Re: Thyroidectomy/Thyroid Burden Measurements -Forwarded -Reply

[FRAMEP@ORAU.GOV]

I asked Mike Stabin if he would care to comment on iodine dosimetry
appropriate for an individual lacking a thyroid. 

Paul Frame
framep@orau,gov

forwarded message:

Sure, what the heck.  

Actually, I have a student who is just finishing up her Master's at
Colorado State and her (non-thesis) project is on this very topic.  The
best kinetic model for iodine (MIRD Dose Estimate Report No. 5) has a
two component whole body retention curve - for a euthyroid individual
with 25% thyroid uptake, about 25% is retained in the body (the thyroid)
with a 65 day biological half-time, and the other 75% is eliminated through
the urine with about a 6 hr half-time.  So the natural thing to do for an
athyroidic individual (and what we did here for quite some time) is just to
assume that you have 100% cleared with a 6 hr biological half-time. 
Then it came to our attention, from some whole body counting data on
some athyroidic individuals,  that there is a longer term retention
component previously unaccounted for.  From the notes I have here, it
appears to comprise about 0.1% of the total body clearance, and has a
half-time of around 120 hrs, in three subjects.  When the dosimetry is
performed with and without this long term component, there is a small,
but noticeable difference in the doses to many organs (around 3% or
so).  We are currently finishing the write-up on this,  So, for the time
being, it is probably adequate to just assume that any activity entering the
bloodstream is for the most part uniformly distributed in the whole body
(there are uptakes in the stomach, liver, and intestines that should be
included) and removed with a 6 hr biological half-time.  Using these
assumptions, I come up with the following dose estimates for oral or
intravenous administration (if intakes are by inhalation, modifications
would be necessary):

                                           TOTAL DOSE    
TARGET ORGAN              mGy/MBq  rad/mCi 

1)  Adrenals                    2.28E-02 8.43E-02 
2)  Brain                          1.09E-02 4.05E-02 
3)  Breasts                      1.22E-02 4.51E-02 
4)  Gallbladder Wall         3.33E-02 1.23E-01 
5)  LLI Wall                      3.70E-02 1.37E-01 
6)  Small Intestine            2.32E-01 8.58E-01 
7)  Stomach                     3.46E-01 1.28E+00 
8)  ULI Wall                      4.24E-02 1.57E-01 
9)  Heart Wall                   1.87E-02 6.94E-02 
10) Kidneys                     2.28E-02 8.44E-02 
11) Liver                          9.61E-02 3.56E-01 
12) Lungs                        1.55E-02 5.75E-02 
13) Muscle                       1.87E-02 6.92E-02 
14) Ovaries                      4.04E-02 1.50E-01 
15) Pancreas                    3.55E-02 1.31E-01 
16) Red Marrow               1.91E-02 7.08E-02 
17) Bone Surfaces           1.79E-02 6.62E-02 
18) Skin                             1.25E-02 4.63E-02 
19) Spleen                         2.51E-02 9.28E-02 
20) Testes                         2.14E-02 7.92E-02 
21) Thymus                       1.35E-02 4.99E-02 
22) Thyroid                        1.23E-02 4.54E-02 
23) Urin Bladder Wall         8.07E-01 2.99E+00 
24) Uterus                          5.58E-02 2.06E-01 
27) Total Body                    2.37E-02 8.77E-02 
28) EFF DOSE EQUIV         1.09E-01 mSv/MBq  4.04E-01 rem/mCi
29) EFF DOSE                     1.14E-01 mSv/MBq  4.22E-01 rem/mCi

Mike Stabin
Radation Internal Dose
  Information Center
P.O. Box 117
Oak Ridge, TN 
423-576-3449  ph
423-576-8673  fax
stabinm@orau.gov