Occupational Intakes of Radionuclides Part 1 - ICRP

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DRAFT REPORT FOR CONSULTATION
information (e.g., rates of bone restructuring); considerations of mass balance;
predictions of theoretical models based on fundamental physical, chemical, and
mathematical principles (e.g., a theoretical model of deposition of inhaled particles in
the different segments of the lung); experimental data derived with anatomically
realistic physical models (e.g., hollow casts of portions of the respiratory tract used to
measure deposition of inhaled particles); and in vitro data (e.g., dissolution of
compounds in simulated lung fluid). Among these supplemental sources of
information, mass balance and quantitative physiological data (P) have particularly
wide use.
Sources of uncertainty in applications of human data
(376) It is desirable to base a biokinetic model for an element on observations of the
time-dependent distribution and excretion of that element in human subjects (H1
data). Some degree of this type of direct information is available for most essential
elements, as well as for some important non-essential elements, such as caesium, lead,
radium, uranium, americium, and plutonium. Depending on the degree of biological
realism in the model formulation, it may be possible to supplement element-specific
information for human subjects with quantitative physiological information for
humans on the important processes controlling the biokinetics of the element of
interest. For example, in ICRP Publications 67 (1993), 69 (1995a), and 71 (1995b),
long-term removal of certain radionuclides from bone volume is identified with bone
turnover.
(377) Although it is the preferred type of information for purposes of model
construction, H1 data often have one or more of the following limitations: small study
groups, coupled with potentially large inter-subject variability in the biokinetics of an
element; short observation periods, coupled with potentially large intra-subject
variability; use of unhealthy subjects whose diseases may alter the biokinetics of the
element; paucity of observations for women and children; collection of small,
potentially non-representative samples of tissue; inaccuracies in measurement
techniques; uncertainty in the pattern or level of intake of the element; atypical study
conditions; and inconsistency in reported values. In some cases, inconsistency in
reported values may provide some of the best evidence of the uncertain nature of the
data.
(378) An important tool in the development of biokinetic models for radionuclides
has been the use of reference organ contents of stable elements, as estimated from
autopsy measurements on subjects chronically exposed at environmental levels or at
elevated levels encountered in occupational exposures (ICRP, 1975). Such data are
commonly used to adjust parameter values of biokinetic models or introduce new
model components to achieve balance between reported values of intake, total-body
content, and excretion of stable elements. Such balance considerations can provide
useful constraints on model parameters, provided the data have been collected under
carefully controlled conditions. However, such balance considerations often have
been based on data from disparate sources of information and unreliable measurement
techniques and in some cases may have led to erroneous models or parameter values.
(379) A confidence statement based on H1 data would reflect a variety of factors,
such as the reliability of the measurement technique(s), the number and state of health
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