Plutonium Biokinetics in Human Body A. Luciani - Kit-Bibliothek - FZK

1.4.2 COMMENTS ON THE ICRP 67 MODEL
The ICRP 67 model was designed mainly for dosimetry purposes. It allows to
calculate dose coefficients considering explicitly the dose to the urinary system and bladder as
suggested in the 1990 recommendations of the ICRP [26]. Activities in urine and feces can be
also estimated and directly compared with the measurements of bioassay samples, normally
collected after any Plutonium exposure. However, the modelling of Plutonium excretion,
particularly in urine, seems to be plagued by some shortcomings. This may be suspected from
the assumed transfer of activity from one of the soft tissue compartment (ST1) directly to the
urinary bladder. This pathway was introduced in order to account for “...an apparent increase
with time in fractional clearance of circulating plutonium...”, as explicitly stated in the
publication where the model was presented [74]. Some considerations are given here in
relation to this assumption.
The assumed transfer of activity from the soft tissue compartment to the urinary
bladder content allows one to obtain good fits to empirical data, particularly at long times, for
which it was expressively introduced, but lacks justification by any known physiological
process. The same ICRP, commenting this assumption doesn’t provide an idea of a
physiological phenomenon that could justify this hypothesis. As the soft tissue compartment
ST1 represents the retention of Plutonium in solid soft tissues (as muscle, fat, etc.), it is
difficult to imagine a physiological process where a transfer of materials from such tissues to
the urine occurs by-passing the blood.
Furthermore it is important to underline that the aforementioned assumption is not a
marginal correction of the model, but plays a significant role in its ability to predict the
urinary excretion. This is well pointed out in Figure 1.4.2.
Fractional activity per unit time [d -1 ]
1E-2
1E-3
1E-4
1E-5
1E-6
from blood and urinary path to urinary
bladder content
from ST1
to urinary bladder
content
1 10 100 1000 10000 100000
Days after injection [d]
Figure 1.4.2 Daily transfer of Plutonium activity to the urinary bladder from the soft tissue
compartment ST1 and from blood (directly and through the urinary path
compartment).
Here an acute systemic contamination (injection) due to an injection of Plutonium was
assumed and the Plutonium transferred per unit time from blood, directly and through the
urinary path, and from the ST1 soft tissue compartment to the urinary bladder was calculated.
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