Researchers have warned doctors to ensure that CT scans carried out on children are clinically justified, after a government-funded study found that exposure to ionising radiation during such scans could triple the risk of under-15s developing brain cancer or leukaemia later in life.

Two or three scans would be sufficient to heighten the risk to such levels for brain cancer, while five to 10 would similarly lift the risk of leukaemia, according to a study funded by the Department of Health and the US National Cancer Institute.

Brain tumours and leukaemia, while the most common childhood cancers, are still relatively rare. While the risk of developing them after scans remains low in absolute terms, and CT scanning could be vital in saving young people's lives, the figures reported in Lancet Online will encourage doctors to seek alternative diagnostic procedures to computed tomography scans where possible. These include ultrasound and magnetic resonance imaging.

Children are more sensitive to radiation than older people. The research, involving experts from the UK, US, the Netherlands, South Korea and Canada, is the first to provide direct evidence of a link between exposure to radiation from CT scans in childhood and cancer risk, and to quantify that risk.

The study looked at data covering nearly 180,000 patients under 22 who had not already been diagnosed with brain tumours or leukaemia and who had had CT scans between 1985 and 2002 at seven in 10 UK hospitals; estimated doses of radiation energy absorbed through the brain and bone marrow (which varies by age and body part scanned); and data linked to cancer incidence and mortality across the UK between 1985 and 2002. From this they calculated "excess incidence" of leukaemia and brain tumours.

The researchers found the relative risk of leukaemia increased 0.036 times for every milligray (mGy) of radiation exposure. For brain turmors, the increased risk per milligray was 0.023. One mGy is the amount of radiation energy absorbed by 1kg of human tissue or other material. For children under 10 receiving head scans, about one extra case of leukaemia and one extra brain tumour per 10,000 patients would be expected after the procedure.

CT scans were introduced in the 1970s and their use in the UK, for examining patients for a range of conditions including tumours, bone disorders and internal injuries, rose by 68% between 1998 and 2008. In 2007, an estimated 72m scans were carried out in the US.

Lead researcher Mark Pearce, from the University of Newcastle, said: "The immediate benefits of CT outweigh the potential long-term risks in many settings and because of CT's diagnostic accuracy and speed of scanning, notably removing the need for anaesthesia and sedation in young patients, it will, and should, remain in practice for the foreseeable future."

But, said Pearce, "further refinements to allow reduction in CT doses should be a priority, not only for the radiology community, but also for manufacturers. Alternative diagnostic procedures that do not involve ionising radiation exposure, such as ultrasound and MRI, might be appropriate in some clinical settings.

"Of utmost importance is that where CT is used, it is only used where fully justified from a clinical perspective."

In a comment accompanying the research, Andrew Einstein, from Columbia University medical centre in New York, said it confirmed that CT scans "almost certainly produce a small cancer risk", but with their use rising, "we must redouble our efforts to justify and optimise every CT scan".

The DoH said: "Clinicians should make decisions on a case-by-case basis, making sure scans are given only when the benefits outweigh the risks.

"The UK uses lower levels of radiation in CT scans than other countries. We also have clear regulations to ensure a CT scan is only carried out when clinically justified. These regulations are based on expert knowledge of anatomy and medical radiation physics to ensure that scans are not given unnecessarily."

© Guardian News and Media 2012

[CT scan via Tyler Olson / Shutterstock]