Results strengthen the foundation for current radiation protection standards

More precise estimates of cancer risks associated with prolonged, low level exposure to ionising radiation among nuclear industry workers are published by The BMJ today.

The results suggest a linear increase in the relative rate of cancer with increasing radiation exposure and strengthen the scientific basis for current radiation protection standards.

Ionising radiation is an established cause of cancer, but information on radiation risk has come mainly from studies of people exposed to acute, high doses of ionising radiation, such as Japanese atomic bomb survivors. Research into associations between exposure to moderate or low dose radiation and risk of cancer began in the 1950s but estimates remain uncertain.

So an international team of researchers set out to investigate whether exposure to prolonged low doses of ionising radiation are associated with an increased risk of cancer.

The study involved 308,297 nuclear industry workers from France, the United Kingdom, and the United States. The workers, most of whom were men, were monitored for external radiation exposure, and were followed-up for an average of 27 years.

Risk estimates were then calculated for deaths from all cancers excluding leukemia. Factors such as age, duration of employment, and socioeconomic status were taken into account.

The estimated rate of mortality from all cancers excluding leukaemia increased with cumulative dose by 48% per gray (Gy). Similar associations were found within each country. Based on these estimates, the researchers suggest that about 209 of the 19,064 observed deaths due to cancer other than leukaemia were excess deaths associated with external radiation exposure.

The risk per unit dose was similar to estimates derived from studies of Japanese atomic bomb survivors, note the researchers, contrary to the belief that high dose rate exposures are more dangerous than low dose rate exposures.

This is an observational study in which bias cannot be ruled out, they say. For example, they cannot rule out the possibility that smoking and occupational asbestos exposure may have influenced the results.

“This study provides evidence of a linear increase in the excess relative rate of cancer mortality with increasing exposure to ionising radiation at the low dose rates typically encountered in the nuclear industries in France, the UK, and the USA,” they write. The findings can also help strengthen the foundation for radiation protection standards, they conclude.

This study “adds to a growing body of evidence suggesting associations between exposure to moderate or low dose radiation and risk of cancer,” writes Mark Little from the US National Cancer Institute, in an accompanying editorial.

But should we conclude, as this study does, that exposures at lower dose rates are just as risky as those at higher dose rates, he asks?

He suggests this conclusion may be unwarranted, pointing to several uncertainties, in particular the effect of the lower (but biologically more risky) energy of radiation received by these workers than in groups exposed at high dose rates such as the Japanese atomic bomb survivors. He also highlights a few weaknesses, including the lack of information on smoking and occupational exposure to asbestos, which “could conceivably confound the association between radiation dose and cancer risk.”

He argues that the excess solid cancer risks associated with radiation in this cohort are modest. “For the average worker, the lifetime risk of cancer death is likely to be increased by about 0.1% from a baseline risk of cancer death of about 25%.” However, he adds, “it is equally clear that the excess risks are unlikely to be zero.”

http://www.bmj.com/cgi/doi/10.1136/bmj.h5359

http://www.bmj.com/cgi/doi/10.1136/bmj.h5405

Expert reaction to risk of cancer and occupational exposure to ionising radiation, as published in the BMJ*

Prof Dudley Goodhead, SRP Member and Visiting Scientist at MRC Harwell, said:

“This large study by Richardson and co-workers is a very thorough analysis of the cancer mortality of nearly a third of a million nuclear workers in France, the UK and the USA, who were exposed to external radiation protracted over many years.  Over the many decades of follow up of these workers there were almost 18 thousand deaths due to solid cancer, of which about 200 are estimated to be due to external radiation exposure. 

“Notable findings from the study include an apparent linear increase in cancer risk with increasing radiation dose, even for the low dose range below 100 mGy, and risk coefficients that are similar in magnitude to those obtained for survivors of the atomic bombs in Japan, who, in contrast, had received a brief high dose-rate exposure.  Conclusions from the present study appear to be robust against a variety of potential biases and confounding factors, although of course not all possibilities can be evaluated. 

“Overall the study adds to the evidence that cancer risk from ionizing radiation increases approximately linearly with total dose, even down to low doses, and that the magnitude of risk is not greatly dependent on the period of time over which the exposure occurs. 

