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The rapid growth of cardiac imaging by computed tomography has spurred greater attention to risks from cardiac CT radiation, as well as other medical imaging studies that employ ionizing radiation.
In an observational study, researchers assessed radiation exposure among patients who underwent cardiac CT angiography at 50 sites worldwide. Greater than fivefold variation in radiation doses was observed, and, surprisingly, experience in performing cardiac CT and hospital procedure volume were not associated with lower radiation doses (JW Gen Med Feb 24 2009).
In a second study, investigators estimated lifetime risk for developing cancer from cardiac CT studies. At a dose of 2.3 millisieverts (mSv), a single study at age 55 would result in a lifetime excess cancer risk of 8 cases per 100,000 men and 20 per 100,000 women. Lung cancer accounted for most of the excess risk (JW Gen Med Aug 18 2009).
In a third study, researchers used insurance claims data from nearly 1 million adults and found that 70% of them underwent at least one imaging procedure during 3 years. Procedures with the largest contributions to cumulative radiation doses included CT of the abdomen and pelvis (30%), myocardial perfusion imaging (22%), and CT of the chest (8%; JW Gen Med Aug 26 2009).
Finally, investigators found that patients knew extremely little about radiation doses associated with CT examinations. Fifteen percent of patients didn't know that CT studies involved radiation, and most patients didn't know that the radiation dose of a single abdominal/pelvic CT study exceeds annual background radiation dose. For the record, typical radiation doses are 3 mSv for annual background, 0.5 mSv for mammograms, 10 mSv for abdominal/pelvic CT, and 3–15 mSv for cardiac CT (Arch Intern Med 2009; 169:1069).
Some people argue that ionizing radiation from medical imaging might not raise cancer risk in a meaningful way, given that most research has involved extrapolating risk from data derived from atomic bomb survivors. Nonetheless, these findings and others are altering the practice of medical imaging substantially. Manufacturers and imaging specialists have responded by developing new technologies and protocols to lower radiation exposure. Meanwhile, some of our patients receive astonishingly high numbers of CT scans — many of questionable clinical value — during relatively short periods. Before ordering imaging procedures that involve ionizing radiation, clinicians should consider carefully whether such studies are likely to enhance clinical decision making, given that no safe limit of radiation is likely to be established.