Even though individual cancer risk from airport x-ray machines is probably small, the large number of Americans passing through these backscatter x-ray devices every year could result in a slightly increased population risk, one researcher says.
It's possible that there will be a billion passenger scans in the U.S. alone this year, said David Brenner, PhD, DSc, of the Center for Radiological Research at Columbia University Medical Center, in New York.
And that could raise concerns "about the long-term consequences of an extremely large number of people all being exposed to a small radiation-induced cancer risk."
That's especially relevant in light of the fact that there are "current practical alternatives that do not involve ionizing radiation" -- such as millimeter wave scanners, which rely on radiofrequency to create an image, he said.
Brenner and David Schauer, ScD, CHP, executive director of the National Council on Radiation Protection and Measurements (NCRP), offered their views on the subject in Radiology.
Schauer pointed out the uncertainty of trying to estimate such a population risk, which Brenner acknowledged, saying that there are only statistical models to figure out what the risks may be, and that it may be hard to get an accurate readout.
For example, the machines give an effective dose of about 1 microSievert, but research has only shown estimated risks for doses of 5 milliSieverts and up -- which is already orders of magnitude greater than scanner doses.
"Critics who have expressed concerns about a population risk from use of security screening systems appear to be relying solely on the use of the linear-nonthreshold dose-response model for ionizing radiation, which hypothesizes that any increase in dose, no matter how small, results in a small increase in cancer risk," Schauer wrote.
"Strict reliance on the linear-nonthreshold hypothesis without regard to realistic radiation protection goals would unnecessarily hinder the beneficial uses uses of ionizing radiation that have been justified and optimized," he added.
Schauer cautioned that people should only be exposed to ionizing radiation screening when a threat exists that can be detected, and when appropriate actions can be taken.
In his modeling, Brenner estimated that there could be 100 extra cancers per year in a population exposed to a billion scans.
Still, individual risk is likely very small, thus making the scans "safe," Brenner said, although frequent fliers and those who are more sensitive to radiation -- children, for instance -- may be at greater risk.
A pilot or flight attendant who gets, on average, 240 to 380 scans per year, gets an effective dose of 300 microsieverts per year, while a frequent flyer may get 200 microSieverts. These are small, but much higher than the 6 microSieverts that more sporadic passengers will get.
Brenner said the principles of ALARA (as low as reasonably achievable) require making every reasonable effort to minimize ionizing radiation exposure in each individual situation. Therefore, comparisons with other risks aren't necessarily relevant -- for instance, the fact that flying involves other radiation exposures, which certain groups have often cited in arguing in favor of the x-ray machines.
"One would not ignore the radiation exposures associated with computed tomographic scans simply because domestic radon exposure involves larger effective doses," he wrote.
He said non-ionizing alternatives should be used, and the millimeter-wave technology is of comparable specificity, sensitivity, cost, and speed.
"Whatever the actual radiation risks associated with x-ray backscatter machines, the ALARA principle clearly implies that a comparable technology that does not involve x-rays is a preferable alternative," he wrote.
Source Reference: Brenner DJ "Are x-ray backscatter scanners safe for airport passenger screening?" Radiol 2011; 259: 6-10.
Source Reference: Schauer DA "Does security screening with backscatter x-rays do more good than harm?" Radiol 2011; 259: 12-16.