“There is strong scientific evidence from basic studies that the passage of even a single particle of ionizing radiation (such as an electron) through a cell has a very low probability of producing the types of DNA damage that can lead to permanent genetic alterations to the cell.  The biochemical repair systems of a cell are very efficient at repairing most DNA damage, but failures do occur at a very low probability.  In this way the risk of genetic alterations increases even at low doses and in proportion to the dose (that is, the number of particle tracks).

“Although there are many biological processes and many years between these early cellular events and the rare occurrence of a radiation-induced cancer, the results of this epidemiological study by Richardson et al. suggest that the linearity with dose is preserved to a large extent, including at low doses of protracted radiation.  This implication is generally consistent with current policy and practices in radiation protection, for which the linear no-threshold (LNT) assumption is applied.  However, for radiation protection it is usually assumed also that the risk is reduced (halved) for protracted, as compared to acute, exposure --- this aspect is challenged by the results of the current study.”

Prof Richard Wakeford, SRP Member and Professor in Epidemiology at the Centre for Occupational and Environmental Health, University of Manchester, said:

"INWORKS is the next logical step in the study of workers regularly exposed to slightly increased levels of radiation over a long period - it is the most powerful study of radiation workers to date.

"The study finds a risk of cancer that increases with increasing radiation dose, at a level compatible with that assumed for the purposes of radiological protection, and provides important direct evidence of the effects on health of low level radiation exposure.

"However, as observed by Mark Little in his editorial, there is a need to appropriately account, in studies such as INWORKS, for the influence of other factors (such as smoking) on cancer risk before a precise estimate of the impact of radiation on workers can be obtained."

Dr Barrie Lambert, SRP Member and Independent Radiobiologist, said:

“This is a meta-study including epidemiological studies from France, US and the UK and in that sense the data are not new.  The authors of the three studies are included as co-authors of this paper and thus, presumably, endorse the conclusions.

“The study compiled data on solid cancer mortality amongst nuclear workers in relation to their total external (photon) radiation dose lagged by 10 years.  The authors then compared the excess relative risk per unit dose with similar risks derived from the Japanese A-bomb survivors (H&N).  They concluded that the risks were statistically ‘similar’. This suggests that the risks at high doses and dose rates are no greater than at lower (worker) doses and dose rates.

“This again is not a very new deduction as the risks from the UK NRRW study (now in its third iteration) are consistent with this and as far back as 2006 the BEIR VII committee of the US NAS suggested that the H&N data could be shown to be consistent with a dose rate reduction factor of 1 (or in fact,2).  Traditionally, however, the ICRP have used a factor of 2 to extrapolate from high dose rate to low dose rate and by implication in the setting of radiation limits.

“Thus this meta study, assuming it is reasonably statistically sound (it might have some drawbacks) adds more evidence to the theory that, in terms of cancer mortality, reducing the dose rate does not necessarily reduce the effect and this might have implications for radiation protection.”

Dr Heather Williams, Senior Medical Physicist for Nuclear Medicine at Central Manchester University Hospitals, said:

“This a large study of over 300,000 people who received a significant radiation dose in their line of work. The study concludes that the risk of dying from cancer increases in proportion to radiation dose, so very small exposures carry a very small risk, large exposures carry a large risk, and the risk increases linearly (in a straight line) in between.

“The straight line shown in the paper links 10 points, and there is considerable uncertainty about each of these points. This is because it is very challenging to deduce radiation dose to the body from readings on the radiation monitoring badges worn by nuclear workers, and to then link increasing radiation dose to the body to an increase in the risk they will die from cancer. However, there will probably be more uncertainty in this information than is shown on the graph, as this study does not fully take into account other things that could increase the risk of death from cancer.

“Examples of things that haven’t been taken into account include exposure to chemicals that are known to cause cancer, exposure to radiation outside of work during long-haul flights or certain medical procedures, or variations in healthcare which may mean some workers who develop cancer are more likely to die of it than others.

“There is a lot of controversy about exactly how risky low radiation doses are compared to high radiation doses, with some claiming the body can cope well with very small doses and others claiming that there is always a small risk associated with a small dose. In the absence of firm evidence, those working to ensure safe use of radiation (such as myself, a Medical Physicist in the NHS) err on the side of caution and assume that the very small amounts of radiation we use carry a very small amount of risk, and hence work to reduce radiation exposure wherever possible. Whilst there are serious questions about how reliable the conclusion of this study is, it seems to suggest that being cautious remains the right approach.”

Declared interests

Richard Wakeford sits on the Technical Working Party of the Compensation Scheme for Radiation-linked Diseases.