Since September 2001 and, more particularly, since an incident in 2009 when a would-be terrorist smuggled plastic explosives onto an aeroplane in his underwear, security at airports worldwide has understandably become more rigorous. Whole-body scanners have become common at US airports within the last year. One of the two types of such machine in regular use, the backscatter X-ray scanner, emits small doses of X-rays similar to those in routine clinical use, and some opponents of whole-body scanners have focused on the potential increase in cancer risk due to these X-rays as a cause for concern.

The cancer risk arising from X-rays is caused by their emission of ionising radiation. All people, however, are exposed to background levels of this radiation in daily life, and most will also receive some from medical procedures. Pratik Mehta and Rebecca Smith-Bindman of the University of California, San Francisco, CA, USA, have extrapolated known dose-risk models to determine the increase in cancer risk caused by the radiation emitted by these scanners.

Backscatter X-ray machines used in airport security have been estimated to provide an ionising radiation dose of about 0.03-0.1 microsieverts (mSv) per scan, which is equivalent to about 3-9 minutes' dose of background radiation. This is clearly tiny, but it is also instructive to compare it to other common sources of radiation. Flying itself exposes people to increased levels of radiation, and the extra dose obtained from a single backscatter X-ray scan is equivalent to that from no more than a few minutes' extra flight time. It is also completely dwarfed by the dose provided by some common medical procedures: a woman would need to go through approximately 4,000 airport scans to receive the dose obtained through a single mammogram.

Mehta and Smith-Bindman qualified their calculations by pointing out that the doses of radiation emitted by scanners were much lower than those considered in obtaining the "linear no-threshold" models generally applied to radiation risk calculations. These models assume that there is a linear relationship between dose and risk and that no dose, no matter how small, which is not associated with an increase in risk. They also based their calculations on predictions of breast cancer risk, as radiation is known to accumulate in breast tissue and as the dose-risk relationship for this tumour type is well understood.

Using an accepted average figure of approximately 0.08 extra cancer cases arising from each sievert of exposure, Mehta and Smith-Bindman calculated risk separately for all flyers, frequent flyers and five-year-old frequent flyers. They estimated the risk for all flyers to be minuscule: assuming that 100 million individuals take 750 million flights per year, six extra cancer cases will arise in this population as a result of exposure to whole-body scanning. This, however, has to be taken in the context of a total of forty million cancers expected to arise in this general population. An extra four cancers were also expected to arise in one million "frequent flyers" estimated to take ten flights a week for a year. Even in a population known to be at increased risk from radiation – young girls – "frequent" flying (one round trip per week for a year) the predicted excess risk was only one case of breast cancer in two million girls.

Mehta and Smith-Bindman maintained that although they calculate the additional cancer risk from whole-body scanning to be extremely low, risk-benefit analysis still states that it should only be used if there is some benefit. Furthermore, there is a small but real risk of greatly increased radiation dose due to malfunction. The authors recommended that additional safety tests to minimise the risk of such malfunction would be useful.
 

Article: Mehta, P. and Smith-Bindman, R. (2011). Airport Full-Body Screening: What is the Risk? Arch. Intern. Med., published online ahead of print 28 March, 2011. doi:10.1001/archinternmed.2011.105