Re: I-125 dose calc formulae? -Reply

["Michael P. Grissom" <mikeg@SLAC.Stanford.EDU>]

Al's poser:

For those of us who liked to use MIRDOSE to evaluate possible
doses from I-131 to patients receiving ablation therapy (the 
big dose patients, that is, 100-200 mCi for single dose 
technique--fractionated methods were not used at our clinic), 
the issue of using an euthyroid modeling system was a real 
concern.  Of course, even with a pre-therapy dose distribution 
(5 mCi or so of I-131 for diagnostic scan) in the nucmed 
clinic in order to determine the degree of removal of tissue 
in the thyroid bed, and more importantly for evidence of 
metastatic and/or adventitious tissue, the opportunity to 
calculate the "real" residence times was virtually never there 
(not enough residents and/or graduate students).

Assumptions are, and were demonstrated clinically, that if
a minimal amount of thyroid "equivalent" tissue (tissue with
a high avidity for the I-131) was present, the I-131 dose would
clear through the patient much faster than the effective half
life of about 24 hours seen in a patient with substantial amounts
of thyroid tissue remaining, that is, 12 hours or less.  I
assumed the dose was largely whole-body from I-131 in the vascular
system and, of course, dose to the bladder.  Indeed, in the
patient without thyroid tissue, the bladder ends up the critical
organ!  Hence our instructions to void as much as possible, even to
use a catheter to a shielded collection bag if necessary for non-
ambulatory patients.

Alas, again, patient summary data and residence time estimates
are no longer at hand (the old hospital is closing and the
records have been, or will be, archived along with Indiana
Jone's lost ark!--never enough time to write all those papers...).
I believe, however, that there is an amount of data in the 
literature now.  The problem is the very large variation seen from 
patient to patient.

Ciao,

MikeG.

At 08:53 AM 2/16/96 -0600, you wrote:
>1) For the patients - they've had most of their thyroids removed already.
>They're more concerned about the dose to "everything besides the thyroid".
>It may be small potatoes for a 1 Bq intake, but how about drinking 100 mCi?
>
>2) For the nurse in question - Doing a thyroid bioassay was an exercise in
>futility, because she had had a thyroidectomy!  Aside from the problem of
>measuring uptakes, if any, how would her CEDE be calculated in such an event?
>
>Fun stuff indeed.
>
>Albert Lee Vest           The Ohio State University
-----------------------
Michael P. Grissom
mikeg@slac.stanford.edu
Phone:  (415) 926-2346
Fax:    (415) 926-3030