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

Figure 3.1.4 shows that the use of time dependent bone turnover rates does not remedy the
underestimation of the ICRP67-a model, because practically no significant lifting of the
urinary excretion curve occurs. At around 10.000 days post injection the Plutonium urinary
excretion is increased by about 10% relative to the value of ICRP67-a with constant bone
rates, but it is still smaller by a factor of three than the experimental value at this time (0.0014
%/d on average).
The ICRP 67 model predictions for Plutonium fecal excretion and blood retention
were also compared to the reference data set (Figure 3.1.6 and Figure 3.1.7). It can be pointed
out that the ICRP 67 model overestimates the fecal excretion up to a factor 2 during the first
week after the injection, whereas underestimates up to a factor 5 in the following twenty days.
The general trend of model predictions in this time range seems to be in disagreement with
experimental outcomes because the reference data shows a more gradually decrease of
Plutonium activity in feces. A sufficient agreement with the reference data exists only at
longer term: At around 10,000 days post injection model and experimental data differ by less
than 15%. For the Plutonium activity in blood the situation is similar with an initial
overestimation in the first week post injection, followed by an underestimation in the
following twenty days. However in this case the ICRP 67 model underestimates the reference
data up to about a factor of 20.
The effect of the deletion of the connection between the ST1 soft tissue compartment
and urinary bladder on fecal excretion and blood retention was also investigated. Because of
such modification the activity in the ST1 compartment is completely transferred to the blood
compartment. Therefore an increase of activity in the blood compartment and consequently in
the excreted activity in feces is expected. This can be seen in Figure 3.1.6 and Figure 3.1.7
(ICRP67-a curves). For the fecal excretion the effect is significant only at very long times (at
10,000 days post injection the excreted activity increases by about 30%). No effect is
observable at shorter times when the model predictions and reference data are in total in
disagreement. In case of the blood retention of Plutonium the activity increases at shorter
times: Already at around 30 days it is increased by 50%. However the predictions of blood
retention are still very far from the reference data.
Analogously to the urinary excretion analysis, the effect of age dependent bone
remodelling rates according to ICRP 70 is also shown in Figure 3.1.6 and Figure 3.1.7
(ICRP67-b curves). As in the case of urinary excretion, the effect of such modification on
fecal excretion and blood retention of Plutonium is quite small and, in any case, doesn’t
improve significantly the agreement with the reference data set.
On the basis of the previous considerations it can be summarized that:
• The Plutonium urinary excretion predicted by ICRP67 shows some disagreements with
the reference data set; the fecal excretion and blood retention of Plutonium predicted by
this model shows even more severe disagreement;
• If a physiologically based modelling is assumed by deleting the transfer of activity from
ST1 soft tissue to urinary bladder content compartment, the ICRP 67 predictions for the
Plutonium urinary excretion are significantly far from the reference data. This points out
that the ICRP 67 assumption of such transfer of activity is not a minor correction of the
model but essentially determines the urinary excretion of Plutonium at long times;
• Age dependent bone remodelling rates as suggested by ICRP in Publication 70 were also
considered but they don’t result in a significant improvement.
Therefore a general overhaul of the ICRP 67 model seems to be necessary.
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