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

functions, defined as retained and excreted activity per unit activity of intake, respectively, are
calculated. On the basis of the measurements the intake can be assessed. In case of
environmental monitoring, such as surface or air contamination measurements, assumptions
on resuspension factor and breathing habits of exposed subject are made to infer a
hypothetical intake. The intake assessment is strictly related to the scenario of contamination
such as time of intake, compound, path of intake, etc.
In the second step a dose coefficient, commonly taken as committed effective dose
(see Annex) per unit activity of intake, can be derived on the basis of the biokinetic model.
The same biokinetic model adopted in the phase of intake estimation, must be adopted also in
calculating dose coefficients.
Multiplying the estimated intake by the calculated dose coefficient, the dose is finally
obtained.
Biokinetic studies have been carried out since many years to describe the timedependent
distribution, both after the introduction of a radioactive compound via a specific
pathway, and after the direct uptake into the blood. For this purpose models for the presystemic
and systemic phases of the contamination were developed, respectively. In internal
dosimetry the relevant organs and/or tissues are represented by distinct compartments, and
generally it is assumed that the radionuclide is transported from one compartment to another
at a rate which is proportional to the amount present in the feeding compartment. This type of
behaviour is named first order kinetics [56]. Compartmental models are a general tool for
modelling the behaviour of material obeying first order kinetics, whether radioactive or stable.
For the specific case of radioactive materials, processes of biological transport are
conveniently separated from radioactive decay. Thus biokinetic models normally treat
materials as stable and consequently the transfer rates specified in the model represent only
the biological transport. The effect of radioactive decay is taken into account separately by
considering for each compartment an exit pathway with rate equal to the physical decay
constant.
ICRP has developed models for the pre-systemic phase of the contamination to
represent the behaviour of radionuclides that have entered the human body via inhalation and
ingestion, and also systemic models for the metabolism of the radionuclides after the uptake
into the blood.
The ICRP model for the respiratory tract [57] distinguishes five regions:
• Extrathoracic airways, divided in anterior nasal passage (ET1) and posterior nasal-oral
passage (ET2), including also pharynx and larynx.
• Thoracic airways divided in bronchial (BB), bronchiolar (bb) and alveolar-interstitial (AI)
regions.
Lymphatic tissues (LNET and LNTH) are associated with extrathoracic and thoracic airways,
respectively. Deposition in each region of the respiratory tract is determined by the aerosol
particle size and breathing parameters. The clearance from the respiratory tract is treated as
two competing processes: Particle transport (by mucociliary clearance and translocation to
lymph nodes) and absorption to blood. Particle transport is a function of the deposition site in
the respiratory tract and of particle size. The mechanical transport being time dependent, most
regions are subdivided into several compartments with different clearance rates. Absorption to
the blood is treated as a function of the deposition site in the respiratory tract and of the
physicochemical form of the radionuclide. Specific dissolution rates are normally
recommended. But if no specific information is available, default absorption parameters are
given for three dissolution types, namely type F (fast), M (moderate) and S (slow). In case of
the inhalation of vapours, the respiratory tract deposition is material specific. Values are given
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