CT Radiation Exposure and Cancer

Radiation from CT scans might cause 2% of future cancers.
An estimated 72 million computed tomography (CT) scans were performed in the U.S. in 2007. Two research groups assessed radiation dose from CT scans and future cancer risk.

Smith-Bindman and colleagues studied CT scans performed at four hospitals (private and public, large and small) in San Francisco in 2008. For each of 11 types of CT studies, the researchers estimated the effective dose of radiation, which takes into account "the amount of radiation to the exposed organs and each organ's sensitivity to developing cancer from radiation exposure."

The effective dose varied widely both within and between institutions for each type of CT study. The median effective dose was 2 millisieverts (mSv) for a noncontrast head CT scan and 31 mSv for an abdomen-pelvis study. For comparison, the effective dose is 0.065 mSv from posteroanterior and lateral chest radiography and 0.42 mSv from conventional mammography.

The authors estimated that a coronary angiogram delivers radiation to the breast equivalent to 15 mammograms and to the lung equivalent to 711 chest x-rays. The lag time between exposure and cancer development makes exposure potentially riskier for younger than older patients; the authors estimated that 1 in every 270 women who undergo coronary CT angiography at age 40 will develop cancer.

Berrington de Gonzalez and colleagues used a national database to estimate age-specific cancer risks from CT studies performed in the U.S. in 2007. CT scans performed during the last 5 years of life or after a diagnosis of cancer were excluded from the analysis. Thirty percent of CT scans were performed on patients aged 35 to 54. The authors predict that 2% (29,000) of future cancers will be caused by CT scans performed in 2007, and they estimate a cancer mortality rate of 50%.

Comment:
An editorialist terms these numbers "eye opening" and calls for dose standardization, patient education, and test ordering guidelines. A recent government report shows an eightfold difference in test ordering across states, with no detectable mortality benefit from higher rates of testing. One approach to limiting radiation exposure is to display patients' historical cumulative radiation dose in electronic order entry systems. It is time for someone to shout "stop."

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J. Stephen Bohan, MD, MS, FACP, FACEP
Published in Journal Watch Emergency Medicine January 22, 2010

Citation(s): Smith-Bindman R et al. Radiation dose associated with common computed tomography examinations and the associated lifetime attributable risk of cancer. Arch Intern Med 2009 Dec 14/28; 169:2078.