CT scans triple risk of leukaemia for children

CT scans can triple the risk of children developing leukaemia and brain cancer, a study has found.

Despite a miniscule absolute risk, scientists called for greater efforts to ensure use of the 3D X-rays is justified.

Previously, it was widely assumed that low-dose CT scans posed negligible risks even to children, who are more sensitive to radiation than adults.

This was based on risk estimates derived from survivors of the atomic bombs dropped on Japan at the end of the Second World War.

However, many experts acknowledge that there are crucial differences between exposure to radiation from an atomic bomb and a medical procedure.

For instance, CT scans are usually focused on a particular region, such as the head, rather than the whole body.

Scientists conducting the new research studied data on about 180,000 patients under the age of 22 who had CT scans at British hospitals between 1985 and 2002. Their cancer rates were compared with those of the general population, reported in the NHS registry.

The results showed children younger than 15 would receive enough radiation from two to three head CT scans to triple their risk of developing brain cancer.

Because brain and bone marrow do not absorb X-rays at the same rate, the figures for leukaemia are slightly different. In this case, five to 10 head scans were sufficient to triple the cancer risk.

In general, the scientists calculated the relative risk of leukaemia increased 0.036 times for every extra milligray of radiation exposure.

One milligray is the amount of radiation energy absorbed by 1kg of human tissue or other material. For brain tumours, the increased risk per milligray was 0.023.

The findings are published in the latest online edition of The Lancet medical journal.

They show that the chances of an individual developing cancer after a CT scan remain tiny.

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 during the decade after the procedure.

The study did not look at the reasons why the children underwent CT scans, but ensured it was not because they already had brain tumours or leukaemia.

Lead researcher Mark Pearce from the University of Newcastle, said: “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 [magnetic resonance imaging] 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.”

CT scans are used to examine patients with a range of health conditions including brain tumours, certain bone disorders, and injuries to internal organs.

Commenting in The Lancet, Andrew Einstein, from Columbia University Medical Centre, said: “Pearce and colleagues confirm that CT scans almost certainly produce a small cancer risk.

“Use of CT scans continues to rise, generally with good clinical reasons, so we must redouble our efforts to justify and optimise every CT scan.”