Otto G. Raabe ograabe at ucdavis.edu
Fri Dec 3 10:06:59 CST 2010


At 06:16 AM 12/3/2010, Falo, Gerald A Dr CIV USA MEDCOM PHC wrote:

> From posts to Radsafe and discussions elsewhere, it appears that there
>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:
>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.........

Prof. Otto G. Raabe, Ph.D., CHP
Center for Health & the Environment
University of California
One Shields Avenue
Davis, CA 95616
E-Mail: ograabe at ucdavis.edu
Phone: (530) 752-7754   FAX: (530) 758-6140

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