The study also shows that overall mortality in the UK's 175,000 radiation workers is lower than that in the general population. This "healthy worker effect" has been observed in studies of many other occupational groups.
Dr Colin Muirhead from the Health Protection Agency said: "This is a continuation of a study started in 1976 and it provides reliable information on the health of people working with ionising radiation. The results confirm the cancer risk estimates observed in other studies even though, overall, radiation workers have lower cancer risks than the general population."
This study provides the most precise estimates to date of mortality and cancer risks following occupational radiation exposure and strengthens the scientific evidence for raised risks from these exposures. The cancer risk estimates are consistent with international radiation protection standards, both for leukaemia and for all other cancers combined.
Continued follow-up of these workers will be valuable in determining whether radiation-associated risks vary over time or by age, and enables the study of specific cancers and causes of death in more detail. The research will also make a useful contribution to wider national and international studies on the effects of occupational exposures to ionising radiation.
In a separate paper** published at the same time, researchers at the Childhood Cancer Research Group at the University of Oxford and the Health Protection Agency have studied cancer in the children of female radiation workers. This study does not support an earlier finding of a raised risk of cancer. In a subgroup of women who worked with radiation during pregnancy, a weak statistical association was found between maternal radiation work and childhood cancer, but the evidence is limited by the small numbers involved and the result may be due to chance.
The two papers are published in the British Journal of Cancer and radiation workers and their representatives were informed of the results before publication.
References
* Muirhead CR, O'Hagan JA, Haylock RGE, Phillipson MA, Willcock T, Berridge GLC, Zhang W (2009) Mortality and cancer incidence following occupational radiation exposure: 3rd analysis of the National Registry for Radiation Workers. British Journal of Cancer (2009), Volume 100, Issue 1, pages 206-212.
** Bunch KJ, Muirhead CR, Draper GJ, Hunter N, Kendall GM, O'Hagan JA, Phillipson MA, Vincent TJ, Zhang W. Cancer in the offspring of female radiation workers: a record linkage study. British Journal of Cancer (2009), Volume 100, Issue 1, pages 213-218.
Notes
1. The National Registry for Radiation Workers (NRRW) analysis* was funded by the Health and Safety Executive. Further details of the analysis will be published in due course in an HPA report available on the Agency's website. The authors and HPA particularly wish to thank all of the organisations and individuals participating in the NRRW for their cooperation.
2. Estimates of the long-term health risks from ionising radiation are based largely on epidemiological studies of the survivors of the atomic bombings in Japan and of groups exposed for medical reasons. The desirability of obtaining data on the risks from protracted or low dose radiation exposures has long been apparent. Consequently, the National Registry for Radiation Workers (NRRW) was set up in 1976 by the National Radiological Protection Board, which is now the Radiation Protection Division of the Health Protection Agency (HPA).
3. The organisations that participate in the NRRW are in the nuclear, research and industrial sectors. Two earlier analyses of mortality in the NRRW found some evidence of an association between workers' radiation dose and leukaemia, after excluding chronic lymphatic leukaemia (CLL) which does not appear to be radiation-inducible. However, the estimates of radiation risks derived from these analyses and from a study of nuclear workers in 15 countries, for which NRRW formed the UK component, were imprecise. To obtain more precise information on the risks of occupational radiation exposure, the NRRW cohort has now been expanded to include about another 50,000 workers compared to the previous analysis and the period of follow-up has been extended by nine years, to the end of 2001. Data on cancer registrations have also been included in the NRRW for the first time, together with mortality data.
4. As in previous NRRW analyses, there was a strong healthy worker effect (HWE); overall mortality was 81% of that expected for the general population of England and Wales, having allowed for the effects of age and gender. Total cancer mortality was also reduced, being 84% of that in the general population. The HWE has been seen in many studies of workers, not only radiation workers, and is likely to reflect factors associated with the recruitment and retention of persons in work.
5. To study the possible impact of radiation exposure on health whilst minimising the impact of the HWE, rates of mortality and cancer incidence within the NRRW were analysed according to workers' radiation dose, which was assessed through personal monitoring. Mortality and incidence from both leukaemia excluding CLL and the grouping of all cancers excluding leukaemia increased to a statistically significant extent with increasing radiation dose. Estimates of the trend in risk with dose were similar to those for the Japanese atomic bomb survivors, with confidence ranges that excluded risks more than 2-3 times greater than the A-bomb values and also excluded the possibility of no raised risk. There was some evidence of an increasing trend with dose in mortality from all circulatory diseases combined; for these diseases, other studies of radiation workers have yielded mixed results and the NRRW finding may, at least partly, be due to confounding by smoking. In contrast, both for mortality and incidence, the trend with radiation dose in cancers other than leukaemia was maintained after excluding lung cancer, so indicating that this trend is not an artefact due to smoking.
6. This analysis provides the most precise estimates to date of mortality and cancer risks following occupational radiation exposure and strengthens the evidence for raised risks from these exposures. The cancer risk estimates are consistent with values used to set radiation protection standards and - both for leukaemia and for all other cancers combined - exclude the possibility of radiation risks being more than a few times higher than existing estimates.
7. The separate study** of cancer in the offspring of female radiation workers was conducted by the Cancer Research UK - funded Childhood Cancer Research Group (CCRG) at the University of Oxford and HPA. The study was funded by the Department of Health and follows on from an earlier study (Draper et al, British Medical Journal, 315, 1181-88, 1997) that examined whether the offspring of radiation workers were at raised risk of childhood cancer. This earlier study did not find an association between childhood cancer and paternal preconception radiation exposure. However, there was some suggestion of a raised risk of cancer in the offspring of female radiation workers, although based on small numbers of cases. Consequently, CCRG and HPA have examined the latter topic further, using more recent data. Analysis of the new data did not show an association between childhood cancer and maternal preconception radiation work and does not support the earlier finding of a raised risk in the offspring of female radiation workers. However, when the original and new data were combined, a weak association was found between maternal radiation work during pregnancy and childhood cancer in offspring, although the evidence was limited by the small numbers of cases and controls that were linked to the NRRW.
8. The British Journal of Cancer (BJC)
The BJC is owned by Cancer Research UK. Its mission is to encourage communication of the very best cancer research from laboratories and clinics in all countries. Broad coverage, its editorial independence and consistent high standards have made the BJC one of the world's premier general cancer journals. bjcancer
The British Journal of Cancer
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