Re: Re[2]: Patient Dose Rates

[gfeldman@uci.edu (Gerald Feldman)]

In responding to a question by Steven Rema regarding exposures from nuclear 
medicine patients, George Xu writes:
>
>You might want to get in contact with a Medical physicist at a hospital 
that has
>nuclear medicine capabilities.  Usually external dose rates from patient 
>receiving nuclear medicine studies are measured directly with radiation survey 
>meters.  I know from experience with I-131, depending upon the dose, that the 
>release criteria from the hospital was measuring the dose rate at 1 meter.  
>Radiation surveys were performed folliwing initial dosing and allowing 1 hour 
>for distribution within the body.  Example 150mCi dose of 1-131 would yield a 
>dose rate at 1 meter of approximately 100mRem on the first day; 40 mRem at 1 
>meter on the second day; and less than 6 mRem on the third day.  


-------------------
Just a couple of quick comments:

1) With respect to I-131 patients, George is correct in that one should 
allow time for distribution of the radionuclide.  The isotope should be 
given with the patient NPO for a least several hours; a full stomach will 
increase the uptake time dramatically.  We measure the dose rate from the 
patient immediately after administration, and then take 2 or 3 additional 
readings at 15 minute intervals.  Have the patient void before 
administration of the iodine so that you will have adequate time to obtain a 
"real" reading before the fraction passed to the urine is eliminated.  

2)  George's numbers seem more than a little bit off.  The gamma for I-131 
is 2.2 R/mCi-hr at 1 centimeter (or 22 mR/Hr for 100 mCi at 1 meter) (Table 
I, NCRP #37); thus, 150 mCi as a point source (eg, in the patient's stomach) 
without attenuation from the patient's body should yield 33mR/Hr at most; 
once distributed, the measured exposure rate will be less. 

3)  After distribution and uptake, the location and level of the maximum 
exposure rate will vary from patient to patient, and will have to be found 
emperically.  Factors include whether the thyroid is intact or removed in 
whole or in part by surgery, and whether or not the patient has metastases.

***************************************************************************
  Gerald Feldman, M.S.
  Radiation Safety Officer
  UCI Medical Center
  101 City Drive South, Rt. 107
  Orange, California  92668
  Voice:  (714) 456-5607;     FAX:  (714) 634-8639
  E-mail: gfeldman@uci.edu
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