[ RadSafe ] Cumulated Activity and Dosimetry Question (UNCLASSIFIED)

[Falo, Gerald A Dr USACHPPM]

Classification:  UNCLASSIFIED 
Caveats: NONE

All,

I was reading Mike Stabin's new book "Fundamentals of Nuclear Medicine
Dosimetry," and it got me thinking about cumulated activity.  The
thinking then got me confused, which is more common than I'd like it to
be.

The cumulated activity is defined at the time integral of the activity
in a particular organ or tissue (compartment).  In this integral the
effective (biological + physical) removal rate constant is used.  This
cumulated activity is often called the number of nuclear transitions,
decays, or distintegrations.  This is where I get confused.  By using
the effective removal rate constant in the integrand, the result is the
total number of atoms removed by biological and physical process, not
just radiological decay.  I thought that only those atoms "removed" by
decay would contribute to dose.

For dosimetry, it seems that what is needed is the total number of
decays that occur in the compartment of interest, not the total removed.
So, it seems that the cumulated activity as defined above should be
multiplied by the fraction of the atoms removed that decay, which is the
ratio of the physical removal rate constant (the decay constant) to the
effective (total) removal rate constant.

Is there an underlying assumption that the physical half-life for the
radionuclides used will always be much shorter than the biological
half-life so that the effective removal rate equals the decay constant?

What am I missing here?  I'd appreciate someone pointing out where I've
gone astray.

BTW, I've enjoyed Mike Stabin's book very much so far, and what I've
skimmed ahead to looks excellent.

Thanks,
Jerry
________________________________

The statements and opinions expressed herein are my responsibility; no
one else (certainly not my employer) is responsible, but I still reserve
the right to make mistakes.
 
Don't panic! - Douglas Adams in "The Hitchhiker's Guide to the Galaxy"
 
Gerald A. Falo, Ph.D., CHP
U.S. Army Center for Health Promotion and Preventive Medicine - Health
Physics Program
jerry.falo at us.army.mil
410-436-4852
DSN: 584-4852
Classification:  UNCLASSIFIED 
Caveats: NONE