Studies Quantify Cancer Risks From CT Scans
An estimated 29,000 future cancers could be linked to scans performed in 2007
MONDAY, Dec. 14, 2009 (HealthDay News) -- Commonly performed CT scans are exposing patients to far more radiation than previously thought and in doses that could cause tens of thousands of cancers a year, two new studies claim.
Based on the findings, reported in the Dec. 14/28 issue of the Archives of Internal Medicine, the study authors, joined by Archives editor Dr. Rita F. Redberg, are calling on clinicians to limit radiation exposure to patients.
"CT has become so quick that we are using it so commonly, and we have lowered our threshold for using it -- meaning we use it for patients who really are unlikely to have any underlying disease," said Dr. Rebecca Smith-Bindman, a professor in residence in the department of radiology at the University of California, San Francisco, and lead author of one of the studies. Although it's a "fabulous diagnostic tool," she said she believes "we have lowered it to the point where there may be no benefit in some patients."
Redberg, who specializes in cardiovascular CT imaging, urged clinicians to carefully assess the benefits of each CT scan and inform patients of the known risks of radiation.
"In light of these data, physicians (and their patients) cannot be complacent about the hazards of radiation or we risk creating a public health time bomb," she wrote.
In the United States, the total number of CT (or computed tomography) scans performed annually has swelled from 3 million in 1980 to nearly 70 million in 2007, according to data cited by Smith-Bindman's team.
CT scans combine computer and X-ray technology to produce detailed, cross-sectional images of the body that can help clinicians diagnose and treat medical conditions.
However, because CT scans typically expose patients to a higher dose of radiation than conventional X-rays, the dramatic rise in the use of this technology has become a reason for concern.
In fact, the new data suggest that its overuse may be doing more harm than good.
Smith-Bindman's team collected data from 1,119 patients who received 11 common types of CT scans performed at four San Francisco-area hospitals. For each type of CT scan, the dose of radiation varied widely within and across hospitals. There was a 13-fold variation, on average.
"This kind of range had been reported before in CT scans of the heart, but putting it all together like this for other types of CT scans is something which hasn't been appreciated before," said Dr. Andrew J. Einstein, director of cardiac computed tomography research at Columbia University College of Physicians and Surgeons in New York City.
Because patients come in different sizes, some ability to vary the radiation dose is necessary to produce good diagnostic-quality pictures, Einstein explained. But he found amounts of variability in the study to be excessive.
The dose of radiation for a multiphase abdomen-pelvis CT study ranged from 6 to 90 millisieverts, and the average dose was 31 millisieverts.
Ninety millisieverts, depending on how you count it, is equivalent to "many thousands of chest X-rays," Einstein said. "That's like the background radiation you and I would receive over a 30-year period; it's just a tremendously high dose from one CT scan procedure."
The number of CT scans that would lead to a case of cancer also varied by type of scan and the patient's age and sex. For example, one in 270 women and one in 600 men who have a CT coronary angiography at age 40 will develop cancer as a result. The risks were approximately doubled for 20-year-old patients undergoing CT imaging. For 60-year-olds, the risks were 50 percent lower, the study authors noted.
In a separate analysis, Amy Berrington de Gonzalez, an investigator in the Radiation Epidemiology Branch of the National Cancer Institute's Division of Cancer Epidemiology and Genetics, projected future cancer risk from current CT use according to age, sex and scan type.
Overall, her team estimated that 29,000 future cancers could be related to scans performed in 2007, and that these cases would result in about 14,500 deaths. The highest contributors to those numbers are the scans most frequently performed, including abdomen and pelvis, chest and head exams.
It's estimated that two-thirds of the projected cancers will occur in women, primarily because of the higher frequency of use in women (60 percent) and because of higher breast and lung cancer risks from scans that expose the chest.
While the numbers may be scary, Berrington de Gonzalez said people should realize "that CT scans provide great medical benefits and that, in general, individual risks are small and should be outweighed by the benefits if the CT scan is clinically justified."
To ensure safe use, the authors' recommendations focus on reducing radiation dosages, eliminating unnecessary and repeat examinations, and creating searchable electronic medical records to collect and track CT studies over time.
"Although there is much work to be done, at this point this should be the agenda in radiology quality improvement programs," said Dr. Richard T. Griffey, associate chief for quality and safety in the Division of Emergency Medicine at Barnes-Jewish Hospital and an assistant professor at Washington University School of Medicine in St. Louis.
Clinicians also need more information about patients' radiation exposures over time, he added. "We have what can be a very fractionated health-care system, and doctors may not know what studies their patients have undergone in their own institution, let alone others, over their lifetime," he said. "While the incremental harm of a single study may be impossible to really know, some sense for cumulative radiation exposure would be valuable."
The U.S. Food and Drug Administration has more on CT scans.