Medical X-rays are to blame for many thousands of fatal cancers every year, according to the most comprehensive analysis to date. Medical experts stress that X-rays and CT scans can be very beneficial, but believe the new work shows that they should be used as sparingly as possible.
"They are of enormous benefit for such things as early cancer detection, but medical experts need to be aware of the quantifiable risks of X-rays." says Amy Berrington, of Oxford University, UK, and one of the research team. "If you need an X-ray for medical reasons then I should not worry about it."
But Peter Herzog, of Ludwig-Maximilians University in Munich, Germany, points out that many X-rays may be unnecessary. In some countries, up to a third of chest X-rays may not be required, he says. "In everyday practice, those ordering radiological procedures should think carefully about the benefits and risks to their patients for each examination."
Adrian Dixon, a radiologist and spokesman for the Royal College of Radiologists, believes this already happens in the UK: "This study validates our policy. Our members are very scrupulous about vetting all requests for X-rays and will only carry them out if it is for the benefit of the patient."
Medical X-rays are the biggest source of man-made radiation exposure and are being used with increasing frequency in many countries. In the US, for example, their use has increased by 20 per cent since the early 1980s. The growing use of CT (computed tomography) scans, which also use X-rays, are also adding to exposure.
Global variations
This rise prompted Berrington, with colleague Sarah Darby, to update the seminal 1981 Dole and Peto study of the risks posed by medical X-rays. Dole and Peto estimated that 0.5 per cent of all deaths from cancer in the US were attributable to medical X-rays.
The Oxford team first estimated the radiation dose received by patients for each X-ray. They then collated the numbers of X-rays performed each year in 15 developed nations.
This data was then fed into a computer model for estimating the risks posed by ionising radiation. This "Excess Relative Risk Model" is derived from data gleaned from Japanese atomic bomb survivors.
The authors found substantial world-wide variations in the numbers of cancers attributable to X-rays. The UK had the lowest, with 0.6 per cent of all cancers attributable to medical X-rays. About 0.9 per cent of all cancers in the US are caused by X-rays.
But in Japan, the corresponding figure was 3.2 per cent. Overall, Berrington and Darby estimate that X-ray-based medical imaging causes an extra 18,500 cases of cancer each year across the 15 countries studied.
Herzog cautions that the increased risk of cancer could have been over-estimated by the study, because of its reliance on the data from Japanese atomic bomb survivors. It is the most accurate data available, but the survivors were exposed to many types of radiation, not just X-rays alone.