Formulation of Relationships between the Radiation Exposure of Tissues and the Excretion Rate of Nuclides

The organization of protection against undue occupational exposure to internal radiation involves several processes: 1 1. Decisions as to the highest dose-rates, for the body or for particular organs-, that can be regarded as permissible; 2. Estimation, for all relevant nuclides, of the intakes, and of the body burdens, which would cause any such dose-rates to be reached or sustained; 3. Monitoring of exposed individuals to determine what fraction of a permissible body burden of any nuclide is retained in the body. Techniques of monitoring by whole-body counting, or by data on excretion or exhalation rates, are relevant to the Symposium. Several major problems are involved: (a) For most radionuclides, insufficient metabolic data are available to link tissue dose-rates either with body burdens or with excretion rates, at least as based on adequately large numbers of normal human subjects; (b) The large variability between different subjects in physiological functions, such as excretion or clearance rates, limits severely the inferences that can be made from isolated human observations, and restricts also the deductions as to body burden that can be drawn from the excretion rates observed in any individual; (c) The complex variation of excretion rate with time after a single intake cf many, nuclides prevents any direct deduction of body burden from excretion rate when the time course of intake is unknown, except in the special case of a nuclide which is excreted at a rate which decreases (mono-) exponentially with time since intake; (d) In particular, many radionuclides which are of particular importance because they are highly concentrated and long retained in certain organs, may show initial rapid excretion of the fraction of intake which is not so concentrated, and much slower and more prolonged excretion of the retained part. A small recent intake will thus contribute preponderating to the excretion as compared with that from a much larger and more important burden already localized in the critical organ; (e) The formulations of the International Commission on Radiological Protection (ICRP), which are commonly based on simple exponential models as approximations or in the absence of more detailed metabolic information, may be adequate for estimating the dose commitments attributable to particular intakes or body burdens, but may be much less appropriate for relating excretion rate to body burden in any individual in the case of nuclides which mix in, and are excreted from, multi compartment systems in the body, and particularly in the case of excretion rates observed after recent intake. These situations require special review in the light of evidence as to the earliest phases of turnover. (author)