3 years.
The best epidemiological study of radiation workers ever done is the US nuclear shipyard worker study – NSWS (1980–1988) – supported by the US Department of Energy [12]. I was a member of the Technical Advisory Panel (TAP) that met twice a year to review progress and to suggest improvements. TAP comprised eight well-qualified scientists who unanimously approved the draft of the final report of the NSWS in early 1988. It is unfortunate that the details of this important study have not yet appeared in a peer reviewed scientific journal. I am sure that if the results had supported the linear hypothesis of radiation risk the details would have been published promptly. I am the co-author of a review article on the NSWS that has not yet been accepted by a journal [13].
The scientists who performed the study selected about 28 000 nuclear shipyard workers with the largest cumulative doses. They had a death rate from cancer 15% lower (p<0.01); from non-cancer 31% lower (p<10–16) and from all causes 24% lower (p<10–16) than 32 500 age-matched and job-matched unexposed shipyard workers. No other study of radiation workers has had the important advantage of job-matched controls. The very significant reduction in non-cancer deaths is in agreement with a similar reduction of deaths from non-cancer of British radiologists who joined a radiological society between 1955 and 1979 referred to earlier [6]. Since the nuclear shipyard worker study had not been published, the authors of the 100-year study were apparently unaware of the striking similarity of the results. Neither the 100-year study of British radiologists nor the nuclear shipyard worker study emphasised the strong evidence that moderate dose radiation stimulates the immune system.
It is a mystery to me why some radiologists and other healthcare workers involved with radiation still believe that diagnostic X-ray doses much lower than annual background radiation carry a risk of inducing cancer. None of the above studies proves that moderate dose radiation increases longevity but they do provide strong evidence that moderate dose radiation is beneficial to the health. I have suggested that valuable information on this question of longevity could be obtained from a double blind study using increased background radiation of about 10 mGy year–1 to half of a population of senior citizen volunteers. This dose rate is lower than that to British radiologists in the second quarter of the last century where there was no significant increase in cancer. I have suggested such a double blind might be carried out in the US Gulf States where the population seems to be suffering from "radiation deficiency" [3].
Conclusion
The linear model is often defended as a conservative assumption. It is not conservative if we need a moderate dose rate of radiation to stimulate our immune system. Too little radiation appears to result in an earlier death. The analogy would be to reduce essential trace elements in our diet because they are poisonous in large quantities. The great statistical strength of the studies on the reduction in non-cancer deaths of the British radiologists and the US nuclear shipyard workers should not continue to be ignored.
Audience participation
The question put to the audience on the basis of this lecture was "Considering that a typical adult has billions of cells hit each day by ionizing events resulting from background radiation, is it reasonable to conclude that radiation damage to even one cell may initiate cancer, as assumed by the linear, no-threshold hypothesis?"
Responses: Yes 47%; no 44%; don't know 10%.
References
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