Re: pregnancy post I-131 NaI treatment

["1LT Philip Hypes"<1lt_philip_hypes_at_chppm8__apgea@chppm-ccmail.apgea.army.mil>]

     The intent here is to prevent the fetus from receiving more than 500 
     mrem.  There are tables that give the fetal exposure per millicurie of 
     I-131 administered to the mother.  Of course, those tables are 
     intended to model post-conception administration of the dose, in which 
     case fetal tissue can take up the I-131 that is not taken up by the 
     mother's thyroid.  In the scenario you are dealing with, the primary 
     source of I-131 is the activity released from the mother's thyroid.  I 
     believe it's safe to assume that the activity available for uptake in 
     this scenario will be less than the activity available when the mother 
     is dosed post-conception.  It's still probably worth running few a 
     quick calculation with the dose/activity tables as a conservative 
     reality check on whatever time a reference gives as an acceptable 
     waiting period.
     
     Table 10.7 of the HPRHH gives a fetal dose of 0.1776 rad/mCi for I-131 
     given to a hyperthyroidal mother.  If we let rad approximate rem in 
     this situation, that means a dose of 2.8 mCi will result in 500 mrem 
     to the fetus.  Assuming the mother will be given 150 mCi of I-131, a 
     decay calculation using the 7.6 day effective half life of I-131 (from 
     NCRP 65) indicates that approximately 44 days will have to elapse 
     before there will be less than 2.8 mCi available in the mother's body 
     to potentially expose the fetus.
     
     Considering the conservative aspects of these (rough) calculations, a 
     one month wait might be enough, but I'd be more inclined to say at 
     least two.  There are probably other health factors to be considered, 
     so the person who should actually make the final recommendation to the 
     patient is the patient's doctor.  
     
     My opinions only.
     
     Phil Hypes
     1LT_Philip_Hypes_at_chppm8__apgea@chppm-ccmail.apgea.army.mil
     


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