Re: Iodone Therapy Dose Calculation

["Charles Meyer" <cmeyer@brc1.tdh.state.tx.us>]

> >
> >>I am curious to see if anybody has looked into this before:
> >>
> >>For calculating the external dose to general public from iodine therapy
> >>patients, the NRC Regulatory Guide 8.39 (released April 97), makes a
> >>conservative assumption that in the first 8 hours after administration, 80%
> >>of the iodine is removed from the body only through its physical decay
> >>(page 16).  It then proceeds to an equation that includes the first 8 hour
> >>physical decay of 80% of radioiodine and the dose from the thyroidal
> >>and extra-thyroidal radioiodine uptakes (100%) from 8 hours to infinity
> >>(Equation B-5, page 16).
> >>
> >>The question that I have is what does happen to the 20% of the original
> >>administered iodine in the first 8 hours?! It does not show up in Equation
> >>B-5.  The contribution from the missing 20% does not seem to be
> >>substantial (a few tens of mrem for most patients), but it can make a
> >>difference if you are right around 500 mrem limit.
> >>
> >>Thanks in advance for your comments.
> >>    Ehsan Samei, PhD
> >>    Department of Radiology
> >>    Henry Ford Health System            313-876-2245 voice
> >>    2799 W Grand Blvd                   313-876-7925 fax
> >>    Detroit, MI 48202                   samei@rad.hfh.edu
> >
> >Dear Ehsan
> >
> >I think the NRC calculation may be a bit simplistic. It is possible to
> >carry out much more realistic modelling.
> >
> >We use an EXCEL spreadsheet package we've developed to calculate the
> >exposure of other persons from the patient , based on activity
> >administered, clearance rates, the radiation dose to other persons you are
> >willing to tolerate, and  probable patterns of close contact. Decisions
> >relating to the discharge of such patients and advice to be given (eg
> >relating to travel by public transport , contact with young children,
> >return to work etc) are based on this, and the probable exposures
> >documented.Where there is a particular concern with the larger therapeutic
> >doses, we actually measure individual patient exposures and clearance rates
> >and factor these in - this can be quite important, as there is a large
> >variation between patients.
> >
> >This package is freely available. If you are interested in a copy, let me
> >know.
> >
> >Cheers
> >
> >John Cormack
> >
> >
> >_____________________________________________________
> >John Cormack
> >Chief Hospital Scientist
> >Division of Medical Imaging
> >Flinders Medical Centre
> >Bedford Park
> >South Australia 5042
> >
> >Ph:08-8204-4642 National, 618-8204-4642 International
> >Fax:08-8204-5450 National, 618-8204-5450 International
> >
> 
The physical half life for I-131 is 8 days, f1=1, and the fractional 
uptake for the thyroid is 0.3, leaving 0.7 to the whole body with an 
effective half life of 12 days.  Go figure. 
<><><><><><><><><><><><><><><><><>            
<>    Charles R. (Russ) Meyer                  <>
<> Email: cmeyer@brc1.tdh.state.tx.us           <> 
<>     Phone:(512)834-6688                       <>
<>        Fax:(512)834-6654                        <>
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