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The good doctor needs to review his Health Physics in some more detail. Annual dose limit for public to skin is 50mSv. He's also forgotten that beam penetration does not equate to image penetration.
http://blogs.sfweekly.com/thesnitch/...rsy_ucsf_r.php
We'll start with what the clinicians have to say. Two quotes from Fergie and Ronnie:
"The conclusions are wrong," Ronald Arenson, professor of radiology, tells SF Weekly of his own ins ution's letter. "People who are totally unrelated to radiation wrote it. ... It was senior faculty at UCSF. They're smart people and well-intended, but their conclusions, I think, were off-base. They don't understand how radiation translates to an actual dose in the human body."http://www.fda.gov/Radiation-Emittin.../ucm231857.htm"The airport scanner thing is totally bogus," says Professor Fergus Coakley, chief of the abdominal imaging section at the UCSF radiology department. When you fly, you're "closer to the sun, there's less shielding from cosmic radiation, so being worried about the scan on the way to the plane ride where you're getting extra radiation is bogus."
http://www.tek84.com/downloads/Holt-Letter2010-12-2.pdfRegarding the specific “Red Flag” issues raised in the letter:
First, the letter is correct to note that the TSA-deployed product is a recent model. However, the specification for the x-ray tube for the deployed model is almost identical to the original 1991 product. The stated concern was, “The majority of their energy is delivered to the skin and the underlying tissue.” We agree. However, the concern that “the dose to the skin may be dangerously high” is not supported. The recommended limit for annual dose to the skin for the general public is 50,000 µSv. The dose to the skin from one screening would be approximately 0.56 µSv when the effective dose for that same screening would be 0.25 µSv. Therefore the dose to skin for the example screening is at least 89,000 times lower than the annual limit.
Second, radiation safety protection quan ies are stated as ‘effective dose’. NCRP Commentary No. 16 says, “The purpose of effective dose is to place on a common scale the radiation doses: (1) from different types of ionizing radiation that have different biological effectiveness, and (2) in different organs or tissues that have different radiation sensitivities.” Comparing effective doses from different sources is appropriate. The comparison between the effective dose from cosmic ray exposure or a medical diagnostic chest x-ray and the effective dose from a security screening is intended to be a clear means of risk communication.
The third point relates to a concern “that real independent safety data do not exist.” In fact, independent safety data do exist. Independent measurements have been made on various versions of this product and all results are consistent with the dose specified by the manufacturer. Examples include:
Sandia National Laboratories, measurements made July 1991. Published as Sandia Report: Evaluation Tests of the SECURE 1000 Scanning System (1992), National Technical Information Service, DE92013773
FDA, dose measurements re-verified via computational evaluation, September 15, 1998
N43.17 working group, measurements made at Folsom State Prison on November 15, 1999
FDA & NIST, Assessment for TSA, July 21, 2006
Johns Hopkins University Applied Physics Laboratory (JHU APL), Assessment for TSA, October 2009
Fourth is the concern that “the relevant radiation quan y, the Flux [photons per unit area and time (because this is a scanning device)] has not been characterized.” We disagree that flux is the appropriate quan y. The air kerma (or skin entrance exposure) for one screening can be determined by a direct measurement of the total charge produced in the air contained in an ion chamber during one complete screening when the meter is correctly calibrated. Additionally, measurements to determine the amount of material required to reduce the intensity of the x-ray exposure by half are necessary to convert air kerma (or exposure) to effective dose. These measurements can most practically be made —and indeed have been repeatedly made— at locations where these products are installed and can be made without altering a scanner’s normal operation. These are the same sorts of measurements made to characterize the output of medical x-ray systems.
