formed 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."
SOURCES: Rebecca Smith-Bindman, M.D., professor, residence, radiology, University of California, San Francisco; Amy Berrington de Gonzalez, Ph.D., investigator, Radiation Epidemiology Branch, Division of Cancer Epidemiology and Genetics, National Cancer Institute, Bethesda, Md.; Andrew J. Einstein, M.D., Ph.D., assistant professor, clinical medicine, and director, cardiac computed tomography research, Columbia University College of Physicians and Surgeons, 