Radiation from multiple CT scans in childhood could triple the risk of developing leukaemia or brain cancer later in life, a study suggests.
Although the absolute risk of developing cancer was found to be small, the UK researchers said CT scans should only be used when "fully clinically justified," with radiation doses kept as low as possible.
However, they also added that in cases such as children with major head injuries or life-threatening illnesses, the benefits of CT scans should outweigh the future risk of cancer.
For the study, the team of researchers analysed the medical records of nearly 180,000 young people aged under 22 who underwent a CT scan at an NHS hospital between 1985 and 2002.
They looked at the number and types of CT scan and estimated the radiation dose absorbed by the bone marrow and brains for each scan.
None of the patients had been diagnosed with leukaemia or a brain tumour.
They then compared the patient cancer rates with those of the general UK population, and calculated the "excess incidence" of leukaemia and brain tumours.
The study, published in The Lancet, found that children under the age of 15 who received a radiation dose equivalent to two to three head CT scans were three times more likely to develop brain cancer over the next ten years compared with the general population.
A similar radiation dose to the bone marrow - the part of the body responsible for generating blood cells - increased the risk of leukaemia three-fold compared with the general population, but five to 10 CT scans would be needed to reach this level, the researchers said.
They estimated that for every 10,000 head CT scans performed on children 10 years old or younger, one more case of leukaemia and one more brain tumour would occur than would normally be expected.
The researchers also pointed out that UK regulations mean that CT scans should only be done when clinically justified, but this is not true of other countries where they are used more often.
Dr Mark Pearce, who led the study said: "CT scans are accurate and fast so they should be used when their immediate benefits outweigh the long-term risks.
"However, now we have shown that CT scans increase the risk of cancer, we must ensure that when they are used they are fully justified from a clinical perspective."
"While the risks are small, the medical community needs to consider carefully its use of CT scans and refine their use as a priority. We need to look at worldwide regulation."