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

1.4.1 DESCRIPTION OF THE ICRP 67 MODEL
1.4 THE ICRP 67 MODEL FOR PLUTONIUM
The most recent and updated biokinetic model for Plutonium was presented in 1994
[74]. This model can also describe the metabolism of Americium and Neptunium with some
modifications. It was developed from a previous model [75] with two main goals:
1. To introduce specific pathways for the urinary excretion in order to evaluate the effective
dose to the urinary bladder, explicitly considered in the ICRP’s new list of tissue
weighting factors [26];
2. To implement the new information on the biokinetics of Plutonium in soft tissue that
seems to suggest a greater retention of Plutonium in soft tissues at long times after
exposure than thought previously.
The model is depicted in Figure 1.4.1. The compartments of the gastrointestinal tract
affected in case of a systemic contamination are also given. The systemic model for
Plutonium biokinetics is characterized by a mamillary compartmental structure with a
“central” compartment, the blood, connected to all the other organs and tissues: Liver, soft
tissues, skeleton, gonads, kidneys and the urinary bladder. Each organ or tissue is represented
by one or more compartments. Except for the urinary bladder, all the compartments represent
the kinetics of Plutonium, once it is absorbed in the tissues. The urinary bladder compartment
instead represents an empty organ and is used here to describe the kinetics of Plutonium in the
content of the organ, i.e. the urine.
The general structure was derived from an age-specific biokinetic model for
Americium developed in the early nineties [76]. A second liver compartment was added for
Plutonium. It was introduced mainly on kinetic rather than biological reasons in order to
represent better the long-term retention of Plutonium in the liver. From a physiological point
of view, this second liver compartment can be associated to the reticulo-endothelial system.
The activity from the blood is transferred to the first liver compartment that represents the
short-time hepatic retention. From there the activity is removed to both the gastrointestinal
tract via biliary excretion and to the second liver compartment . The second liver
compartment clears to the blood.
The kinetics of Plutonium in the soft tissues is modelled using three compartments,
ST0, ST1 and ST2. The first, a soft tissue pool including also extracellular fluids and the fast
exchange of Plutonium with blood, represents the early build-up and clearance of material in
soft tissues. The compartments ST1 and ST2 are used to describe the intermediate (up to 2
years) and long term retention (many years), respectively, in those solid soft tissues such as
muscle, skin, subcutaneous fat etc., not explicitly represented in the model.
The skeleton is represented by separate cortical and trabecular parts. In each part
Plutonium kinetics is described using three compartments: Bone surfaces, bone volume and
bone marrow. The activity first reaches the bone surface and then is subsequently transferred
to bone marrow by bone resorption and to bone volume by bone formation. The activity in the
bone volume is also transferred to bone marrow by resorption processes. From the bone
marrow the activity reaches the blood, where then it is again re-distributed among all the
organs and tissues.
The gonads compartment is used to describe the kinetics of Plutonium in testes (male
subjects) and ovaries (female subjects). The activity from blood is transferred to the gonads
with gender specific transfer rates, while it is removed from the gonads with the same rate for
male and female.
25