CT scans may pose a much bigger radiation danger to patients than is generally believed, research has shown.
Some scans may be triggering cancer in as many as one in 80 patients, say American scientists.
Such a level of risk is far higher than the one in 1,000 that is generally quoted.
If the American findings hold true for the UK it could mean the X-ray procedures are causing thousands more cases of cancer than is suggested by current estimates.
CT, or Computed Tomography, scans take cross-sectional X-rays to build up detailed 3D pictures of internal organs, blood vessels, bones or tumours.
All X-rays are associated with a slim increased risk of cancer. CT scans are known to pose a greater risk than ordinary X-rays such as mammograms, but the new research indicates they might be more hazardous than was previously thought.
Results of the US study, published in the journal Archives of Internal Medicine, showed that CT radiation doses were generally higher than typically reported. For individual types of scan, there was an average 13-fold variation between the highest and lowest doses experienced by patients.
“If a physician sent a patient for a particular CT procedure, the dose that patient would have received varied by this much,” said study leader Professor Rebecca Smith-Bindman, of the University of California at San Francisco.
“The risk associated with obtaining a CT is routinely quoted as around one in 1,000 patients who undergo CT will get cancer. In our study, the risk of getting cancer in certain groups of patients for certain kinds of scans was as high as one in 80.”
The typical effective dose delivered by a single CT scan was the equivalent of up to 74 mammograms or 442 chest X-rays, said Prof Smith-Bindman.
A separate team of scientists writing in the same journal calculated that around 29,000 cancers in the US could be related to CT scans performed in 2007.
Some experts have become concerned by the increasing number of CT scans being performed and the risks posed to healthy people undergoing the procedures as part of a general check-up.
Since 1980, the annual number of CT scans carried out in the US has increased from about three million to 70million.
While the technology has improved, so has the speed at which the scans can be conducted. This was a “double edged sword”, making it tempting to carry out “multiphase” studies, said Prof Smith-Bindman.
The researchers concluded: “The radiation exposure associated with CT has increased substantially over the past two decades and efforts need to be undertaken to minimise radiation exposure from CT, including reducing unnecessary studies, reducing the dose per study and reducing the variation in dose across patients and facilities.”