June 19, 2012
New Findings Demonstrate That Even Lower Doses of Radiation Than Those Used In X-Rays and CT Scans Cause Cancer
A statistically significant link between risk of childhood leukemia and the gamma rays we are all exposed to from our natural environment has been detected in an Oxford University-led study. The research gives credibility to critics of medical imaging techniques and provides evidence contradicting medical authorities who insist that other low doses of radiation, such as from medical X-rays and CT scans are of no consequence to human health.
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 medical imaging.
"This is a very important study showing that we are harming possibly millions of patients every year through diagnostic imaging," said Dr. Ian Martin commenting on the study."The low doses of radiation used as a benchmark in this study are lower than those used in even one medical X-Ray," he added.
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 results of the study contradict the idea that there are no adverse radiation effects at these very low doses and dose rates.
A study published in 2009 in International Journal of Occupational Medicine and Environmental Health found that exposure to low-dose ionizing radiation increases risk of cardiovascular death.
Most patients have little or no awareness of the dangers of ionizing radiation due to medical imaging scans. Moreover, few doctors realize that one CT scan exposes the body to the equivalent of several hundred X-rays. Very few women undergoing mammograms understand that the the radiation emitted by mammography machines actually causes cancer by exposing heart and breast tissue to dangerous ionizing radiation that directly causes DNA damage.
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 leukemia. 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 leukemia,' 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 leukemia is likely to be causal.
What's Your Radiation Dose?
Those occupationally exposed to low doses of ionizing radiation such as employees at nuclear power stations, medical, dental, and industrial workers were more likely to die of cardiovascular disease than those ¬†in the general population.
Another report released in 2009 by the National Council on Radiation Protection and Measurement revealed that our exposure to radiation has increased more than 600 percent over the last three decades. Most of that increase has come from patients' exposure to radiation through medical imaging scans such as CT scans and mammograms.
Background radiation accounts for only a minority of childhood leukemias. The cause of most cases is unknown.
While there is some variation in natural gamma-ray exposure around the UK, the radiation doses are small and there is very little that can be done to mediate or prevent any cancer risk. In this respect it is different from naturally occurring radon gas: radon exposure can be controlled but gamma exposure where you live is inevitable.
'In terms of preventing childhood cancers caused by natural gamma-rays, there's not a lot you can do,' explains Dr Kendall.
'We have estimated that about 15% of the 500 or so cases of childhood leukemia which occur annually in the UK are due to natural background radiation.
"If only 15% of the 500 cases are being caused by natural radiation, can you imagine how many cases are being caused by unneccessary medical imaging?" concluded Dr. Martin.
The National Registry of Childhood Tumours has an essentially complete record of UK cases of childhood cancers, allowing the researchers to compare the radiation exposures for almost 27,500 cases diagnosed between 1980 and 2006 (including over 9,000 childhood leukemias) with a set of almost 37,000 matched control children without cancer.
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 leukemia for every millisievert of natural gamma-ray dose to the bone marrow. While this finding was statistically significant, even with a study of this size there is still some uncertainty around the size of the effect. The relative risk increase is likely to lie within a range from 3% to 22% per millisievert.
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 leukemia linked to radiation exposure from CT scans in childhood was published in The Lancet on 7 June.
Two studies on this topic were published in the December 2009 issue of Archives of Internal Medicine. One of the studies reports that just one scan can deliver enough radiation to cause cancer and predicts that 29,000 new cancers will develop that can be linked to CT scans received in just the year 2007. Making matters much worse is the fact that the use of CT scans in medicine has grown explosively -- more than tripling in the US since the 1990s, with more than 70 million given each year.
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.
Marco Torres is a research specialist, writer and consumer advocate for healthy lifestyles. He holds degrees in Public Health and Environmental Science and is a professional speaker on topics such as disease prevention, environmental toxins and health policy.