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.