The study is the first large-scale examination of radiation doses given to patients who underwent a range of different types of CT procedures. The researchers reviewed the procedures performed on 1,119 patients at four San Francisco Bay Area institutions over five months. They evaluated procedures in three anatomic areas: head and neck, chest, and abdomen and pelvis.

Study findings appear in the December 14, 2009, issue of the "Archives of Internal Medicine."


Study results showed that doses were in general higher than typically reported and that there was a mean 13-fold variation between the highest and lowest radiation dose for each type of CT procedure, which means "If a physician sent a patient for a particular CT procedure, the dose that patient would have received varied by this much," Smith-Bindman explained. As expected, the researchers' estimate of the number of CT scans that would result in a cancer varied widely by sex, age, and type of scan. They estimated that it would take fewer CT scans to result in a cancer among women compared with men.

They estimated, for example, that 1 in 270 women who underwent a CT coronary angiogram at age 40 years will develop cancer from the procedure, compared to 1 in 600 men. For a routine head CT scan at the same age, the estimated risk is 1 in 8,100 for women and 1 in 11,080 for men. In patients around age 20 years of age, the risks were approximately twice as high. "The risk associated with obtaining a CT is routinely quoted as around 1 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 1 in 80," noted Smith-Bindman.

The research team used "effective dose" to quantify the radiation exposure associated with each CT scan because it is one of the most frequently reported measurements. Then to translate the variation of doses into cancer risks, the team utilized data from a National Academy of Science National Research Council report that assessed the health risks from exposure to ionizing radiation. The report was published in 2006 under the title "Biological Effects of Ionizing Radiation VII Phase 2."

Depending on the location and technical parameters of the individual CT scanner, the effective dose received by a patient could substantially exceed the median, according to study results. "This is particularly important because the threshold for using CT has declined, and CT is increasingly being used among healthy individuals, in whom the risk of potential carcinogenesis from CT could outweigh its diagnostic value," the research team noted in the "Archives" article.

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