RE: Individual monitoring device question

["Sorensen, Scott" <ssorensen@saint-lukes.org>]

William G. Nabor Asked:

		Is there a reason for this?  So it can be seen by the
regulators?  If the
		lead apron is shielding the person wearing it, wouldn't it
be more accurate
		for the dosimeter to reflect this fact.

As pointed out by others, EDE equations allowing credit for lead apron
shielding effects (e.g., Webster equation) are only applicable for
diagnostic x-ray radiation environments.  The average energy scattered
during fluoroscopy procedure ranges between 30-50 keV.  At these levels, the
photoelectric effects dominates (large cross section) and a 0.5 mm Pb
equivalent lead apron provides effective shielding.  However, at the higher
energies associated with many radioisotopes (e.g., Cs-137, I-131), the
Compton effect becomes dominant, having a much smaller cross section.  Thus,
one would need to wear a thicker apron than the widely available 0.5 mm Pb
equivalent to achieve similar protective effects.  Since I'm not aware of
the use of thicker (and much heavier) aprons, the NRC may have some basis to
deny dose adjustments.  

The NRC rule provides a worse-case risk estimate (for comparison against
dose limits). When correctly applied, the ANSI standard would allow a more
realistic risk estimate to be made.  Anyone care to comment on the impact on
epidemiological studies by using the NRC dosimetry data? 

As an aside, radionuclides are being used more and more in areas where only
X-rays have been used in the past (e.g., brachytherapy in cardiac cath
labs).  One should be careful in taking credit via the Webster equation (and
others) during these procedures since the energies involved are much greater
(and also NRC rules apply).

-Scott Sorensen, RSO
 Saint Luke's Hospital of Kansas City


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