Ionizing radiation from backscatter x-ray scanners deployed at airports around the United States pose little cancer risk. In fact, the flight itself poses a greater radiation risk, according to a new analysis.
"Based on what is known about the scanners, passengers should not fear going through the scans for health reasons, as the risks are truly trivial," wrote Pratik Mehta of the department of public health at the University of California, Berkeley, and Dr. Rebecca Smith-Bindman, who is the director of the radiology outcomes research lab at the University of California, San Francisco. "If individuals feel vulnerable and are worried about the scans, they might reconsider flying altogether since most of the small but real radiation risk they will receive will come from the flight and not from the exceedingly small exposures from scans."
The researchers estimated the cancer risk associated with exposure to radiation from a backscatter x-ray scan for a very large number of travelers, for a smaller group of more frequent travelers, and for 5-year-old girls – because children are more vulnerable to the effects of radiation exposure. The article was published online March 28 (Arch. Intern. Med. 2011 [doi:10.1001/archinternmed.2011.105]) and will appear in the July 25 issue.
The researchers estimated that there would be an additional six cancers over the lifetime of 100 million passengers taking 750 million flights per year. However, "these cancers need to be considered in the context of the 40 million cancers that would develop in these individuals over the course of their lifetimes due to the high underlying cancer risk."
The researchers assumed that among 1 million frequent flyers, who take 10 trips per week for a year for 6 hours each trip, there would be an additional four cancers from the backscatter scans. However, these cancers "need to be considered in the context of the 600 cancers that could occur from the radiation received from flying at high elevations and in context of the 400,000 cancers that would occur in these 1 million individuals over the course of their lifetimes," they wrote.
The researchers used a breast dose of 0.049 mcSv/backscatter scan and an increased risk of breast cancer of 9,140 cases/100,000 5-year-old girls exposed to a sievert of radiation. "We estimate that for every 2 million girls who travel one round trip per week, one additional breast cancer would occur from these scans over their lifetime. This increase of one cancer per 2 million young girls needs to be put into the context of the 250,000 breast cancers that will occur in these girls over the course of their lifetimes owing to the 12% lifetime incidence of breast cancer," the researchers pointed out.
Backscatter x-ray scanners expose an individual to 0.03 to 0.1 mcSv/scan, which amounts to the same dose as 3-9 minutes of exposure to naturally occurring radiation. Importantly, "naturally occurring radiation is higher at the altitudes of commercial air flights because of the greater proximity to the sun." Air travel is associated with an exposure of about 0.04 mcSv/per minute of flight time. In comparison, backscatter x-ray scans deliver radiation that is equivalent to 1-3 minutes of flight time.
Some question the usefulness of these estimates though, given the extremely small dose from a single scan. "It certainly is controversial," Dr. Richard L. Morin said in an interview. "A great number of scientists would say that while no one would disagree with the calculation itself, the problem is with the model." Dr. Morin is the chairman of the American College of Radiology’s safety committee and a professor of radiology at the Mayo Clinic in Jacksonville, Fla.
"There’s really no significant proof of an effect when you’re down at this level. ... In general, most scientific bodies would not make risk estimates of numbers this small because there’s lack of evidence," he said.
Still, the issue is concerning to the public because the Transportation Security Administration has stationed 486 scanners in 78 airports in the United States, according to Mr. Mehta and Dr. Smith-Bindman. Two types of scanners are in use at airports: Millimeter-wave scanners emit very low energy waves, while backscatter scanners use very low-dose x-rays. These machines are more commonly used in the United States.
"In contrast to x-rays used for medical imaging in which variation in transmission of x-rays through the body is used to generate an image, backscatter scanners detect radiation that reflects off of the person imaged ... with the backscatter technology, all of the energy of the scan is absorbed by the most superficial tissues of the body, such as the skin," the authors noted.