Radiology: Experts debate safety of x-ray scanning at U.S. airports
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David J. Brenner, PhD, from the Center for Radiological Research at Columbia University in New York City, argued that while the extensive deployment of x-ray backscatter scanners is likely safe for most individual passengers, it presents long-term population concerns. Meanwhile, David A. Schauer, ScD, from the National Council on Radiation Protection and Measurements (NCRP) of Bethesda, Md., suggested that extrapolating very small individual risks to large populations distorts risk.
Both authors agreed that millimeter wave technology provides a viable alternative to backscatter scanners because it entirely eliminates radiation at the same cost and functionality as x-ray scanning.
In December 2009, in response to the “Underwear Bomber,” the TSA bolstered its advanced imaging technology (AIT) program and recommended the use of AIT as a primary screening method at U.S. airports. The new policy could translate into one billion whole-body scans annually.
“The effective doses [associated with backscatter scanners] are extremely low, of the order of 1 uSv (microsievert),” offered Brenner. The 1 uSv figure is based on a mathematical formula to estimate dose rather than tests of actual backscatter x-ray systems, according to a spokesperson from the Radiological Society of North America (RSNA). Manufacturers of the scanners claim that the equipment meets the lower 0.1 uSv threshold established by NCRP.
However, recent TSA maintenance checks indicating potential discrepancies may mean that doses are higher than the 0.1 uSv threshold.
Risk is ambiguous as well. Brenner acknowledged, “We do not know with any certainty the magnitude of individual cancer risks associated with such low doses.”
Nevertheless, Brenner relied on a standard cancer mortality risk of 5 percent per sievert to estimate a 10-7 lifetime cancer mortality for a trip involving two 1 uSv security screens, while recognizing the major uncertainties associated with such low level exposure. Brenner referred to arguments that individual risk at extremely low doses is zero. Also, he pointed to indicators that “suggest that low-dose radiation risks could be higher than those anticipated on the basis of extrapolating risks estimated at higher doses.”
According to Brenner’s extrapolation, one billion annual scans at individual cancer risk per scan of 10-7 could result in 100 cancers. A “risk of 10-7 multiplied by 109 exposures no longer represents a trivial population risk,” he wrote.
In a second Radiology article on the topic, Schauer referred to the primary objectives of radiation protection: justification, optimization and limitation, while advocating for strict regulatory control of backscatter scanners to ensure use consistent with these principles.
The NCRP, he noted, recommends an administrative control of 0.25 mSv effective dose per year be employed for individuals undergoing security screening. The council also stated that general use backscatter systems adhere to an effective dose of 0.1 uSv or less per scan. About the exposure, he suggested that “such systems can be used without regard to the number of scans per individual in a year.”
As TSA re-checks equipment and Congress mulls health concerns and other issues related to backscatter x-ray security scanning, Schauer concluded. “The time is right for countries like the United States to take a comprehensive look at the use of x-rays for medical and nonmedical imaging applications to ensure their use is consistent with the goals and objectives of radiation protection.”