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

The comparison of model’s predictions with the available measurements points out
that the experimental data for the urinary excretion of Plutonium are fitted quite well, up 1000
days post intake, by the central curve representing the mean Plutonium urinary excretion for a
population of subjects characterized by different Plutonium biokinetics. At intermediate time
(about 2,000 days post intake) the measurements are characterized by an enhancement of the
excretion that is at the limit of the confidence interval for the 16 th and 84 th percentiles. Finally,
at 6,000 days post intake, the measurements for the urinary excretion of Plutonium are on
average out of the confidence interval for the 16 th and 84 th percentiles and at the limits of the
confidence interval for the 1 st and 99 th percentiles.
This seems to point out that it can be still theoretically possible to find a combination
of transfer rates, in the range of the values representing possible inter-subject differences, that
can describe the observed trend of Plutonium urinary excretion measurements observed for
the subject here analyzed.
3.2.5.2 The research of the best fitting individual parameters
The search of which transfer rates and the evaluation of those values that could explain
the Plutonium urinary excretion of the investigated subject was performed by complying with
a main constraint: only transfer rates values in the range that turned out to be representative
for inter-subject biokinetic differences were considered. In fact the assumption of a
particularly great, or small, value for certain specific transfer rates could easily generate
almost whatever desired trend for the activity distribution of Plutonium in the compartments
of the model. However if the assumed value of the transfer rate is out of the realistic range of
values expected considering the possible differences in the individual biokinetics, it should be
concluded that the model can not describe the radionuclide metabolism in this specific
subject. Therefore extensive modification of the model should be considered.
For the purpose of the present paragraph the results of the sensitivity analysis for
Plutonium biokinetics performed in the paragraph 3.1.5.3 were extensively applied.
The transfer rates determining Plutonium urinary excretion specifically at long time
with a scarce importance for the excretion at shorter time were firstly considered because they
are the best candidates for simulating a long-term enhancement of Plutonium urinary
excretion. On the basis of the sensitivity analysis performed for the systemic and the
respiratory tract transfer rates, type S, (Figure 3.1.22 and Figure 3.1.26), the leading
parameters are:
• for the systemic model: The transfer from the Liver 2 compartment; the transfer from
blood to the ST2 soft tissue compartment; the cortical remodelling rate;
• for the respiratory tract model: The clearance from the alveolar-interstitial compartments.
These transfer rates were modified on the basis of the results of the sensitivity analysis
in order to generate always an enhancement of the urinary excretion. Therefore the transfer
rates for which an increase of the value causes an enhancement of Plutonium excretion (i.e.
with a positive sensitivity coefficient S i) were increased, whereas those for which an increase
of the value causes a diminution of Plutonium excretion (i.e. with a negative sensitivity
coefficient S i) were decreased.
Quantitatively the modification of the value of such transfer rates was carried out
respecting the constraint presented at the beginning of the paragraph. The systemic transfer
rates were increased or decreased by multiplying or dividing them by a factor 3. As the
uncertainty analysis showed that the variation range of the systemic transfer rates can be
realistically described by a lognormal distribution with a GSD = 1.75, the variation by a factor
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