Exposing a child to the nuclear radiation from two or three computed tomography(CT) head scans can triple its risk of developing brain cancer later in life, according to a 20-year study published on Thursday.
The study also found that a child exposed to the cumulative radiation of between five and 10 CT scans is three times more likely than an unexposed child to develop leukaemia.
While the absolute risk of cancers developing after a CT scan is still small, the researchers said radiation doses should be kept as low as possible and alternatives to ionising radiation should be used whenever possible.
"It's well known that radiation can cause cancer but there is an ongoing scientific debate about whether relatively low doses of radiation, like those received from CT scans, do increase cancer risks, and if so the magnitude of those risks," said Amy Berrington de Gonzalez of the National Cancer Institute at the United States National Institutes of Health, who worked on the study with scientists from Britain and Canada.
"Ours is the first study to provide direct evidence of a link...and we were also able to quantify that risk."
CT imaging is a diagnostic technique often used on children with possible head injuries.
The number of CT scans has increased rapidly in the United States and other wealthy countries, particularly in the past decade, because new uses are constantly being identified, such as scanning for possible appendicitis.
The risk of developing cancer comes from the ionising radiation used in CT scans. The risk is higher in children, who are more radiosensitive than adults.
"This work emphasises the very great importance of only using this form of imaging when it has a strong medical justification," Bruce Armstrong, professor of public health at Sydney University, said in an emailed comment on the study.
No radiation in ultrasound
One alternative to a CT scan is ultrasound, which involves no radiation, but is less accurate. A study published last month found that it may also be safe to postpone CT scans in some cases of childhood head injuries.
In the current study, published in the Lancet medical journal, researchers studied almost 180,000 patients who had a CT scan between 1985 and 2002 in a British hospital.
They extracted the number and types of CT scan from the records and estimated the radiation dose absorbed by the brain and bone marrow in milli-Grays (mGy) from each scan. These data were then linked to data on cancer cases and deaths in the UK National Health Service Registry between 1985 and 2008.
A total of 74 out of 178,604 patients were diagnosed with leukaemia and 135 of 176,587 were diagnosed with brain cancer, and the researchers calculated the relative risk of leukaemia increased by 0.036 for every mGy received, while the increased risk of a brain tumour was 0.023.
Mark Pearce of Newcastle University, who led the study, said scientists should make it a priority to work on further refining CT technology so that radiation doses are reduced as much as possible.
"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," he added.
David Spiegelhalter, an expert in the understanding of risk at Cambridge University, who was not directly involved in the research, said its results needed to be seen in context.
"This study suggests there is around a 1 in 10,000 chance that a young person's CT scan will give them leukaemia over the next 10 years," he said in an emailed comment. "This is important, but the CT scan may be even more important - a judgment has to be made."