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Opinion |
1 From the Department of Radiology, University of Texas-Houston Medical School, 6431 Fannin St, Houston, TX 77030. Received April 8, 2003; accepted April 9. Address correspondence to the author.
Index terms: Opinions • Radiations, exposure to patients and personnel • Radiology and radiologists
Dr Cameron poses an intriguing argument that counters contemporary and popular assumptions about the health effects of occupational exposure to radiation (1). He asserts the hypothesis that radiation workers enjoy greater longevity than counterparts who do not work with radiation. He cites several publications that bear witness to the validity of this hypothesis. These include studies of British radiologists (2) and American naval shipyard workers (3). He also cites a study of American radiologists (4) that provides some supporting evidence to his hypothesis but for which there are some contradicting trends. He claims that longevity is the best indicator for risk analysis.
Dr Cameron’s observations regarding British radiologists and American naval shipyard workers merely and correctly point out what is recorded in black and white. In the shipyard studies, radiation workers experienced significantly less mortality from all causes when compared with the most appropriate control groups. In the study of British radiologists, the radiologists had lower mortality from noncancer causes. Does this insight expose an underlying prejudice that pervades the scientific literature regarding radiation effects, that is, do we choose to see only that for which we look and do we unconsciously, but perhaps with an unwitting intention, ignore those observations that contradict our instincts?
For example, Dr Cameron points out, with reference to the study of British radiologists, that the authors state that "[t]here was no evidence of an effect of radiation on diseases other than cancer even in the earliest radiologists." In fact, there was a statistically significant finding of a potential effect; it just went in the unexpected direction. The data demonstrate that decreases in noncancer death rates existed in all registration groups and that they were statistically significant across three of the four registration groups. Is this not evidence that there might be some underlying benefit to occupational exposure to radiation? Should this observation be ignored? Scientifically speaking, the statistical significance of this observation of a lower death rate deserves attention and the big question is, "Why is this so?"
Many will no doubt assert that the lower noncancer mortality rates in radiologists are caused by selection bias or the "healthy worker effect." This argument states that the exposed group represents a healthier subpopulation because of biases in the selection process that qualify them for their work. It also might mean that the control group is not as healthy because of selection biases in their lifestyles. A casual disregard claiming "healthy worker effect," also pointed out by Daunt (5), goes to the core of the scientific dilemma. Is this explanation science or is it prejudice?
Working with exposed and control populations for epidemiologic research is a mind-boggling dilemma. Try as they may, scientists cannot design control populations that are identical in all characteristics to exposed groups. The British study clearly demonstrates how selection of a control group can bias results. In that study, comparisons of mortality rates in radiologists with rates in the general population, as well as with rates in specific social classes, gave different results than did the comparison with rates in male medical practitioners. Because the lifestyles of radiologists and those of other physicians are markedly different, a control group of male medical practitioners is probably not a valid control group either. For example, surgeons, internists, and obstetricians all experience a considerably greater degree of patient contact than do radiologists. This places nonradiologists in a more frequent contact with contagious diseases, blood-borne pathogens, and body fluids, for instance. They are also more frequently exposed to certain chemical vapors. It also might be surmised that surgeons and obstetricians experience a greater degree of emergent care, which leads to increase in stress and sleep deprivation. This would raise the question as to whether noncancer mortality rates in radiologists are lower because they die less frequently of illnesses contracted from direct patient care. On the other hand, my experience in the 1970s and 1980s was that radiologists seemed to smoke more while on the job, due in part to their markedly reduced patient contact. Finally, at least in the United States, many nonradiologists are exposed to radiation more often than are some radiologists. This would include some surgeons, anesthesiologists, and especially cardiologists. In other words, despite attempts to equalize study and control groups, this is not physically possible. The "healthy worker" selection bias argument could go either way, depending on which way one wants it to go. A study of longevity would certainly demonstrate which group lives longer, but the reasons for this would not be obvious.
Dr Cameron also finds supporting evidence for his hypothesis in the study of American naval shipyard workers, which mysteriously was never published, despite the extensive efforts to perform a large well-controlled study of occupational exposures. While some scientists may believe that hormesis explains the lower cancer mortality rates and the lower overall mortality rates found among radiation workers in the study, the National Council on Radiation Protection and Measurement, or NCRP, (6) discounts this possibility by stating that "[t]he fact that there was a difference for total mortality, not just for radiosensitive cancers, supports the interpretation that selection factors were operative." This statement blatantly assumes that radiation has no potential beneficial effect on the occurrence of other diseases. The NCRP could have chosen to say that an alternative interpretation that cannot be ruled out is that occupational radiation might be a health-enhancing factor. Such a statement is contrary to our common prejudice.
The study of American radiologists supports Dr Cameron’s hypothesis only for the early years of occupation following commencement of the profession. As radiologists age and increase their cumulative radiation exposure, the study suggests that they experience a greater risk of mortality.
Dr Cameron has chosen three studies to support his arguments, but there exists a much broader base of data. The study of nuclear industry workers is one (7). This study interestingly demonstrates some statistically significant trends in cancer mortality with increasing occupational dose, and it also demonstrates a statistically significant and increasing trend for circulatory diseases. There is no indication of a significant health improvement with increasing occupational exposure.
It is not likely that Dr Cameron’s call to investigate longevity will be completed anytime soon. In the face of all this evidence regarding low-level benefits or risks, what should we believe? As a radiation management professional in the medical field, my principal concern is to ensure the most reasonably safe working environment for employees. The truth about the potential health risks or health benefits from low levels of occupational exposure to ionizing radiation is sufficiently well known that the following statement can be made:
In studies of large populations of workers occupationally exposed to ionizing radiation, there is evidence that there may be slight increases in mortality rates from some malignancies, particularly leukemia. But there is also significant evidence that workers in radiologic fields generally have a lower mortality rate from causes other than cancer when compared with groups of workers in similar fields. While the cause of the increased leukemia rate is thought to be exposure to radiation, the causes of the reduced overall mortality rate may also be due to chronic exposure to radiation, although underlying selection biases in control groups are a likely explanation for this finding. Occupational exposures to ionizing radiation, with use of current standards of protection, do not represent an inordinate risk to the workforce population as a whole.
The above discussion applies only to populations of workers and should not be construed to apply to specific individuals. The subpopulation of subspecialty medical workers, who are exposed to radiation doses that represent a significant fraction of the maximum allowed by regulation, must not be cavalier in their attitudes toward occupational exposures. Findings of all studies of early radiologists suggest an increased risk for cancer mortality. Although Dr Cameron suggests that there is a nullifying effect from health benefits of radiation exposure, this remains an unproven hypothesis and does not justify any alteration in our code of practice. Therefore, managing occupation doses to ALARA (as low as "reasonably" achievable) is still the recommended behavior.
As a final caveat, the above discussion applies only to occupational exposure to radiation. Radiation exposure of the public is an entirely different issue, primarily due to the incorporation of children in the potentially exposed group. I know of no evidence to suggest that children might benefit from chronic exposure to low-level radiation.
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