Children and teenagers exposed to scans were 24 per cent more likely to have cancer than those who had never had such scans.

Youngsters who have CT scans are more likely to get cancer than those who do not, a study suggests.

Researchers found that children and teenagers exposed to scans were 24 per cent more likely to have cancer than those who had never had such scans.

But the authors said that the incidence of cancer in children is “extremely small” to begin with, so a 24 per cent increase makes this risk just slightly less small.

A computerised tomography (CT) scan uses X-rays and a computer to create detailed images of the inside of the body. Patients who undergo such scans are exposed to a small amount of radiation, but most of the time, the benefits of having a scan outweigh any potential risks.

Of 680 000 Australians exposed to a CT scan from birth to 19 years, cancer incidence was increased by 24 per cent compared with the incidence in over 10 million unexposed people, the new research found.

The authors of the study, published on bmj.com, cautioned that the increased incidence may be attributed to the fact that patients could have had the scans because of pre-existing cancer.

They wrote: “The increased incidence of cancer after CT scan exposure in this cohort was mostly due to irradiation. Because the cancer excess was still continuing at the end of follow-up, the eventual lifetime risk from CT scans cannot yet be determined.

“Radiation doses from contemporary CT scans are likely to be lower than those in 1985-2005, but some increase in cancer risk is still likely from current scans. Future CT scans should be limited to situations where there is a definite clinical indication, with every scan optimised to provide a diagnostic CT image at the lowest possible radiation dose.”

Lead author John Mathews, from the Melbourne School of Population and Global Health at the University of Melbourne, said: “CT scans were very useful in providing detailed three-dimensional pictures to diagnose or exclude disease in internal organs and in most cases, the benefits of having a scan clearly outweigh the risk of a later cancer.

“Nevertheless, our 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.

“As an individual patient, your risk of cancer from a CT scan is very low.

“Nevertheless, it is clear from our study that if we reduce the number of scans performed in a large population, and continue to reduce the doses from individual scans, there will be a small but corresponding reduction in the number of cancers in later years.”