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RE: I-125 bioassay
In order to find the IRF for I-125 in the thyroid using NUREG 4884, you will
need to use the intake retention function that is listed for stable iodine
on page B-758, and account for the decay by using the decay constant. If you
are going to calculate alot of IRFs for different times, I would recommend
you program the equation into a programmable calculator, such as an HP 42s,
so you don't have to do as much work. The stable intake retention function
for iodine in the thyroid is:
IRF = (0.286*e^(-0.00605*t)) + (0.015*e^(-0.0602*t)) - (0.301*e^(-2.77*t)),
where t = number of days from intake.
(My email program doesn't allow me to make equations very pretty, so I
apologize for the way it looks, but hope it still gets the point accross.)
(The negative sign indicates recirculation of iodine in the body, i.e. it
leaves the thyroid, and may come back, before it is excreted.)
To get the IRF for I-125, use the above equation to get the stable IRF and
then multiply by e^(-0.0115*t) to correct for decay. (t is in days.)
I guess it would have been easier if the thyroid retention function was
calculated in NUREG 4884, but it wasn't. For that reason, you would have to
go the same route for I-131 to determine how much of an intake is in the
thyroid for thyroid counting. I hope this helps.
Lorna Bullerwell, Radiological Safety Specialist
Cornell University
Department of Environmental Health & Safety E-mail: ljb1@cornell.edu
2Laboratory Safety Section Phone: (607) 255-8816
118 Maple Avenue, Ithaca, NY 14850 Fax: (607) 255-8267
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