Age dependent dose factors for external rad.

[eh_carbaugh@ccmail.pnl.gov]

     To be honest I hadn't given age dependence for external any thought 
     until the question.  But I will attempt a response:
     
     The reason age dependent factors are needed for intakes of 
     radioactivity is because of the variations that occur in biokinetics 
     from birth to adulthood.  A becquerel of radioactivity deposited in an 
     infant liver can mean something very different from an adult liver (or 
     any other organ, for that matter). Because dose coefficients are 
     expressed per unit intake, that difference carries through to the 
     intake.
     
     While the biokinetics (i.e., distribution, retention time) varies with 
     the age, the tissue weighting factor does not.  Or rather, I should 
     state that to my understanding there is no variation in the tissue 
     weighting factor with age at present.  Thus the age dependency for 
     intakes draws its basis from the biokinetics.
     
     In external radiation exposures, the biokinetics is not an issue.  
     Since dose is expressed as energy deposited per tissue mass, the dose 
     is age indepedent.  (Now we'll see if I use my terms right - but I'm 
     sure someone will correct me if I don't:)  In a beam the fluence is 
     constant and a target tissue will receive a dose based on the total 
     energy absorbed in the total mass exposed.  If the mass is smaller 
     (i.e., a child, or a single tissue) the energy absorbed will also be 
     smaller for a given fluence.  However, as long as the tissues are the 
     same density and the fluences are the same the absorbed dose will be 
     the same.  Thus age dependence is not an issue.  
     
     Now, if the organ and tissue weighting factors were to change with 
     age, it would be a whole new ball game.  I suspect the reality is that 
     they probably do change a bit, but I'm unaware of any ICRP position or 
     discussion on that.  
     
        Gene Carbaugh
        Internal Dosimetry
        Pacific Northwest National Laboratory
        eh_carbaugh@pnl.gov