Evolution of the ICRP's biokinetic models

The International Commission on Radiological Protection (ICRP) has usually taken a pragmatic approach to its formulation of biokinetic models for internally deposited radionuclides. With a few exceptions, the models in ICRP Publication 30 and other ICRP documents dealing with occupational exposures are formulated as mathematically convenient 'retention functions' that address only the initial uptake and net rate of decline of radionuclides in a few major repositories in the body. By contrast, some of the models of ICRP Publication 56, Age-Dependent Doses to Members of the Public from Intake of Radionuclides, involve several organs, tissues, and fluids and depict feedback of material from systemic pools to blood plasma, loss of systemic activity by specific excretion pathways, and certain physiological processes known to influence the distribution and translocation of the given radionuclides. This paper discusses the reasons for this shift toward biological realism and illustrates some advantages and limitations of the different modelling approaches. Although attention is focussed on the ICRP's biokinetic models, the discussion applies in most regards to a similar movement toward biological realism seen in the ICRP's new respiratory tract model and in biokinetic models used in medicine and chemical risk analysis.