They found that for every 1,400 CT scans before the age of 20 there was one extra case of cancer over the following 10 years.
This small increase in cancer risk must be weighed against the undoubted benefits from CT scans in diagnosing and monitoring many different health conditions.
In most cases, the benefits of having a scan clearly outweigh the risk. But these new findings will remind doctors to order CT scans only when there is a definite medical reason, and to insist that CT scans use the lowest possible X-ray dose.
The study findings are reported in the BMJ medical journal. The research team, led by Professor John Mathews at the University of Melbourne in Australia, also involved investigators at eight other organisations in Australia as well as scientists in the Clinical Trial Service Unit at Oxford University, and the International Agency for Research on Cancer in Lyon, France.
Professor Mathews says: ‘CT scans have great medical benefits. In the same way that standard X-rays are helpful in bone fractures, CT scans can provide detailed three-dimensional pictures to diagnose or exclude disease in any suspect part of the body.
‘As an individual patient, your risk of cancer from a CT scan is very low. In the vast majority of cases the benefits of a CT scan in diagnosing a condition or guiding treatment will outweigh the risks. I’d certainly have a CT scan if a doctor said “I think you should have a scan” and explained why I needed it.
‘Nevertheless, it is clear from our study that it is important for doctors to use CT scans only where they are necessary. By reducing the number of scans performed in a large population, there will be a small but corresponding reduction in the number of cancers in later years.’
CT scans use multiple X-ray images to produce detailed images of structures inside the body including the internal organs, blood vessels, bones and tumours.
It is already well known that large doses of radiation can damage DNA and increase the risk of a later cancer. However, the radiation doses from CT scans are very small, and there has been uncertainty about whether such small doses would really cause cancer, and whether any small increase in risk could be measured reliably.
This new Australian study was able to answer this question by linking anonymised Medicare records of CT exposures for the entire population of young Australians, aged 0-19 years between 1985 and 2005, to cancers diagnosed up to the end of 2007. It is not yet known what will be seen with longer follow-up.
There were 866,580 CT scans given to 680,211 people in this cohort of 10.9 million young people. Of 60,674 cancers diagnosed, there were 3,150 among those who had received one or more CT scans, compared with 2542 cancers that would have been expected if there was no effect of CT scans on subsequent cancer risk. This gave 608 extra cancers associated with CT scan exposure for an average follow up period of almost 10 years.
‘In a group of 10,000 young people, we would expect 39 cancers to occur during the next 10 years,’ explains Professor Sarah Darby, a Cancer Research UK-funded researcher at Oxford University and a collaborator on the study, ‘but if they all had one CT scan, up to 6 extra cancers would occur.’
‘We don’t know for sure that all the extra cancers are caused by CT scans,’ adds Dr Graham Byrnes of the International Agency of Research in Lyon, ‘but whichever way you hold the data it looks like most of them are.’
CT scans increased the risk of a whole range of cancers, including brain tumours, many solid tumours and blood cancers. However, the site of the later cancer was most often the site that had also been exposed to the earlier CT scan.
The risk of being diagnosed with cancer increased with the number of CT scans. Younger children had a greater proportional increase in risk of cancer following a CT scan, but more CT scans were carried out in the teenage years. The study was carried out just in children and young people, but there may be some risk from CT scans in grown-ups as well.
The average radiation dose per CT scan has been falling in recent years because of improving technology and protocols, but the number of CT scan use worldwide has been going up at the same time.
Professor Darby of the Clinical Trial Service Unit at Oxford University pointed out that the use of CT scans varies from country to country. ‘In the UK we are doing quite well. The number of CT scans per head of population is relatively modest compared to other similar countries,’ she says.
For more information in the UK please contact:
Professor Sarah Darby on +44 (0)1865 743864 or firstname.lastname@example.org
Or the University of Oxford press office on +44 (0)1865 280530 or email@example.com
For more information in Australia please contact:
Professor John Mathews on +61 411 021 013 or firstname.lastname@example.org
For more information in France please contact:
Véronique Terrasse, IARC media officer, email@example.com +33 (0)4 72 73 83 66
Notes for Editors:
- The BMJ are also issuing a press release.
- The paper ‘Cancer risk in 680,000 people exposed to computed tomography scans in childhood or adolescence: data linkage study of 11 million Australians’ is to be published in the journal BMJ on 21 May 2013.
- The researchers did not include CT scans carried out less that 1 year before a cancer diagnosis, to exclude those that may have been involved in diagnosing the cancer and wouldn’t have had time to cause the cancer.
- The study was carried out by a large team that was led by Professor John Mathews of the University of Melbourne. It included researchers from a total of nine Australian organisations and also Professor Sarah Darby of the University of Oxford and Dr Graham Byrnes of the International Agency for Research on Cancer.
- The study was funded by the National Health and Medical Research Council of Australia.
- Oxford University’s Medical Sciences Division is one of the largest biomedical research centres in Europe, with over 2,500 people involved in research and more than 2,800 students. The University is rated the best in the world for medicine, and it is home to the UK’s top-ranked medical school.
From the genetic and molecular basis of disease to the latest advances in neuroscience, Oxford is at the forefront of medical research. It has one of the largest clinical trial portfolios in the UK and great expertise in taking discoveries from the lab into the clinic. Partnerships with the local NHS Trusts enable patients to benefit from close links between medical research and healthcare delivery.
A great strength of Oxford medicine is its long-standing network of clinical research units in Asia and Africa, enabling world-leading research on the most pressing global health challenges such as malaria, TB, HIV/AIDS and flu. Oxford is also renowned for its large-scale studies which examine the role of factors such as smoking, alcohol and diet on cancer, heart disease and other conditions.