Re: Effective Dose Equivalent

["Charles Potter" <CAPOTTE@sandia.gov>]

     I think there are some relevant points here, so I'll throw my two 
     cents in.
     
     First of all, there is a TDE limit of 50 rem in 10CFR835.202(a)(2).  
     This involves adding the deep dose to the CDE to the particular organ 
     or tissue.  This limit is probably more relevant in the scenarios you 
     are discussing.  I see your conceptual problem with equating the 
     localized dose with a whole-body dose and don't have an explanation 
     for it.
     
     Second of all, I believe that at the low levels we are used to dealing 
     with, the dose equivalent concepts are more useful for score keeping 
     in relation to the appropriate limits that have been set, i.e., 5 rem 
     TEDE, 50 rem TDE, rather than an evaluation of actual risk.  This is 
     what keeps us (dosimetrists especially) employed.  I do believe that 
     at larger TEDE values, i.e., 10-100 rem, the dose equivalent vs. risk 
     argument is valid. (fire away!)
     
     The concept of dose equivalent definitely breaks down when large acute 
     intakes/exposures happen.  The quality factor is more useful in 
     describing the risk of long-term radiation induced effects such as 
     cancer, cataracts, etc..  For exposures on the order of hundreds of 
     rads (1 Gy or more), the more imminent risks are of acute radiation 
     sickness.  Therefore, at these levels, the equivalent dose concept is 
     not important as compared to the absorbed dose concept.
     
     Gus Potter
     Sandia National Laboratories
     CAPOTTE@sandia.gov
     (505) 844-2750
     
     The opinions expressed here are solely to incite more delightful chatter on 
     the LNT topic :)


______________________________ Reply Separator _________________________________
Subject: Effective Dose Equivalent
Author:  xat@inel.gov at hubsmtp
Date:    6/3/96 4:14 PM


 The concept of total effective dose equivalent or TEDE requires addition of
external dose to internal dose.  The internal dose is calculated by 
multiplying the organ committed dose equivalent by an appropriate weighting 
factor.  The weighting factor is chosen so that the risk of fatal cancer or 
hereditary effects or non fatal cancer hypothetically caused by the 
committed organ dose equivalent would be the same as the hypothetical risk 
from exposure of the whole body to a dose equal to that of the organ 
multiplied by the weighting factor.  However, I have a conceptual problem 
with that idea, particularly for large doses.
     
For example:  Suppose the testicles receive a dose of 40 Sv and the rest of 
the body receives zero dose (remember this is hypothetical).  The equivalent 
dose is 10 Sv (weighting factor of 0.25 in 10 CFR 835).  10 Sv will almost 
certainly result in death.  40 Sv to the testicles will result in sterility, 
but probably nothing else (at least I can't find any risk numbers for fatal 
testicular cancer as a function of dose).  So, how are the 10 Sv and the 40 
Sv equivalent in risk?
     
A similar problem appears if the testicles are exposed to 4 Sv (equivalent 
dose of 1 Sv).  4 Sv could cause sterility.  1 Sv could have a significant 
risk of cancer.  Are those two risks equivalent?  Even worse: the DOE 
Radiological Control Manual requires establishing special control levels for 
individuals whose total lifetime dose in rem exceeds their age in years 
(based on an NCRP recommendation).  Would it make sense to establish such a 
level for such a person?
     
What am I missing here?
     
Is there an upper limit of dose above which the concept of equivalent dose 
does not apply?  Maybe 50 mSv per year?