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.

All Flyers

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."

Frequent Flyers

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.

Five-Year-Old Girls

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.

The safety of backscatter x-ray machines has been questioned because these scanners use ionizing radiation, which can cause damage to the body. The potential damage depends on the dose; at low doses, radiation causes biological damage that cells can quickly repair. At moderate doses, the cells can become cancerous or changed in ways that lead to other abnormalities such as birth defects. At even higher doses – such as those in radiation oncology – the cells cannot be replaced quickly enough, and serious health problems can occur.

While the doses of ionizing radiation emitted by backscatter machines are exceedingly low, it’s not clear whether the machines pose a health risk. The risk of cancer is an important consideration given that 750 million passengers board airplanes each year. "Even a small risk per person could potentially translate into a significant number of cancers," Mr. Mehta and Dr. Smith-Bindman wrote.

Estimating the cancer risk associated with backscatter x-ray machines is difficult. Estimations of this risk must be based on extrapolation of data from published studies that have demonstrated a cancer-radiation association but were based on much greater levels of radiation exposure. It is usually assumed that cancer risk is directly proportional to dose (linear dose-risk) and that every exposure carries some risk

Another difficulty is that exposure from backscatter machines is concentrated mostly in the skin, and there currently is no model for understanding the relationship between skin exposure and risk of skin cancer. However, "the backscatter x-rays will be concentrated in breast tissue, so the breast exposure from these scans can be used to accurately predict breast cancer risk," the investigators observed.

The researchers estimated backscatter x-ray exposure for three groups and made a few assumptions to calculate the risk. They assumed that all passengers undergo a full-body scan for each trip; that 100 million unique passengers will take 750 million flights per year; and that scan exposure is 0.1 mcSv. They extrapolated the estimates from the linear dose-risk relationship model. In addition, they assumed an increase of 0.08 cancers/Sv of exposure.

Concerns about the scanners remain "in part because the TSA does not permit independent assessment of the machines," the authors noted. It would be prudent for the TSA to permit additional testing to verify the safety of the devices, they added.

"In medicine, we try to balance risks and benefits of everything we do, and thus while the risks are indeed exceedingly small, the scanners should not be deployed unless they provide benefit – improved national security and safety – and consideration of these issues is outside the scope of our expertise." However, "if the scanners are not deemed efficacious, they should not be used," they wrote.

Radiologists tackled the question of potential long-term public health threats posed by backscatter x-ray scans in the April issue of Radiology.

David J. Brenner, Ph.D., argued in one commentary article that from a public health policy perspective, we should have some concerns about the long-term consequences of an extremely large number of people, regardless of how small the individual exposure risk is (Radiology 2011 [doi:10.1148/radiol.11102347]).

"The risks for any individual going through the x-ray backscatter scanners are exceedingly small. However, if all air travelers are going to be screened this way, then we need to be concerned that some of these billion people may eventually develop cancer as a result of the radiation exposure from the x-ray scanners," Dr. Brenner said in a press release. Dr. Brenner is the director of the Center for Radiological Research at Columbia University, New York.

In a separate commentary article, David A. Schauer, Sc.D., argued that summing negligible average risks over large populations or time periods distorts the risk (Radiology 2011 [doi:10.1148/radiol.11102376]). "There is no scientific basis to support the notion that a small risk to an individual changes in any way for that individual as others around him are also exposed to the same source of radiation," he said in a press release. Dr. Schauer is the executive director of the National Council on Radiation Protection and Measurement.

Dr. Schauer does advocate for strict regulatory control of backscatter scanners in order to ensure that benefits exceed cost or harm; that exposures are kept as low as reasonably achievable; and that individual doses are limited. In fact, the NCRP has recommended that these systems not exceed an effective dose of 0.1 mcSv of ionizing radiation per scan.

Both Dr. Brenner and Dr. Schauer agree that scanners using millimeter wave technology should be considered first options for screening passengers because there is no ionizing radiation risk.

Mr. Mehta and Dr. Smith-Bindman reported that they have no financial disclosures. Dr. Morin reported that he has no relevant financial disclosures.

The NRCP had a grant from the Food and Drug Administration for this commentary by Dr. Schauer. Dr. Schauer had no other relevant financial disclosures. Dr. Brenner had no relevant financial disclosures.

Radiation From Backscatter X-Ray Scans Poses Little Cancer Risk