Reliability of the ICRP'S dose coefficients for members of the public: IV. basis of the human alimentary tract model and uncertainties in model predictions

The biokinetic and dosimetric model of the gastrointestinal (GI) tract applied in current documents of the International Commission on Radiological Protection (ICRP) was developed in the mid-1960s. The model was based on features of a reference adult male and was first used by the ICRP in Publication 30, Limits for Intakes of Radionuclides by Workers (Part 1, 1979). In the late 1990s an ICRP task group was appointed to develop a biokinetic and dosimetric model of the alimentary tract that reflects updated information and addresses current needs in radiation protection. The new age-specific and gender-specific model, called the Human Alimentary Tract Model (HATM), has been completed and will replace the GI model of Publication 30 in upcoming ICRP documents. This paper discusses the basis for the structure and parameter values of the HATM, summarises the uncertainties associated with selected features and types of predictions of the HATM and examines the sensitivity of dose estimates to these uncertainties for selected radionuclides. Emphasis is on generic biokinetic features of the HATM, particularly transit times through the lumen of the alimentary tract, but key dosimetric features of the model are outlined, and the sensitivity of tissue dose estimates to uncertainties in dosimetric as well as biokinetic features of the HATM are examined for selected radionuclides.     

Dose coefficients for the embryo and fetus following intakes of radionuclides by the mother

The International Commission on Radiological Protection has recently issued Publication 88, giving dose coefficients for the embryo, fetus and newborn child from intakes of selected radionuclides of 31 elements by the mother, either before or during pregnancy. The biokinetic models used for calculating these doses were based upon the available human data and the results of animal experiments. This paper summarises the approach used for the development of biokinetic and dosimetric models. It also compares the estimates of dose received by the offspring with those received by the reference adult. The main findings are that, in general, doses to the offspring are similar to or lower than those to the reference adult. For a few radionuclides, however, the dose to the offspring can exceed that to the adult. The reasons for these variations in comparative doses are examined.     

Practical experience of the application of ICRP models in internal dose assessment

The introduction of the Ionising Radiations Regulations 1999 in the UK, which came into force on 1 January 2000, led to significant changes in internal dose assessment. Before this date, assessments were based on the methodology from ICRP Publication 26 and, in general, made use of simple models such as those detailed in ICRP Publication 30. However, the introduction of the new Regulations required the use of ICRP Publication 60 methodology, and, at the same time, the latest ICRP biokinetic models were introduced. Many of these newer models were considerably more complex than the ones they replaced. In particular, the use of 'recycling', where activity is constantly recirculated between different organs, meant that the models could not simply be implemented by use of the Skrable formula, as detailed in ICRP Publication 30. This paper outlines two aspects of the application of these latest ICRP models. First, the problems encountered during implementation of these models are detailed, and secondly, it covers the practical experience of using the resulting computer programs for internal dose assessment.     

Reliability of the ICRP's dose coefficients for members of the public. 1. Sources of uncertainty in the biokinetic models

During the decade following the Chernobyl accident, the International Commission on Radiological Protection (ICRP) developed dose coefficients (doses per unit intake) for ingestion or inhalation of radionuclides by members of the public. The level of uncertainty in those coefficients varies considerably from one radionuclide to another, due largely to differences in the level of understanding of the biological behaviour of different elements in the human body. This paper is the first in a series that examines the sources and extent of uncertainties in the ICRP's biokinetic and dosimetric models for members of the public and the dose coefficients derived from those models. The present paper describes the different types of information generally used to develop biokinetic models for radionuclides, the main sources of uncertainty associated with each type of information, and the approach used in subsequent papers in this series to quantify the uncertainties in biokinetic and dosimetric estimates.     

The internal dosimetry code PLEIADES

The International Commission on Radiological Protection (ICRP) has published dose coefficients for the ingestion or inhalation of radionuclides in a series of reports covering intakes by workers and members of the public, including children and pregnant or lactating women. The calculation of these coefficients divides naturally into two distinct parts-the biokinetic and dosimetric. This paper describes in detail the methods used to solve the biokinetic problem in the generation of dose coefficients on behalf of the ICRP, as implemented in the Health Protection Agency's internal dosimetry code PLEIADES. A summary of the dosimetric treatment is included.     

