Occupational Intakes of Radionuclides Part 1 - ICRP

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DRAFT REPORT FOR CONSULTATION
database to different subgroups of the population and to times outside the period of
observation (for example, a parameter value found to depend on the rate of bone
remodelling can be varied with age on the basis of age-specific data on bone
remodelling rates); facilitates the extrapolation of biokinetic data from laboratory
animals to man, in that it helps to focus interspecies comparisons on specific
physiological processes and specific subsystems of the body for which extrapolation
may be valid, even if whole-body extrapolations are not; facilitates the extrapolation
of biokinetic data from an element to its chemical analogues, in that the degree of
physiological similarity of chemical analogues may vary from one physiological
process to another (for example, the alkaline earth elements show similar rates of
transfer from blood to bone but much different rates of transfer to non-exchangeable
sites in bone); links excretion with exchanges of activity among body tissues and
fluids, so that the same model can be used for dose calculation and bioassay
interpretation; allows modelling of the differential biokinetics of parent radionuclides
and their radioactive progeny produced in the body; and allows the addition of
compartments and pathways to the model for purposes of extending the model to new
applications, as was demonstrated in the ICRP documents on doses to the embryo and
fetus (ICRP, 2001) and to the nursing infant (ICRP, 2004) from intakes of
radionuclides by the mother.
(219) On the other hand, the level of physiological realism in the systemic biokinetic
models currently used in radiation protection, including those recommended in the
present report, should not be overstated. Even the most sophisticated models represent
a compromise between biological realism and practical considerations regarding the
quantity and quality of information available to determine parameter values. For
example, the recycling models applied to bone-seeking radionuclides in the
Publication 72 series all include soft-tissue compartments representing fast,
intermediate, and slow exchange with blood for all soft tissues not explicitly
identified in the models. These soft tissue compartments typically are defined on a
kinetic basis rather than a physiological basis, i.e., the compartment sizes and
turnover rates are set for reasonable consistency with data on accumulation and loss
of elements by soft tissues. For some elements, these soft tissue compartments appear
to be associated with specific sites or processes, but the associations generally are not
confirmed by available information. For example, biokinetic studies of calcium
suggest, but do not establish, that the rapid-turnover pool in soft tissues may
correspond roughly to interstitial fluids plus some rapidly exchangeable cellular
calcium (Heaney, 1964 Harrison et al, 1967;Hart and Spencer 1976); the intermediate
turnover rate may stem from a composite of several pools with slower exchange rates,
including mitochondrial calcium, cartilage calcium, and exchangeable dystrophic
calcium (e.g., arterial plaque and calcified nodes) (Heaney, 1964; Borle, 1981); and
long-term retention in soft tissues may be associated with relatively nonexchangeable
dystrophic calcium that gradually accumulates in the human body (Heaney, 1964).
(220) For many elements it is not feasible to develop genuine physiological system
models due to inadequate information on the processes that determine the systemic
behaviour of these elements. Even for relatively well understood elements the model
components are often intended only to represent the net result of multiple processes.
For example, in the model for bone-surface-seeking radionuclides shown in Figure 18
93