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RADAR - Overview of the Dose Assessment System

Internal and external dose assessment are related concepts. Both involve the assessment of absorbed doses to organs and tissues of the body to evalute possible biological effects. Note that we are using the term 'dose assessment' here, and not 'dosimetry'. The term 'dosimetry' has been used colloquially for many years, as 'internal dosimetry' was developed as a science following 'external dosimetry' during the atomic age. 'External dosimetry', meaning the measurement of doses received by people exposed to radiation, is actually 'dosimetry', in that it involves measurements (the suffix 'metry' signifying that you are measuring something). Both internal and external dose assessment involve only the theoretical calculation of doses, using models, and thus this term is somewhat more appropriate. If you want to go on calling this 'dosimetry', no one will really object, but we will use this slightly more accurate terminology.

For internal dose assessment, there are two primary systems in use now, the ICRP 30/60 system for radiation workers and the general public, and the MIRD system for nuclear medicine patients. When one studies the various documents of these two groups, there appear to be dozens of equations and terms to learn. In reality, both systems are using the same basic equation, they are just calling the various terms by different names, and grouping together various parameters for ease of use. But both systems are completely identical, so why don't we just use one system for both? OK, you convinced me, let's do that. For a more complete description of the equivalence of all existing internal dose assessment systems, see the document Demystifying Internal Dose Calculations, which was written for the Health Physics Society as part of their Professional Enrichment Program lecture series. If you don't care for that much detail, then you can probably just accept that the following equation underlies all of the stuff published in these longer documents:

HT = total dose equivalent to target region T (rem or Sv),
NS = number of disintegrations that occurred in source region S,
fi = fraction of radiations with energy Ei emitted per nuclear transition,
Ei = energy emitted as radiation i (MeV),
fi(TS) = fraction of energy emitted in S that is absorbed in T,
Qi = the quality factor (in the ICRP system 'radiation weighting factor') for radiation i,

(unofficially - rem/rad or Sv/Gy, officially - dimensionless),
mT = mass of target region (g or kg), and
k = a proportionality constant that makes the units work out, for example rad-g/mCi-hr-MeV or
Gy-kg/MBq-sec-MeV. The derivation is tedious, but simple, from first principles.

What's so simple about that, you ask? That looks horrible. In it's raw form, yes there are a lot of terms, but the concepts are pretty simple. We just have to find out where the disintegrations occurred in the body (the source regions) and add up all the contributions to the targets we are interested in, and we have dose estimates for the targets. What others have done, and what we will also do, is to simplify this equation by breaking it into two parts - one part that describes the biokinetics and one part that describes the physics - and just have an equation with two terms, not all the ones you see above. Do you like this better?

HT =  NS x DF(TS)   

Here, DF is the Dose Factor for source region S irradiating target region T, and represents just a combination of all the other terms in the first equation. This is basically what MIRD and ICRP did previously; MIRD called the DF an S value (don't ask me why) and ICRP called it an SEE (Specific Effective Energy - don't ask me why). We are going to call it a DF, OK? Just another name. But some of these other published documents also introduced dozens of other complicating terms that we don't think are needed, so we're just going to use these two terms for both systems, and then comes the part that engineers always like - we just need to plug in some numbers!

For external dose assessment, the situation is much the same, except that we probably only have one source per problem, so we can simplify the equation even further:

HT =  NS x DF(TS)   

The DFs will be different in magnitude and somewhat in form than those for internal dose assessment, but will have all of the same terms.

The Internal Dose Assessment System

The External Dose Assessment System