A generic biokinetic model for Carbon-14

The generic biokinetic model currently recommended by the International Commission on Radiological Protection (ICRP) for the treatment of systemic radiocarbon assumes uniform distribution of activity in tissues and a biological half-time of 40 d. This model is intended to generate cautiously high estimates of dose per unit intake of C-14 and, in fact, generally predicts a much higher effective dose than systemic models that have been developed on the basis of biokinetic studies of specific carbon compounds. The simplistic model formulation precludes its application as a bioassay model or adjustment to fit case-specific bioassay data. This paper proposes a new generic biokinetic model for systemic radiocarbon that is less conservative than the current ICRP model but maintains sufficient conservatism to overestimate the effective dose coefficients generated by most radiocarbon-compound-specific models. The proposed model includes two systemic pools with different biological half-times representing an initial systemic form of absorbed radiocarbon, a submodel describing the behaviour of labelled carbon dioxide produced in vivo, and three excretion pathways: breath, urine and faeces. Generic excretion rates along each path are based on multi-phase excretion curves observed in experimental studies of radiocarbons. The generic model structure is designed so that the user may adjust the level of dosimetric conservatism to fit the information at hand and may adjust parameter values for consistency with subject-specific or site-specific bioassay data.     

Biokinetic models for the behaviour of carbon-14 from labelled compounds in the human body: can a single generic model be justified?

The published data on the biokinetic behaviour of 27 14C-labelled compounds in humans or animals have been reviewed. Effective doses have been calculated for those compounds for which this information had not been published and doses have been compared to that calculated with the International Commission on Radiological Protection (ICRP) default model for carbon compounds of unknown composition. The compound-specific effective doses for a few natural human biochemical substrates are quite close to the ICRP default dose coefficient, but for the remainder of the compounds considered the doses are smaller by factors ranging from about 5 to 200. Comparison of the dosimetric data suggests that although the ICRP default model will overestimate the dose for very many compounds it could remain useful as a guide for general prospective radiological protection purposes. However, a comparison of the biokinetic information indicates that the ICRP default model would not be reliable for the interpretation of bioassay data.     

A model for the transfer of alkaline earth elements to the fetus.

A biokinetic model has been developed for the transfer of calcium, strontium, barium and radium to the human fetus. For the mother, ICRP models were adapted for pregnancy to include increases in gastrointestinal absorption, urinary excretion and bone turnover rates. The fetus was modelled with blood, soft tissue and bone compartments. Fetal requirements for Ca were determined by skeletal calcification, and recyling between fetal and maternal blood was inlcluded. Daily transfer of Sr, Ba and Ra to the fetus was taken to be lower than for Ca by factors of 0.6 for Sr and 0.4 for Ba and Ra. For acute intakes in late pregnancy at 35 weeks after conception, when maximum transfer occurs, the model predicts whole-body fetus:mother concentration ratios (C(F):C(M)) of 18 for Ca, 8 for Sr and 2 for Ba and Ra, respectively. Estimates of committed equivalent doses to the red bone marrow of offspring, including in utero and postnatal dose, after maternal ingestion in late pregnancy, were greater than corresponding doses in adults by factors of 20-31 for 45Ca, 2-3 for 90Sr and 3-4 for 226Ra but slightly lower (0.8-1.9) for 133Ba.     

Assessment of environmental radon hazard using human respiratory tract models

Radon is a natural radioactive gas derived from geological materials. It has been estimated that about half of the total effective dose received by human beings from all sources of ionizing radiation is attributed to 222Rn and its short-lived progeny. In this paper, the use of human respiratory tract models to assess the health hazard from environmental radon is reviewed. A short history of dosimetric models for the human respiratory tract from the International Commission on Radiological Protection (ICRP) is first presented. The most important features of the newest model published by ICRP in 1994 (as ICRP Publication 66) are then described, including the morphometric model, physiological parameters, radiation biology, deposition of aerosols, clearance model and dose weighting. Comparison between different morphometric models and comparison between different deposition models are then given. Finally, the significance of various parameters in the lung model is discussed, including aerosol parameters, subject related parameters, target and cell related parameters, and parameters that define the absorption of radon from the lungs to blood. Dosimetric calculations gave a dose conversion coefficient of 15 mSv/WLM, which is higher than the value 5 mSv/WLM derived from epidemiological studies. ICRP stated that dosimetric models should only be used for comparison of doses in the human lungs resulted from different exposure conditions.     

