AIM: CT scans may lead to 'thousands' of future cancer cases
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In the U.S., about 70 million CT scans were performed in 2007, up from three million in 1980, according to Rebecca Smith-Bindman, MD, of the University of California, San Francisco, and colleagues.
“While CT scans can provide great medical benefits, there is concern about potential future cancer risks because they involve much higher radiation doses than conventional diagnostic x-rays,” wrote Smith-Bindman and colleagues. A chest CT scan exposes the patient to more than 100 times the radiation dose of a routine chest x-ray. “The risks to individuals are likely to be small, but because of the large number of persons exposed annually, even small risks could translate into a considerable number of future cancers,” they wrote.
In one study, Smith-Bindman and colleagues studied 1,119 patients undergoing the 11 most common types of diagnostic CT scans at four institutions in 2008. Using hospital records, the researchers calculated the radiation dosage involved with each scan and then estimated lifetime risks of cancer that could be attributed to those scans.
Radiation dosage varied widely between different types of CT studies; median doses ranged from 2 mSv for a routine head CT scan to 31 mSv for a multiphase abdomen and pelvis scan. “Within each type of CT study, effective dose varied significantly within and across institutions, with a mean [average] 13-fold variation between the highest and lowest dose for each study type,” the authors wrote.
The estimated number of CT scans that would lead to the development of one cancer case also varied by type of CT scan and also by each patient’s age and sex. For instance, an estimated one in 270 women and one in 600 men who undergo CT coronary angiography at age 40 will develop cancer as a result. One cancer case will likely occur among every 8,100 women and 11,080 men who had a routine head CT scan at the same age.
“For 20-year-old patients, the risks were approximately doubled, and for 60-year-old patients, they were approximately 50 percent lower,” the authors wrote.
“The radiation exposure associated with CT has increased substantially over the past two decades, and efforts need to be undertaken to minimize radiation exposure from CT, including reducing unnecessary studies, reducing the dose per study and reducing the variation in dose across patients and facilities,” they concluded. “Patient outcomes studies are needed to help define when CT leads to the greatest benefit and when these studies may have no impact, [to determine] where the radiation risk may be greater than the benefit expected from the examinations. Understanding exposures to medical radiation delivered through actual clinical studies is a crucial first step toward developing reasonable strategies to minimize unnecessary exposures.”
In another study, Amy Berrington de González, DPhil, of the National Cancer Institute in Bethesda, Md., and colleagues constructed a risk model to estimate age-specific cancer risks for each scan type. They derived the data from previous reports of radiation-associated cancer risks, national surveys and insurance claims.
“Overall, we estimated that approximately 29,000 future cancers could be related to CT scans performed in the U.S. in 2007,” the authors wrote. This includes an estimated 14,000 cases resulting from scans of the abdomen and pelvis; 4,100 from chest scans; 4,000 from head scans; and 2,700 from CT angiography. One-third of these projected cancer cases would occur following scans performed on individuals age 35 to 54 years, compared with 15 percent due to scans performed in children and teens. Two-thirds of the cancers would be in women.
“Changes made to practice now could help to avoid the possibility of reaching the level of attributable risk suggested above (2 percent). Our detailed estimates highlight several areas of use in which the public health impact may be largest, specifically abdomen and pelvis and chest CT scans in adults aged 35 to 54 years,” the authors concluded. “Further work is needed to investigate the balance of the risks and benefits from CT scan use and to assess the potential for dose or exposure reduction.”
“The articles in this issue make clear that there is far more radiation from medical CT scans than has been recognized previously, in amounts projected to cause tens of thousands of excess cancers annually,” wrote Rita F. Redberg, MD, of the University of California, San Francisco, in an accompanying editorial. “Also, as these scans have become more sensitive, incidental findings lead to additional testing (and often more radiation), biopsies and anxiety.”
“Although a guiding principle in medicine is to ensure that the benefit of a procedure or therapy outweighs the risk, the explosion of CT scans in the past decade has outpaced evidence of their benefit,” Redberg wrote. “Although there are clear instances when CT scans help determine the treatment course for patients, more and more often patients go directly from the emergency department to the CT scanner even before they are seen by a physician or brought to their hospital room.”
“To avoid unnecessarily increasing cancer incidence in future years, every clinician must carefully assess the expected benefits of each CT scan and fully inform his or her patients of the known risks of radiation,” Redberg concluded.