Occupational Intakes of Radionuclides Part 1 - ICRP

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of the subjects, representativeness of the subjects and biological samples, consistency
in data from different studies, knowledge concerning the level and pattern of intake,
and the relevance of the information to the situation being modelled. For example,
confidence in a parameter value based on H1 data would be reduced if the data were
determined in a study on any of the following study populations: several seriously ill
subjects with known intakes, several healthy subjects with poorly characterized
intakes, or one healthy subject with known intake.
Uncertainty in interspecies extrapolation of biokinetic data
(380) Interspecies extrapolation of biokinetic data is based on the concept of a
general biological regularity across the different species with regard to cellular
structure, organ structure, and biochemistry. Mammalian species, with cell structure,
organ structure, biochemistry, and body temperature regulation particularly close to
those of man, are expected to provide better analogies to man that do non-mammalian
species with regard to biokinetics of contaminants.
(381) Despite the broad structural, functional, and biochemical similarities among
mammalian species, interspecies extrapolation of biokinetic data has proven to be an
uncertain process. Similarities across species often are more of a qualitative than
quantitative nature, in that two species that handle an internally deposited
radionuclide in the same qualitative manner may exhibit dissimilar kinetics with
regard to that substance. Moreover, there are important structural, functional, and
biochemical differences among the mammalian species, including differences in
specialized organs, hepatic bile formation and composition, level of biliary secretion,
urine volume and acidity, the amount of fat in the body, the magnitude of absorption
or secretion in various regions of the digestive tract, types of bacteria in the digestive
tract, and microstructure and patterns of remodelling of bones.
(382) In general, the choice of an animal model will depend strongly on the
processes and subsystems of the body thought to be most important in the biokinetics
of the radionuclide in humans, because a given species may resemble humans with
regard to certain processes and subsystems and not others. For example, data on
monkeys or baboons may be given relatively high weight for purposes of modelling
the distribution of a radionuclide in the skeleton due to the close similarities in the
skeletons of non-human primates and humans. Data on dogs may be given relatively
high weight for purposes of modelling the rate of loss of a radionuclide from the liver
due to broad quantitative similarities between dogs and humans with regard to hepatic
handling of many radionuclides.
(383) A physiologically based model provides the proper setting in which to
extrapolate 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 invalid.
Depending on the process being modelled, it may be preferable to limit attention to
data for a single species or small number of species, or to appeal to average or scaled
data for a collection of species.
(384) The degree of confidence that can be placed in a model value based on animal
data depends on the quality and completeness of the data and the expected strength of
the animal analogy for the given situation. Thus, one must consider potential
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