Patients undergoing a full-body computed tomography (CT) scan are exposed to a radiation dose equivalent to that received by some survivors of the Hiroshima and Nagasaki atomic bombs, say US researchers.
This exposure has been linked with a significant rise in cancer mortality and younger patients are especially at risk.
CT scanners rotate around the body taking a series of cross-sectional X-ray “slices”, which are compiled by computer to produce a 3D portrait of internal organs and structure. The radiation dose received is often 500 times that of a conventional X-ray and nearly 100 times that of a mammogram.
While those patients referred for diagnostic CT scans by medical practitioners should see benefits that far outweigh the risks, those who self-refer may be increasing their risk of cancer unduly.
Atomic blasts
David Brenner and his colleagues at Columbia University Medical Center in New York City found that the body absorbs a radiation dose of about 13 milliSieverts from each scan.
That is equivalent to the dosage absorbed by people living 2.4 kilometres from the centre of the World War II atomic blasts in Japan, he says. Brenner adds that some scanners can produce a third more radiation than this.
Elective CT scans, where healthy patients choose to undergo full-body screenings to detect potential health problems, are increasing in popularity. This is especially true in the US, where they make up a growing proportion of the 65 million or so CT scans performed each year.
Chat show host Oprah Winfrey recently had the procedure done on her television show, boosting the scan’s popularity. In Los Angeles, the number of centres offering CT scans have more than doubled in the past year.
The risk from one CT scan increases your chance of dying of cancer by 0.08%, but the risk is cumulative. Brenner explains that “a 45-year-old who has a CT scan every year until he is 75 has a one in 50 increased risk of dying of cancer. That’s comparable to your risk of dying in a road accident.”
Vulnerable cells
Younger people have a greater risk since radiation-induced cancer takes a long time to develop, and older people are more likely to die from other causes during this period. Also, younger people possess greater numbers of dividing cells, which are more sensitive to the effects of radiation, Brenner adds.
A separate study, carried out at Yale University School of Medicine, published in May 2004, found that only 7% of patients given CT scans were told about the possible risks and benefits of the procedure.
The study also discovered that “patients, emergency department physicians and radiologists alike are unable to provide accurate estimates of CT doses regardless of their experience level".
Elective CT scans, costing between $600 and $1500, are heavily promoted in the US as being able to find hidden diseases such as cancers at an early stage of development, but they have proved controversial.
False positives – where the scans have indicated a possible health problem where none exists – typically involve more extensive, costly and stressful tests. The US Food and Drug Administration advises against elective scans, saying there are no proven health benefits.