Women are at particular risk from CAT scans as they are more sensitive
to the effects of radiation
Having a CT - or CAT - scan puts patients at far greater risk of developing cancer than previously thought, scientists claim.
The radiation generated by the scans - an increasingly popular diagnostic tool - may trigger the disease in as many as one in 80 patients.
This is far higher than the often used figure of one in 1,000 - with women at particular risk as they are more sensitive to the effects of radiation.
Researchers now believe the dose of radiation delivered by a CT scan can vary wildly according to where on the body it is used.
In some cases, they suggest, a single scan can be the equivalent of 442 chest X-rays.
Although the scans - CT stands for computed tomography - can be lifesaving in detecting disease or brain damage, the new U.S. study will heighten fears the 'worried well' paying for them privately could be suffering unnecessary harm.
If the American findings were applied to the UK, it could mean CT procedures are causing thousands more cancer cases than previously thought.
The scans allow doctors to build detailed 3D images of internal organs, blood vessels, bones or tumours.
They were already known to carry a greater risk than ordinary X-rays, such as those used for breast screening, but the latest research suggests a bigger problem.
It found the dose of radiation received was larger than thought, although this varied according to the part of the body being scanned and the age and sex of the patient.
The researchers concluded there was an average 13-fold variation between the highest and lowest doses experienced by patients, says a report in the journal Archives Of Internal Medicine.
University of California Professor Rebecca Smith-Bindman, who led the study, said: '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 dose delivered by a single CT scan was the equivalent of 74 mammograms or 442 chest X-rays, the professor said.
Researchers reviewed 1,119 patients in San Francisco who had been scanned in three body areas - the head and neck, the chest, and the abdomen and pelvis.
The scientists then worked out the radiation dosage of each scan and estimated the associated lifetime risk of cancer.
A heart examination might involve three scans, looking at different phases of the pumping cycle.
Prof Smith-Bindman said: 'This increases the information that we can get from the CT procedure, but increases the radiation dose by a factor of three.'
She said doctors need to reduce unnecessary use of scans.
In the UK, the Committee on Medical Aspects of Radiation in the Environment does not recommend whole body CT scanning for healthy people because the risks outweigh the benefits.
It advises against CT scans for spinal conditions, osteoporosis and assessing body fat as there are better tests that do not rely on radiation.
