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    Multiple CT Scans Raise Cancer Risk

    Increase in Cancer Risk Is Small but Significant, Study Shows
    WebMD Health News
    Reviewed by Louise Chang, MD

    March 31, 2009 -- As many as 7% of patients treated at a large U.S. hospital received enough radiation exposure from repeated CT scans to increase their cancer risk, according to a new study.

    One in three patients included in the study from Harvard University’s Brigham and Women’s Hospital had undergone five or more CT examinations and one in 20 had more than 22 CT scans.

    The findings confirm a modest but clinically significant increase in cancer risk associated with multiple CT scans, researchers say.

    “We found that while most patients accrue small cumulative cancer risks, 7% of the patients in our study had enough recurrent CT imaging to raise their estimated cancer risk by 1% or more above baseline levels,” says lead researcher Aaron Sodickson, MD, PhD.

    Use of CTs Growing

    Approximately 68 million CT exams were performed in the United States in 2007, up from 62 million the previous year.

    Unlike conventional X-rays, CT -- short for computed tomography -- provides a detailed, three-dimensional image of internal organs, which helps physicians diagnose and track the spread of disease.

    “We know that many patients have multiple scans,” Sodickson says. “But we really haven’t had good information on the individual patient’s risk. That is what we tried to do in this study.”

    Thanks to an electronic database, the researchers were able to track the CT histories of more than 31,400 patients who had a CT scan in 2007 at Brigham and Women’s Hospital or Harvard’s Dana-Farber Cancer Center.

    They found that:

    • 5% of the patients in the study had received more than 22 CT scans, and 1% had received more than 38 examinations.
    • 15% of the patients had received estimated cumulative radiation doses that were higher than the radiation exposure from 1,000 chest X-rays.
    • 4% of patients had lifetime exposures equivalent to 2,500 conventional chest X-rays.

    Using a cancer risk assessment model, the researchers found that 7.3% of the study participants had an elevated risk of cancer because of radiation from CT scans.

    The risk was very small for most patients, totaling just 1% over the average lifetime risk of 42%.

    But about 1% of the patients in the study had CT-related elevations in risk of between 2.7% and 12%.

    “CT is an amazing diagnostic tool, and the last thing we would want to see is patients refusing needed scans because they are worried about cancer,” Sodickson says. “But patients and their physicians also need to be aware that there are risks and those risks add up over time.”

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