I-125 bioassay

[ehsnet!hps (HPS)]

Reply-to: Roger.Moroney@p0.f13.n233.z1.fidonet.org (Roger Moroney)
Fido-To: hps

I have a question in regards to action levels for I-125 bioassays and 
interpretation of the results. Based on reg guide 8.9 rev 1, I am using an 
evaluation level of 0.02 ALI and an investigation level of 0.1 ALI. This 
comes out to 0.3 uCi and 1.5 uCi respectively for a quarterly period. (I 
just took the 60 uCi ALI and divided by 4). That part seemed fairly straight 
forward. Are these similar to what others are using? The next question is in 
using NUREG CR 4884 intake retention fractions (IRF) :( . There is no IRF 
listed for I-125 in the thyroid. In the text, it suggests using the IRF for 
the stable isotope and multiplying by the decay factor. Well, I looked for 
IRF for stable I-127 in the thyroid but no such luck. I then used the IRF 
for I-131 in thyroid, divided by I-131 decay factor to arrive at what should 
be stable IRF, and then multiplied by I-125 decay factor. When the IRF's are 
plotted for 40 days on a semi-log graph, I-125 lays in between the stable 
and I-131 lines and everything looks reasonable. The specific example given 
in the text used I-125. Upon closer inspection, I discovered that the phys. 
1/2 life was used and not the effective 1/2 life in calc the decay factor. 
It does make some difference as I went back and used the effective half life 
and compared the results. Which should it be? Any help on this would be 
appreciated.
 Thanks,
 Roger Moroney

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