Occupational Intakes of Radionuclides Part 1 - ICRP

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DRAFT REPORT FOR CONSULTATION
and its precursors (Leggett, 1985, 1992), the depiction of burial of activity in bone
volume is intended to approximate the net result over time of a number of known or
suspected burial processes occurring at different rates. Activity depositing in bone
remodelling units, either in the formation period or in the transitional period between
resorption and formation, may be buried relatively quickly. Delayed burial of surface
activity may result from ‘local recycling’ during bone restructuring processes; that is,
some of the surface activity removed by osteoclasts during bone remodelling may be
redeposited almost immediately at closely adjacent sites of new bone formation that
are supplied by the same blood vessels. Such local redeposition of mineral ions is
thought to occur, particularly in cortical bone (Parfitt and Kleerekoper, 1980). Burial
of surface deposits may also occur as a result of ‘bone drift’, a phenomenon in which
new bone is deposited on previously formed bone without any prior resorption
process. Bone drift occurs on a larger scale in immature bone than in mature bone, but
drift within bones and expansion of bone volume via periostial-endosteal drift
continues throughout life in humans (Epker and Frost, 1965a,b; Frost 1986; Priest et
al, 1992). ‘Drifting osteons’ are observed at all ages within human cortical bone, and
their count is used in forensics for age-at-death estimation.
(221) The systemic biokinetic models used in this series of reports generally follow
the physiologically descriptive modelling scheme applied on a more limited scale in
the Publication 72 series. That is, the model structures include one or more
compartments representing blood, depict feedback of activity from extravascular
repositories to blood (i.e., they are recycling models), and, as far as practical, depict
the main physiological processes thought to determine the systemic biokinetics of
individual elements.
(222) The systemic biokinetic models for some elements, such as iodine and iron,
are developed within model structures specifically designed to describe the unique
behaviour of these elements in the body. The models for most elements, however,
have been constructed within one of the two generic model structures applied in the
Publication 72 series to bone-seeking radionuclides (Figure 18 and 19), or variations
of those structures. This was done not only for bone-seeking elements but for a
number of elements that show relatively low deposition in bone (e.g., cobalt and
ruthenium) because the main repositories and paths of movement of those elements in
the body are included in one or the other of these two structures. In some cases, the
model structure as applied in the Publication 72 series has been modified slightly to
accommodate specific characteristics of an element or to reflect the limited
information on certain aspects of the biokinetics of an element. This is illustrated in
Figure 20, which shows the model applied in this series to cobalt. The structure
shown in Figure 20 is a variation of the structure for bone-surface seekers (Figure 18),
although it could also be viewed as a variation of the structure for bone-volume
seekers (Figure 19). In either case, the model for the skeleton has been simplified
because of the limited information on the skeletal behaviour of cobalt, and two nonspecific
blood pools are used to represent two components of retention of cobalt in
blood.
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