March 31, 2009 -- As many as 7% of patients treated at a large U.S. hospital received enough radiation exposure from repeated CT scans to increase their cancer risk, according to a new study.
One in three patients included in the study from Harvard University’s Brigham and Women’s Hospital had undergone five or more CT examinations and one in 20 had more than 22 CT scans.
The findings confirm a modest but clinically significant increase in cancer risk associated with multiple CT scans, researchers say.
“We found that while most patients accrue small cumulative cancer risks, 7% of the patients in our study had enough recurrent CT imaging to raise their estimated cancer risk by 1% or more above baseline levels,” says lead researcher Aaron Sodickson, MD, PhD.
Use of CTs Growing
Approximately 68 million CT exams were performed in the United States in 2007, up from 62 million the previous year.
Unlike conventional X-rays, CT -- short for computed tomography -- provides a detailed, three-dimensional image of internal organs, which helps physicians diagnose and track the spread of disease.
“We know that many patients have multiple scans,” Sodickson says. “But we really haven’t had good information on the individual patient’s risk. That is what we tried to do in this study.”
Thanks to an electronic database, the researchers were able to track the CT histories of more than 31,400 patients who had a CT scan in 2007 at Brigham and Women’s Hospital or Harvard’s Dana-Farber Cancer Center.
They found that:
- 5% of the patients in the study had received more than 22 CT scans, and 1% had received more than 38 examinations.
- 15% of the patients had received estimated cumulative radiation doses that were higher than the radiation exposure from 1,000 chest X-rays.
- 4% of patients had lifetime exposures equivalent to 2,500 conventional chest X-rays.
Using a cancer risk assessment model, the researchers found that 7.3% of the study participants had an elevated risk of cancer because of radiation from CT scans.
The risk was very small for most patients, totaling just 1% over the average lifetime risk of 42%.
But about 1% of the patients in the study had CT-related elevations in risk of between 2.7% and 12%.
“CT is an amazing diagnostic tool, and the last thing we would want to see is patients refusing needed scans because they are worried about cancer,” Sodickson says. “But patients and their physicians also need to be aware that there are risks and those risks add up over time.”
CT Overuse a Concern
Radiology professor G. Donald Frey, MD, of the Medical University of South Carolina, tells WebMD that the radiology community has been working hard to identify areas where CT scans are overused.
He points out that the use of whole-body CT scanning to screen for illness in healthy people has all but disappeared over the past several years.
Briefly popular early in the decade, body-scanning clinics offered the promise of finding cancer, heart disease, and many other illnesses in their nascent stages, at a cost of about $1,000 a scan. But the claims did not pan out, and most of the clinics soon closed.
The focus now is on the use of CT scans for minor complaints and in the emergency medicine setting, Frey says.
“It is clear that CT is being overused, but it is not so easy to tell where this is happening,” he says. “I would encourage patients not to put pressure on the physicians to order CTs, and I would encourage physicians to be aware of the appropriate indications for ordering them.”
A study published earlier this year in the Journal of the American Medical Association found a wide variation in radiation from clinic to clinic for CT scans performed to identify cardiovascular disease.
Researchers reported that exposures from a single CT scan were as much as six times higher at some clinics than others.
In response to the study, the American Heart Association issued an advisory to physicians warning against the use of CT scans to screen asymptomatic patients.
The fact that physicians often have no idea how many previous scans a patient has had has hampered efforts to reduce exposures in patients.
Databases that keep track of previous CT scans, like the one used in the Harvard study, are not common, but Sodickson says they should be.
He and his colleagues are developing a computerized program that keeps track of the total number of scans a patient has had and also quantifies the risk.
In the meantime, patients can advocate for themselves by knowing how many CT scans they have had and sharing that information each time an additional scan is ordered, he says.