No subject

is some misunderstanding about how the backscatter x-ray body scanners
work and the doses that result from being screened by these systems.
One of the most common misconceptions is that these systems operate at
an unusually low kVp; they do not.  The x-ray energies for these systems
are within the normal medical diagnostic imaging range.  Therefore, the
distribution of doses to the skin and other organs is not significantly
different than for normal medical diagnostic imaging. The second issue
is that of skin dose. The equivalent dose to the skin is not abnormally
large.  A more detailed discussion is below.

In Dr. Raabe's post of 24 November, he calculated the skin equivalent
dose as being about 25 uSv (2.5 mrem) by dividing the effective dose of
0.25 uSv (25 urem) by the tissue weighting factor for the skin (0.01).
This calculation assumes that the skin is the only organ that receives a
dose.  This is not the case.

The doses reported in the FDA response to the UCSF concerns are
determined from an evaluation of the Secure 1000 system which can be
found at http://www.tsa.gov/assets/pdf/nist_rapiscan_secure_1000.pdf.
In appendix B of the report PCXMC (a Monte Carlo code used to calculate
the dose distribution from diagnostic x-ray procedures) was used to
calculate the organ absorbed doses and the associated effective dose for
an entrance skin exposure of 1 mR. As can be seen from the modeling
results for an adult, the organ absorbed doses range from 0.000199 to
0.06834 mGy.  Then, the effective dose calculated by summing the
products of the organ absorbed doses, radiation weighting factor (1),
and the associated tissue weighting factors (from ICRP-60).

The values reported in the FDA response (available here:
http://www.fda.gov/Radiation-EmittingProducts/RadiationEmittingProductsa
ndProcedures/SecuritySystems/ucm231857.htm) were derived as follows:
the organ absorbed dose for the skin from the PCXMC simulation in the
report is 0.005543 mGy (0.005543 mSv, equivalent dose) and the effective
dose is 0.002463 mSv (note that the appendix gives everything in mGy,
but effective dose is actually in mSv).  Therefore, the ratio of skin
absorbed dose to effective dose is approximately 2.25 uGy/uSv.

As per ANSI N43.17-2009, the maximum effective dose permitted for a
general-use system is 0.25 uSv (25 urem) per screening.  Therefore, the
estimated maximum skin dose is 0.25 uSv * 2.25 uGy/uSv =3D 0.56 uGy.
Organ absorbed doses for other tissues can be estimated in the same way.
The FDA further assumed a radiation weighting factor of one for 50 kVp
x-rays and reported the skin equivalent dose as 0.56 uSv.  Therefore,
the values in the FDA response should be interpreted as 0.56 uSv (56
urem) equivalent dose to the skin with an effective dose of 0.25 uSv (25
urem) with the understanding that there are doses to other organs that
are not presented in the FDA response but are part of the calculation of
effective dose.

The manufacturer of the systems being used by TSA states that the
effective dose is less than 0.05 uSv (5 urem).  Following the
methodology above (0.05 uSv * 2.25), the estimated skin dose would be
approximately 0.11 uSv (11 urem).

Again, the key point is that these systems do not deliver abnormally
large doses to the skin.  The dose distribution is well understood from
work that has been done in medical imaging.  The reasoning in the
message posted to Radsafe on 22 November by Mike Brennan is absolute
correct in that "the big difference is where we put the detectors, and
how much energy is needed to get a usable number of photons to the
detector."  By using large detectors, these systems are able to produce
a clear image with very, very small doses.

For those that are interested, the FDA, Center for Devices and
Radiological Health website provide a considerable about of information
and links to several technical reports including the one cited above.
For those that need to evaluate these systems or answer specific
questions, I would highly recommend becoming familiar with the technical
information provided on the FDA web site
(http://www.fda.gov/Radiation-EmittingProducts/RadiationEmittingProducts
andProcedures/SecuritySystems/ucm227201.htm) as well as ANSI N43.17-2009
standard, ISCORS Technical Report 2008-1, Guidance for Security
Screening of Humans Utilizing Ionizing Radiation (GSSHUIR) Report,
available at http://www.iscors.org/doc/GSSHUIR%20July%202008.pdf, and
NCRP Commentary No. 16.

*************************************************************
The statements and opinions expressed herein are not necessarily those
of the US Army, Department of Defense, or the US Government.
=20
Gerald A. Falo, Ph.D., CHP
US Army Public Health Command (Provisional)
jerry.falo at us.army.mil
410-436-4852
DSN: 584-4852

Classification:  UNCLASSIFIED=20
Caveats: NONE