Studies in this week's Archives of Internal Medicine suggest that CT scans could be responsible for tens of thousands of new cancers in the coming decades, and that radiation exposure from CT scans is highly variable -- and often exceeds recommended limits.

According to one of the two new studies, CT scans performed in 2007 alone might be responsible for 29,000 future cancers, as well as 15,000 deaths.

Amy Berrington de Gonzalez, D.Phil., a researcher in radiation epidemiology at the National Cancer Institute in Bethesda, Md., and colleagues combed through commercial insurance databases and Medicare claims. Using that data to estimate radiation dose exposure from CT imaging, as well as models of the effect of CT radiation on the human body, they predict that the 72 million CT scans in 2007 could trigger 29,000 excess cancers over the next 20 to 30 years. At a 50 percent mortality rate, these cancers would kill 15,000 annually.
A separate study in the same issue of Archives of Internal Medicine, led by Rebecca Smith-Bindman, M.D., a professor of radiology at the University of California, San Francisco, found another reason to worry about CT scans. The researchers discovered widespread variation in dosages for similar CT procedures at different health centers, with on average a 13-fold difference between the highest and lowest doses.

The doctors say they couldn't find any definite or predictable reason for the large variation in exposures. In any case, even the typical dosages for some procedures seemed unnecessarily high. In an accompanying editorial, Rita Redberg, M.D., an Archives of Internal Medicine editor also from UCSF, notes that the median dose for a CT coronary angiogram, 22 mSv, was four times higher than it should be if one follows contemporary guidelines.

And this high dosage, say the authors, has consequences. They predict that among 40-year-old women, one out of every 270 CT coronary scans, the riskiest in the study with each one equal to about 309 chest X-rays, will cause cancer.

Cancer-CT link contested

However, the studies don't lack critics. On Tuesday, the American College of Radiology issued a statement saying that the studies were flawed, in part, because the effects of radiation from X-rays on humans aren't well understood.

To estimate cancer-risk from CT scans, Dr. Berrington's study used data largely based on the effects of radiation on survivors of the atom bombs in Japan.

"An atomic bomb blast is a very different thing for a person to be caught in than getting a CT scan," Shawn Farley, an ACR spokesman, tells DOTmed News. ACR says that the two can't be compared, as victims of an atomic bomb blast get instant whole-body exposure, receiving not just X-rays, but particulate radiation, neutrons and gamma rays.

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