Childhood leukaemia risk from very low exposure to ionising radiation
Natural gamma rays linked to childhood leukaemia, University of Oxford, 12 June 12, A small but statistically significant link between risk of childhood leukaemia and the gamma rays we are all exposed to from our natural environment has been detected in a very large study led by Oxford University researchers.
Exposure to gamma rays from natural sources in the environment isn’t something that can readily be altered, but the study adds to our understanding of the small cancer risks associated with other low doses of radiation, such as from medical X-rays and CT scans. The findings demonstrate that there are small effects of radiation at very low doses.
Guidelines on exposure to low doses of radiation have largely been based on estimated risks from models using data from Japanese survivors of the atomic bombs, where radiation exposures were brief and very much higher. As a result, there have been some long-standing uncertainties about the extrapolation of these risks to low radiation doses.
The researchers conclude that the size of the increased risk of childhood leukaemia with natural gamma-ray exposure is consistent with these models and supports their continued use in radiation protection. The results of the study contradict the idea that there are no adverse radiation effects, or might even be beneficial effects, at these very low doses and dose rates.
The Oxford University researchers, along with colleagues from the US National Cancer Institute, The University of Manchester and the Health Protection Agency, have published their findings in the journal Leukemia.
The case-control study, based on tens of thousands of records from a UK national cancer registry, is the largest such study ever conducted on links between childhood cancers and natural background radiation levels.
It has needed a study of this very large size to be able to reliably identify the small effect of background radiation on childhood leukaemia. Previous studies have lacked the size and statistical power to be able to detect any link.
‘We found a statistically significant correlation between natural gamma-rays and childhood leukaemia,’ says lead researcher Dr Gerald Kendall of the Childhood Cancer Research Group at Oxford University. ‘What is new in our findings is the direct demonstration that there are radiation effects at these very low doses and dose-rates.’
The researchers believe that the association between natural gamma-rays and childhood leukaemia is likely to be causal.
Gamma rays in background radiation come largely from naturally occurring radioactive isotopes of uranium, thorium and potassium in the environment. In the UK, children have an annual radiation dose of roughly 0.7 mSv (millisievert) to bone marrow from natural gamma-rays…..
Cumulative radiation exposures from birth to cancer diagnosis were estimated for where the mother was living at the time of the child’s birth.
The team found that there was a 12% increase in the risk of childhood leukaemia for every millisievert of natural gamma-ray dose to the bone marrow. …..
Dr Kendall adds: ‘The findings are relevant to understanding the risks from low radiation exposures such as medical X-rays and CT scans; planning for the disposal of nuclear waste; and the risks from the exposures received by people living near Chernobyl or Fukushima.
‘The risk estimates used by those involved in radiation protection for such situations have tended to rely on models that extrapolate risk from data on Japanese survivors of the atomic bombs where radiation exposures were very high. Our findings are consistent with these models.’
Professor Richard Wakeford of The University of Manchester, a co-author of the study, said: ‘Radiation protection measures assume that even low doses of radiation pose some, albeit small, risk of cancer. Naturally occurring gamma-rays provide an ever-present, very low-level source of exposure to radiation, but this very large epidemiological study suggests that even at these very low levels there is a very small risk to health. However, the results are what would be expected from previous scientific evidence, and indicate that the current assumptions underlying radiation protection are about right.’
A separate paper finding an increase in risk of leukaemia linked to radiation exposure from CT scans in childhood was published in The Lancet on 7 June.
Dr Kendall of Oxford University believes the increase in risk that the authors found to be associated with the radiation dose received from a CT scan is ‘certainly compatible’ with the findings of this study…..
The study was funded by the Department of Health for England and Wales, the Scottish government, and the charity CHILDREN with CANCER. http://www.ox.ac.uk/media/news_stories/2012/120612.html
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