Children exposed to multiple CT scans could be up to three times likelier to contract cancer of the blood, brain or bone marrow later in life, according to research published Thursday.
Writing in The Lancet medical journal, a team of scientists in Canada, Britain and the United States said the cancer risk, in absolute terms, appears to be small.
But they appealed for radiation doses from CT scans to be kept at a minimum and for alternatives to be used where appropriate.
The researchers claimed their study was the first to provide direct evidence of a link between exposure from CT radiation in childhood and later cancer risk.
"Of utmost importance is that where CT is used, it is only used where fully justified from a clinical perspective," said lead author Mark Pearce of Newcastle University's Institute of Health and Society.
As a vital diagnostic technique, use of the CT scan has increased rapidly in the past 10 years, particularly in the United States, the researchers said.
"However, potential cancer risks exist due to the ionising radiation used in CT scans, especially in children who are more radiosensitive than adults."
Computerized tomography -- commonly known as CT -- is an X-ray technique that produces images of the body's internal structures in cross sections.
The researchers studied nearly 180,000 people who underwent a CT scan as children or young adults (under 22) in Britain between 1985 and 2002.
Of these, 74 were subsequently diagnosed with leukaemia and 135 with brain cancer according to data for the period 1985 to 2008.
The team calculated that compared to patients who received a radiation dose of less than five milli-Grays (mGy), those who were given a cumulative dose of 30 mGy had about three times the risk of developing leukaemia (cancer of the blood or marrow) later in life.
Those who received 50 to 74 mGy had thrice the risk of brain tumours.
The study did not compare children who had been scanned against those who had not been scanned.
Put into context, this means that among every 10,000 patients who received one CT scan before the age of 10, there would be one extra case of leukaemia and one extra brain tumour per 10 mGy of radiation in the 10 years after exposure.
"Further refinements to allow reduction in CT doses should be a priority, not only for the radiology community, but also for manufacturers," said Pearce.
"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."
Commenting on the study, Andrew Einstein of the Columbia University Medical Centre in New York said Pearce and his colleagues' work confirmed 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."