Structure of a physiologically based biokinetic model for use in 14C and organically bound tritium dosimetry

Physiologically based biokinetic (PBBK) dosimetry models for beta emitters such as 3H and 14C must include rapid turnover compartments which, while they may be minor in terms of dose commitment, can dominate bioassay measurements at early times after intake. In this paper, a consistent PBBK model structure will be described for use in dose assessments for organic 14C and organically bound tritium (OBT), and also for 14CO2, based on the literature of human carbon metabolism, and on direct measurements of human excretion. CO3/HCO3- is a central compartment in carbon metabolism. The 14CO2 biokinetic model described in ICRP Publication 80 for the calculation of dose coefficients was found to omit early components of excretion necessary for the accurate interpretation of bioassay results. Recommendations on the requirements on dosimetry models for intakes of 14C and OBT are made.     

The computation of ICRP dose coefficients for intakes of radionuclides with PLEIADES: biokinetic aspects

The ICRP has published dose coefficients for the ingestion or inhalation of radionuclides in a series of reports covering intakes by workers and members of the public including children and pregnant or lactating women. The calculation of these coefficients conveniently divides into two distinct parts--the biokinetic and dosimetric. This paper gives a brief summary of the methods used to solve the biokinetic problem in the generation of dose coefficients on behalf of the ICRP, as implemented in the Health Protection Agency's internal dosimetry code PLEIADES.     

Prediction of the variation in risks from exposure to radon at home or at work

On the basis of a review of recent epidemiology, the ICRP recently issued a statement outlining a new approach to radon. The ICRP indicates that the Publication 65 dose conversion convention will be replaced using the dosimetric approach currently used for other radionuclides. Moreover, the ICRP indicates that the dose conversion factor is expected to increase by about a factor of 2. This paper independently examines the risks associated with exposure to radon and decay products through estimation of lifetime excess absolute risks per WLM for a variety of epidemiological risk projection models and baseline cancer and mortality rates. This paper suggests that current ICRP dosimetric models do not reflect the effect of smoking and suggest that basic risk estimates and dose conversion factors be based on risks to non-smoking populations with recognition that lifestyle choices, especially smoking, have a large effect on the risk from exposure to radon.     

Effect of uncertainty in transfer rates for the ICPR publication 67 biokinetic model on dose estimation for 239Pu from results of individual monitoring. International Commission on Radiological Protection

The radiation dose due to internal exposures from 239Pu is mainly estimated by measuring actual urinary or faecal excretion of activity and comparing those values with the standard excretion rates calculated from the models of the International Commission on Radiological Protection (ICRP). Recently, on the other hand, uncertainties in the ICRP's models and parameters are under consideration because of the paucity of human data. In addition, there is a possibility of variation between individuals. A code has been developed to reproduce the ICRP's dose coefficients and excretion rates for 239Pu, which is one of the most important elements for occupational exposure. By using this code, the respective transfer rates for the ICRP Publication 67 biokinetic model were modified, and the effect owing to these changes on present hazard assessment was investigated. As a result, it was shown that dose estimates for workers exposed to 239Pu were not very sensitive to changes in these transfer rates.     

The biokinetics of ruthenium in the human body

The International Commission on Radiological Protection (ICRP) is updating its biokinetic and dosimetric models for workers and subsequently will revisit its models for members of the public. This paper summarises the biokinetic database for ruthenium and proposes a new biokinetic model for systemic ruthenium. In contrast to the ICRP's current model, the proposed model depicts recycling of ruthenium between tissues and blood and a non-uniform distribution of systemic ruthenium. The paper also points out inconsistencies between the ICRP's respiratory model for RuO(4) vapour and reported data, and inconsistencies between the ICRP's default gastrointestinal (GI) uptake value and data for some forms of ruthenium. Dosimetric implications of the proposed systemic model and the findings for inhaled RuO(4) vapour and GI uptake of ruthenium are examined.     

