Re: Embryo/fetal doses

["Mike Bohan" <mike.bohan@yale.edu>]

        Reply to:   RE>>Embryo/fetal doses

Andy and the rest of you RADSAFERs,

Were your dose estimates, skin entrance dose, or were they corrected for depth
dose?  

For anyone who has to do these analyses, you must remember the tissue 
attenuation, which is quite substantial at diagnostic x-ray energies, to
correctly estimate fetal doses.  To use skin entrance dose, based on TLD or in
air measurements, will significantly over-estimate the fetal dose which could
easily be misinterpreted.

For example, according to FDA Publication 89-8031, "Handbook of Selected Tissue
Doses for Projections Common in Diagnostic Radiology",  even a worst case direct
AP abdominal x-ray with a HVL of 3.0 mmAl, the resultant uterine(fetal) dose
would be only 34.5% of the skin entrance exposure.  For off axis projections the
fetal dose would be even a small fraction of this.  A PA abdominal  projection,
(the normal fluoro arrangement) the correction factor is 17.3% of the entrance
exposure.

The current recommendations of the American College of Radiology is that for
fetal doses below 0.1 Gy (10 Rad), at any stage of pregnancy, the evidence of
risk is small.  In NCRP report No 54, there is discussion of the 10 Rad level,
but in the summary of conclusions and recommendations they state " For doses of
less than 5 Rad received at the period critical for the induction of any one
specific type of mal-development, it is unlikely that an increase in this type
of malformation could be measurable in human populations".  Report 54 also
includes a method to estimate fetal doses in Dx radiology.

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Michael J. Bohan, RSO   |  e-mail: mike.bohan@yale.edu
Yale-New Haven Hospital |    Tele: (203) 785-2950
Radiological Physics    |     FAX: (203) 737-4252
20 York St. - WWW 204   |    As usual, everything I say may be plausibly
New Haven, CT    06504  |    denied at my employer's convenience ...
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