Screening Individuals with Backscatter X-Ray Systems

Daniel J. Strom

Installation of backscatter x-ray systems to scan passengers, visitors, and prisoners has been proposed. If hundreds of millions of people are scanned, can the population dose reach levels where a few cancers are possible?

Two ideas should be understood in order to answer this question. First, the question includes neither consideration of the benefits from introducing backscatter x-ray scanning, nor consideration of all alternatives offering the same or similar benefits. Secondly, statistical collective risk is irrelevant when individual risk is below some very low level. These two ideas are discussed below.

1. Valid comparisons require considering alternatives that provide the same or similar benefits.

In radiation protection, no exposure is justified unless it produces a positive net benefit (ICRP 1991; NCRP 1993). This philosophy is part of radiation protection regulations of all agencies in the United States and the European Union and is almost universally adopted throughout the world. In the case of screening passengers, visitors, or prisoners, the benefit is increased security and the possibility of preventing terrorist attacks. In modern industrialized countries, the decision whether the benefit justifies the risk, if there indeed is a nontrivial risk, is generally left to society or to government, not to technology companies.

An example of a decision involving trivial radiation doses is the use of smoke detectors containing small sources of radioactive americium-241 (²⁴¹Am). The benefits of saving lives from fires clearly outweigh the trivial risk from radiation exposure to those who occupy dwellings and offices with smoke detectors.

Alternative technologies offering the same benefit as backscatter x-ray scanning might be metal detectors combined with pat-down searches or strip searches. Indeed, in the California prison system, all visitors, employees, and prisoners must undergo either a backscatter x-ray scan or a strip search on entering and leaving the prisons. The strip-search alternative is not without risk to the person doing the searching, as he or she is potentially exposed to infectious disease carried by the person being searched. As of 2005, one manufacturer of a transmission x-ray system also claims to offer the same or better security benefits as backscatter x-ray security scans and strip searches.

The alternative of not scanning offers no benefit. Thus, the increase in security justifies the increase, if any, in risk.

It should be noted here that this risk is being imposed on members of the public by government for a societal benefit. For example, scanning all airline passengers with backscatter systems, and stopping a terrorist plot such as the attack on the World Trade Center, would have tremendously benefited those who were affected. Governments typically make such decisions in many public-health and national-security cases.

2. When individual risk is negligible, theoretical collective risk is not meaningful.

The kind of collective risk thinking in the question has been considered of little use by mainstream scientific bodies.¹ When individual risk falls below some level, say one in a million lifetime risk of contracting cancer, that risk is "below regulatory action" for agencies that protect public health.

The use of collective dose and collective risk has been the subject of several recent studies (Institut de Protection et de Sûreté Nucléaire 2002; ICRP 1997; NCRP 1995) and is discussed in the draft 2005 recommendations of the ICRP (2004). These discussions are in agreement that extending tiny individual doses to large populations over long time periods ignores the fact that individual risks may be trivial.

The NCRP, an organization chartered by Congress in 1964, defines a "Negligible Individual Dose" as 1 millirem (this is the same as 10 microsieverts [µSv]) per year for each source or practice (NCRP 1993).

At 0.00001 millirem (1 µSv) per scan, it would require 100,000 screening scans in one year to reach the NCRP's Negligible Individual Dose.

The U.S. Nuclear Regulatory Commission (NRC), the states, and other regulatory agencies worldwide recognize an annual limit on doses to the public of 100 millirems (1,000 µSv) per year from all sources and practices combined (excluding medical and dental practices). This limit is 100 times higher than the NCRP Negligible Individual Dose. There is general consensus that limiting doses from a single source or practice (for example, backscatter x-ray examinations of passengers, visitors, prisoners, etc.) to 25 millirems (250 µSv) per year will achieve this goal. For example, this is the limit in the U.S. NRC's Standards for Protection against Radiation for License Termination (U.S. NRC 2005).

Note that the doses from security x-ray scans are literally tens of thousands of times lower than doses to parts of the body from multislice computed tomography (CT) scans that have been in the headlines recently (Einstein et al. 2007).

Conclusions

The Health Physics Society has an official position statement on the use of ionizing radiation for security-screening purposes (http://hps.org/documents/IonRadPS.pdf). In it, the Society states that this use is justified when the individual radiation dose per scan is within the requirements of ANSI/HPS N43.17 (ANSI 2002).

We must question the potential for adverse health effects when introducing a new technology, especially those involving exposure to ionizing radiation. When placed in the context of the benefit of increased security for all, the comparison of backscatter x-ray screening with alternative technologies that provide the same improvement in security shows that the benefits far outweigh the risks. Furthermore, the risk to any individual from frequent backscatter x-ray scans is truly trivial, so that the notion of collective risk, spread out over a huge population, is not meaningful.

Reference List

American National Standards Institute. Radiation safety for personnel security screening systems using x-rays. ANSI Standard N43.173; April 2002.

Einstein AJ, Henzlova MJ, Rajagopalan S. Estimating risk of cancer associated with radiation exposure from 64-slice computed tomography coronary angiography. JAMA 298(3):317-323; 2007.

Institut de Protection et de Sûreté Nucléaire. Dose collective. EDP Sciences, Les Ulis Cedex A, France; 2002.

International Commission on Radiological Protection. 1990 recommendations of the International Commission on Radiological Protection. Oxford: Pergamon Press; ICRP Publication No. 60. Annals of the ICRP 21(1-3); 1991.

International Commission on Radiological Protection. Individual monitoring for internal exposure of workers. Replacement of ICRP Publication No. 54. Oxford: Pergamon Press; ICRP Publication No. 78. Annals of the ICRP 27(3/4); 1997.

International Commission on Radiological Protection. Draft 2005 recommendations of the International Commission on Radiological Protection. Oxford: Elsevier Science; 2004.

National Council on Radiation Protection and Measurements. Limitation of exposure to ionizing radiation. Bethesda, MD: NCRP; NCRP Report No. 116; 1993.

National Council on Radiation Protection and Measurements. Principles and application of collective dose in radiation protection. Bethesda, MD: NCRP; NCRP Report No. 121; 1995.

United States Nuclear Regulatory Commission. Standards for protection against radiation. Title 10, Code of Federal Regulations, Part 20. Washington, DC: U.S. Government Printing Office; 2005.

Footnote

¹ For example, the National Academy of Sciences-National Research Council (NAS-NRC), National Council on Radiation Protection and Measurements (NCRP), International Commission on Radiological Protection (ICRP)

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