Worker inhalation dose coefficients for radionuclides not previously identified in ICRP publication 68

While inhalation dose coefficients are provided for about 800 radionuclides in International Commission on Radiological Protection (ICRP) Publication 68, many radionuclides of practical dosimetric interest for facilities such as high-energy proton accelerators are not specifically addressed, nor are organ-specific dose coefficients tabulated. The ICRP Publication 68 dosimetry concepts are used, along with updated radiological decay data and metabolic data, to calculate committed equivalent dose coefficients [h(T)(50)] and committed effective dose coefficients [e(50)] for radionuclides produced at the Oak Ridge National Laboratory's Spallation Neutron Source.     

Surface-seeking radionuclides in the skeleton: current approach and recent developments in biokinetic modelling for humans and beagles

In the last decade, the biokinetics of surface-seeking radionuclides in the skeleton has been the object of several studies. Investigations were carried out to determine the kinetics of plutonium and americium in the skeleton of humans and beagles. As a result of these investigations, in recent years the models presented by ICRP in Publication 67 for humans were partially revised, particularly the skeletal part. The aim of the present work is to present recent developments in the biokinetic modelling of surface-seeking radionuclides (plutonium and americium) in beagles and humans. Various assumptions and physiological interpretations of the different approaches to the biokinetic modelling of the skeleton are discussed. Current ICRP concepts and skeleton modelling of plutonium and americium in humans are compared to the latest developments in biokinetic modelling in beagles.     

An intake of americium oxide powder: implications for biokinetic models for americium

A worker inhaled 241AmO2 powder. Air sampling showed low activities but a nose blow revealed 92 Bq. Results from faecal sampling and lung and whole-body monitoring indicated an intake of about 200 Bq, but urine sampling, though commencing only 1 d after intake, showed below-threshold activities (< 0.2 mBq). This conflicts with predictions based on the ICRP Publication 67 biokinetic model for americium and the ICRP Publication 66 model for the human respiratory tract, if default lung parameters are used.     

A biokinetic model for predicting the retention of 3H in the human body after intakes of tritiated water

Scarce published data on the long-term excretion of tritiated water from the human body have been re-evaluated in order to develop a biokinetic model describing the retention in the human body of 3H from tritiated water (HTO) that could be used for both prospective and retrospective radiation protection. A three-component exponential function is proposed to describe the elimination of 3H from HTO with biological half-times of 10 d (99.00%), 40 d (0.98%) and 350 d (0.02%) respectively. The model predicts a committed effective dose of 1.7 x 10(-11) Sv Bq(-1), comparable with that of the current ICRP Publication 56 and 72 models, and estimates the retention of 3H to within a factor of about 2 of the measured values up to 40 d after intake and about 5 at times longer than 100 d. The derivation of the model and the uncertainties associated with the various parameters are discussed.     

IMBA Professional Plus: a flexible approach to internal dosimetry

IMBA (Integrated Modules for Bioassay Analysis) is a suite of software modules that implement the current ICRP biokinetic and dosimetric models for estimation of intakes and doses. The IMBA modules have gone through extensive quality assurance, and are now used for routine formal dose assessment by Approved Dosimetry Services throughout the UK. HPA has continued to develop the IMBA modules. In addition, several projects, sponsored by organisations both in the USA and in Canada, have resulted in the development of customised user-friendly interfaces (IMBA Experttrade mark 'editions'). These enable users not only to use the standard ICRP models, but also to change many of the parameter values from ICRP defaults, and to apply sophisticated data handling techniques to internal dose calculations. These include: fitting measurement data with the maximum likelihood method; using multiple chronic and acute intakes; and dealing with different data types, such as urine, faces and whole body simultaneously. These interfaces were improved further as a result of user-feedback, and a general 'off-the-shelf' product, IMBA Professional, was developed and made available in January 2004. A new version, IMBA Professional Plus, was released in April 2005, which is both faster and more powerful than previous software. The aim of this paper is to describe the capabilities of IMBA Professional Plus, and the mathematical methods used.