Recent efforts to incorporate greater anatomical and physiological realism into biokinetic models have resulted in many cases in mathematically complex formulations that limit routine application of the models. This paper describes an elementary, computer-efficient technique for implementing complex compartmental models, with attention focused primarily on biokinetic models involving time-dependent transfer rates and recycling. The technique applies, in particular, to the physiologically based, age-specific biokinetic models recommended in Publication No. 56 of the International Commission on Radiological Protection, Age-Dependent Doses to Members of the Public from Intake of Radionuclides.

A critical review of the literature on the biokinetics of inhaled mercury vapor was performed as part of an accident analysis for the Spallation Neutron Source to be built at Oak Ridge National Laboratory. It was concluded that current models for inhaled mercury vapor do not accurately describe the distribution or residence time of mercury deposited in the respiratory tract. This paper proposes a model that is more consistent with collective information on the fate of inhaled mercury vapor in laboratory animals and human subjects. Compared with the model currently recommended by the International Commission on Radiological Protection (ICRP), the proposed model predicts lower deposition in the bronchi and bronchioles, greater deposition in the alveolar-interstitial region, and a different pattern of absorption to blood. The proposed model yields substantially reduced estimates of lung dose and effective dose for most radioisotopes of mercury inhaled as mercury vapor.

In a previous work we reported that the fraction of the electron energy absorbed in the basal cell layer of the anterior nasal passages was not very sensitive to changes in the surface area or radius of the cylindrical model adopted in Publication 66 of the International Commission on Radiological Protection. These absorbed fraction data are used in calculation of the dose to a 10-microm-thick basal cell layer located at a depth of 40 microm in the epithelial cell layer of the extrathoracic (ET1) region. However, these data may only be applicable to the assumed cylindrical geometry and may not be valid for more realistic ET1 geometries. The nose differs in size and shape from one person to another, its shape is not cylindrical but closer to a truncated elliptical cone, and in most humans the nostrils are elliptical in shape. We propose herein a more realistic geometry model, the frustum of a cone, for the anterior nose region (ET1) as an alternative to the cylinder model provided in ICRP 66. The results of absorbed fraction calculations using MCNP4B with the new model are reported. These absorbed fractions are compared to the values previously obtained using the MCNP4B code and a cylindrical model (10 cm2 surface area). We also investigate the effects of changing the size of the truncated cone to represent variations due to sex and age.

It is generally agreed that Langham's model for urinary excretion of Pu substantially overestimates the systemic burden several years after exposure. Improved estimates can be derived from information obtained since the development of that model, including comparative urine and autopsy data for occupationally exposed persons; reanalyzed and updated data for human subjects injected with Pu; and a large body of general physiological and Pu-specific information on the processes governing the behavior of Pu in the body. We examine modeling approaches based on each of these sets of information and show that the three approaches yield fairly consistent estimates of the urinary excretion rate over three decades after contamination of blood. Estimates from the various approaches are unified to obtain a single set of predicted urinary excretion rates that, in effect, is based on all three bodies of information. A simple method is described for using these excretion rates to estimate intakes and systemic burdens of Pu.

There is an increasing need for age-dependent dosimetric models, and it would be desirable to develop these models in such a way that the uniformity and basic features of the standard man models are retained. Unfortunately, available data concerning the age-dependent retention of nuclides would rarely permit the identification of compartments, uptake fractions, and clearance times using the empirical fitting methods that characterize the development of many adult models. However, in cases where compartments can be made to correspond to physically identifiable processes or subsections within an organ, it may be possible to combine relatively extensive information concerning the development of the human body with generally sparse nuclide-specific information to construct age-dependent compartmental models. In some cases there may be sufficient data to identify trends with age within compartments using empirical fitting techniques, provided the compartments have already been identified on physical bases. To obtain models that describe changes in a continuous manner from birth through adulthood, it may be necessary in many cases to modify existing adult models to consider fewer (or more easily identifiable) compartments. In this article we describe an age-dependent model for retention of ingested 90Sr in bone that exemplifies these concepts.