Occupational Intakes of Radionuclides Part 1 - ICRP

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DRAFT REPORT FOR CONSULTATION
and the following ‘bone-seeking’ elements: calcium, strontium, barium, lead, radium,
thorium, uranium, neptunium, plutonium, americium, and curium. The model
structures for these elements and the structure for iodine, carried over from
Publication 30, depict feedback of material from organs to blood and, where feasible,
physiological processes that determine the biokinetics of radionuclides. Examples of
such physiological processes are bone remodelling, which results in removal of
plutonium or americium from bone surface, and phagocytosis of aging erythrocytes by
reticuloendothelial cells, which results in transfer of iron from blood to iron storage
sites.
(214) The physiologically based modelling scheme applied in the Publication 72
series is illustrated in Figure 18, which shows the generic model structure used for the
actinide elements thorium, neptunium, plutonium, americium and curium. The
systemic tissues and fluids are divided into five main components: blood, skeleton,
liver, kidneys, and other soft tissues. Blood is treated as a uniformly mixed pool. Each
of the other main components is further divided into a minimal number of
compartments needed to model the available biokinetic data on these five elements or,
more generally, ‘bone-surface-seeking’ elements. The liver is divided into
compartments representing short- and long-term retention. Activity entering the liver
is assigned to the short-term compartment (Liver 1), from which it may transfer back
to blood, to the intestines via biliary secretion, or to the long-term compartment from
which activity slowly returns to blood. The kidneys are divided into two
compartments, one that loses activity to urine over a period of hours or days (Urinary
path) and another that slowly returns activity to blood (other kidney tissue). The
remaining soft tissue other than bone marrow is divided into compartments ST0, ST1,
and ST2 representing rapid, intermediate, and slow return of activity to blood,
respectively. ST0 is used to account for a rapid build-up of activity in soft tissues and
rapid feedback to blood after acute input of activity to blood and is regarded as part of
the activity circulating in body fluids. The skeleton is divided into cortical and
trabecular fractions, and each of these fractions is subdivided into bone surface, bone
volume, and bone marrow. Activity entering the skeleton is assigned to bone surface,
from which it is transferred gradually to bone marrow and bone volume by bone
remodelling processes. Activity in bone volume is transferred gradually to bone
marrow by bone remodelling. Activity is lost from bone marrow to blood over a
period of months and is subsequently redistributed in the same pattern as the original
input to blood. The rates of transfer from cortical and trabecular bone compartments
to all destinations are functions of the turnover rate of cortical and trabecular bone,
assumed to be 3% and 18% per year, respectively. Other parameter values in the
model are element-specific.
(215) A variation of the model structure shown in Figure 18 was applied in the
Publication 72 series to calcium, strontium, barium, radium, lead and uranium (Figure
19). These elements behave differently from the bone-surface seekers addressed
above in that they diffuse throughout bone volume within hours or days after
depositing in bone. After reaching bone volume, these elements may migrate back to
plasma (via bone surface in the model) or they may become fixed in bone volume and
are then gradually removed to blood at the rate of bone remodelling. The
compartments in Figure 18 representing bone-marrow and gonads are omitted from
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