The study used anonymised medical records for 11 million young Australians, including 680,000 who were exposed to CT scans between 1985 and 2005.
The Australian researchers, with colleagues at Oxford University and the International Agency for Research on Cancer in France, 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, say the researchers.
The research team, led by Professor John Mathews at the University of Melbourne, report their findings in the BMJ medical journal.
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.
In a group of 10,000 young people, we would expect 39 cancers to occur during the next 10 years. But if they all had one CT scan, up to 6 extra cancers would occur.
Professor Sarah Darby
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 co-author 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 adults 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.
Yinka Ebo, senior health information officer at Cancer Research UK, says: ‘CT scanners can help doctors diagnose and monitor many diseases including cancer, and the radiation doses from modern scanners are likely to be lower than two decades ago.
‘This Australian study found an increased cancer risk following CT scans in children and young adults, which needs to be weighed against the undoubted medical benefits of using CT scanners. UK regulations ensure there should be good clinical reasons for CT scans, but doctors should only use them when necessary and it’s important to try to minimise the radiation dose where possible.’
The study was funded by the National Health and Medical Research Council of Australia.