Re: Back-ground Determination for Thyroi

[stephane_jeanfrancois@Merck.Com (Stephane Jeanfrancois)]

How about thigh measurments for background ? I know that some hospital are 
doing just that .

Stephane Jean Francois Phys. Eng.
RSO Merck Frosst Canada Inc.
stephane_jeanfrancois@merck.com
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From: William G. Nabor
To: jeanfran; Multiple recipients of list
Subject: Re: Back-ground Determination for Thyroid Bioassay
Date:  September 30, 1996 14:19


Our thyroid monitoring system at UCI consists of a Ludlum Model 2200 SCA
with a model 464 computer interface and a model 44-17 2"x2mm NaI low-energy
gamma scintillator probe.  We count a real unexposed person each day in
which a thyroid count is performed.  We do so for two reasons:
     1)  The probe/SCA can detect K-40 gammas (assumed - may be some other
naturally occuring isotope in the body).  Attempts to tweek the SCA window
to screen out these gammas leads to an unacceptable decrease in I-125
sensitivity (we currently have no work with other iodine isotopes).
Therefore, using a plastic phantom for background leads to a "background"
that is siginficantly (by a factor of up to 2) too low.  Even worse for a
bare room count.
     2)  We have found that the natural background can vary by a factor of
up to 3 due to various unknown (cosmic?) factors.  This variance changes
during the day, but on the order of hours, so our background count is done
immediately before (if we know beforehand when a count is to be made) or
immediately after (for unannounced drop-ins) a subject's count.

Since almost all of our people are uncontaminated (may it always be so!), we
are very concerned about the background.  The computer calculates the MDA
based on the count and the background, and the subject is informed if s/he
is below the MDA, or is given the actual calculated uCi amount if above this
figure.  Therefore, the background must be as exact as we can afford to make
it.  Both subject and background person are counted for 10 minutes, this
being the maximum time we can reasonably expect a subject to sit still for.


"Sue M. Dupre" <dupre@phoenix.Princeton.EDU> writes:

>Although we always do a room background measurement prior to any thyroid
>count, it is our procedure, for MDA determination and dose calculation, to
>use a thyroid background based on a recent sampling of counts of unexposed
>thyroids.  We periodically revise this thyroid background, because our
>thyroid counter shows a slow increase in detector background over
>time.
>
**********************************************************************
William G. Nabor
University of California, Irvine
EH&S Office
Irvine, CA,  92697-2725
WGNABOR@UCI.EDU
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