Re: Radiation Detection Ques

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

        Reply to:   RE>Radiation Detection Questions



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Date: 17/09/96 4:40 PM
To: Mike Bohan
From: radsafe@romulus.ehs.uiuc.edu
Hi all!!

I've got a question/problem that I hope some of you can answer/solve.

Concerning I-131 therapy patients. The initial exposure measurement is
done at 1 meter w/ an Ion Chamber.  Assume the reading is around 30 mR/h
~ 1hr post administration of 150mCi I-131 oral solution.  Is it OK to
make the same 1 meter measurement 2 days later, when the patient is
really an extended source of radiation, in order to discharge the
patient if the reading is less than 5 mR/h?

Reply----
Yes, It's not really a point source but for a practical "field" measurement the
geometrical error is not a significant problem.  In addition, after two days,
activity in the blood and non-thyroidal tissues is low and the residual activity
is located in the remaining thyroidal tissue foci. 
 
What would be the difference if a pancake GM detector were to be used
instead of an Ion Chamber for both the initial and final measurements?
Lets also assume that the patient has had a total thyroidectomy.

Reply----
It doesn't make too much difference whether you use a GM or an ion chamber as
long as you use the same instrument for all measurements.  Your really are
making relative measurements and comparing the result to the initial
measurement.  We calculate the activity/dose rate constant from the initial
measurement.  In your example, (150 mCi/30 mR/hr = 5 mCi-hr/mR), consequently
you could release the patient when the dose rate fell below 6 mR/hr (6mR/hr * 5
mCi-hr/mR = 30 mCi).  We like the dose rate to be *both* below 5 mR/hr @ 1 meter
*and* less than 30 mCi I-131 residual by calculation, for conservative "CYA"
purposes.  I prefer using an ion chamber for the measurement, however because I
don't have to worry about demonstrating energy response calibration accuracy for
my 662 keV Cs-137 calibrated GM's.  Non-energy compensated GM's, especially
pancake GM's, might have some overresponse at the 364 keV and lower energies
present in the I-131 spectrum.  This would result in higher than true
measurements.  However, if you calibrated your GM's to Tc-99m at 140 keV, your
GM might underrespond and allow you to prematurely release a patient, if you
were only using the 5 mR/hr @ 1 meter requirement as your threshold.

Some other questions.  

When should a GM be used as opposed to an Ion Chamber and vise-versa in
I-131 cases?  What would be the efficiency difference between a sliding
side-window GM and a thin-window Pancake for I-131 contamination
detection?
-- 
Eric A. Goldman, B.S.,CNMT	


"Gotta run, the cat's caught in the printer."



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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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