Fifth is the assertion that “if the key data (flux-integrated photons per unit values) were available, it would be straightforward to accurately model the dose being deposited in the skin and adjacent tissues using available computer codes [. . .]” In fact, we have done better. FDA and NIST used software called PCXMC to estimate the individual organ doses and to calculate effective dose. This analysis was part of an evaluation performed under contract for TSA. The input information required by the PCXMC program required considerably more information than simply the x-ray flux. Its parameters include 1) the x-ray tube anode angle, 2) anode voltage, 3) total filtration, 4) x-ray field size, 5) location of the field on the body, 6) focus-to-skin distance (FSD), and 7) entrance skin exposure. Every parameter was measured, calculated, or verified by indirect measurement. The modeling results revealed that the dose to the skin is approximately twice the effective dose.
http://www.tsa.gov/assets/pdf/jh_apl_v1.pdfI am writing to you because of continuing misinformation about the safety of these devices. This is typified by the comments made by Dr. Brenner that you reference in your recent letter to the TSA. Similar comments have been made by professors at UCSF and other universities. In particular, these professors argue that the body scanner radiation only penetrates a few millimeters into the body, resulting in the radiation dose to the skin being far higher than the average dose to the entire person. This line of reasoning has lead to a variety of inaccurate claims:
- The FDA has seriously miscalculated the radiation safety of these devices.
- The skin dose is 20 times higher than the effective dose to the entire body.
- It is inappropriate to use the techniques of medical radiography to regulate
body scanners, since the x-rays used in medical imaging penetrate deep
into the body.
- It is inappropriate to compare background radiation to body scanners, since
background radiation also penetrates deep into the body.
- The radiation from body scanners is blocked by clothing, resulting in most
of the exposure being to the skin of the face and head. This presents an
elevated risk of skin cancer.
All of these claims are incorrect, a result of misunderstanding the physics involved. In particular, Dr. Brenner and the other professors have confused Dose Penetration with Imaging Penetration, which are two completely different things. In the attached figures I show measurements taken on a body scanner to help explain this difference.
Dose Penetration is a measurement of how deeply the energy from the x-ray beam is deposited into the body. A simple way to define and measure this parameter is illustrated in Figure 1. As shown, a radiation meter is placed at the subject location within the body scanner. A certain thickness of plastic,
simulating overlying body tissue, is placed in front of the meter and the radiation measurement taken. This procedure is then repeated with other thicknesses of plastic. Figure 4 presents the result of such an experiment, which I conducted a few days ago in preparation for this letter. As shown by the solid curve drawn through the data points, placing 5 mm of plastic in front of the meter reduces the intensity of the x-ray beam from 100% to about 95%. In other words, only about 5% of the total energy of the beam is
deposited in the first 5 mm of depth into the body. Likewise, the curve drops to 50% with a total plastic thickness of about 50 mm. This means that about one-half of the body scanner radiation is deposited within 50 mm of the skin, and one-half is deposited deeper than 50 mm into the body. In comparison, Imaging Penetration describes how deep into the body the acquired image can detect objects. This depends highly on the imaging configuration, that is, where the x-ray source and detectors are placed in relation to the subject. Airport body scanners use backscatter imaging, meaning that they create an image from x-rays that reflect from the first few millimeters of the surface of the body. Only a small fraction of the x-rays that strike the body are used to form this image, with the remaining x-rays being deposited into the body. Figure 2 shows a photograph of a test object used to explain this concept.
This consists of 16 small squares of copper, placed between 16 sheets of 1.588 mm thick plastic. The upper-left copper square is on the surface of the test object. The copper squares immediately to the right are behind 1, 2, and 3 sheets of plastic respectively. On the second row the copper squares are behind 4, 5, 6 and 7 sheets of plastic, respectively, and so on.
Figure 3 shows an image of this test object taken on a body scanner. The key feature is that a copper square is less visible in the image as it is placed deeper into the phantom. The lower curve in Figure 4 shows a graph of these data. The upper-left square, having no overlying plastic, is assigned a darkness value of 100%. The upper-right square, covered by 3 sheets of plastic (4.76 mm), is 77% as dark. About 10 mm (6-7 sheets) of plastic is required to reduce the darkness of the copper square to 50%.
This is a 60 page PDF from Johns Hopkins regarding assessment of the scanners. The important synopsis begins on the bottom of page iii (last paragraph above the two bullet points) and goes to page vi
