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.

Dose coefficients calculated using the new ICRP model for the human alimentary tract.

Publication 100 of the International Commission on Radiological Protection (ICRP) provides a Human Alimentary Tract Model (HATM) to replace the gastrointestinal (GI) model described in Publication 30. The HATM will be used for future calculations of dose coefficients and bioassay predictions, first in a series of publications on occupational intakes of radionuclides, and subsequently in revision of dose coefficients for public exposures. This paper compares dose coefficients calculated using the new model with current values calculated using the GI model for a range of radionuclides. Colon doses are lower using the HATM in all cases considered, in some cases by significant factors. Stomach doses tend to be lower, but are in some cases higher under HATM. The extent to which these changes in doses to gut tissues impacts upon the effective dose varies among nuclides, but there is a tendency for lower effective doses. Special-case applications of the HATM are also described, considering retention on teeth or in the walls of the small intestine. Although the effect of such retention on the regional tissue dose can be large, the effective dose is not greatly changed.

In utero and postnatal haemopoietic tissue doses resulting from maternal ingestion of strontium isotopes from the Techa River.

Reliable estimates of tissue doses to individuals exposed as a result of radioactive releases to the Techa River are essential prerequisites for epidemiological analyses. This paper describes progress made in collaborative studies, sponsored by the European Union, between the Urals Research Center for Radiation Medicine and the UK Health Protection Agency to provide dose estimates to Techa River populations following in utero exposures and infant exposures resulting from breast-feeding. Studies have concentrated on the assessment of internal doses from 90Sr as the main contributor to internal doses to the Techa River populations.

Unexpectedly high activity of 228Th in excretion samples following consumption of Brazil nuts.

A worker provided a routine faecal sample for plutonium and americium analysis. In the course of this analysis 500 mBq of (228)Th was discovered. There seemed no credible occupational route for intake of thorium. Further investigation revealed that the worker consumed approximately 25 g d(-1) of nuts, including Brazil nuts. A sample of these nuts was analysed and found to contain activities of (228)Th in sufficient quantity to account for the faecal activity. However, follow-up urine samples taken from the worker showed 0.6-0.7 mBq of (228)Th. The intake of (228)Th via nuts is insufficient to account for this activity in urine. However, it is likely that the intake of (228)Th was accompanied by similar activity of the parent (228)Ra, and biokinetic calculations show that decay of (228)Ra in vivo would produce sufficient (228)Th to account for the observed urine activity.

The application and adaptation of ICRP internal dosimetry models to the calculation of bone marrow tissue doses from 90Sr for epidemiological studies of Techa River populations.

The operation of the Mayak Production Association in the Southern Urals region of Russia, resulted in releases of large amounts of radioactive effluent into the Techa River during the period 1949-1956. The residents of the riverside communities were thus exposed to both external radiation, and internal radiation following ingestion of contaminated water and foodstuffs. One of the most important radionuclides for internal exposure was 90Sr. This paper gives a brief overview of the models provided by International Commission on Radiological Protection (ICRP), which are of interest for assessing internal doses from 90Sr. The application of these models to the calculation of red bone marrow doses for the fetus and infant from 90Sr intakes by the mother and the infant is illustrated by an example. A hypothetical individual born in 1951 is used as an example for dose calculations. The following doses due to intakes of 90Sr are taken into account: received in utero due to maternal intakes during pregnancy; received after birth from 90Sr accumulated by the fetus in utero; from intakes in breast milk; from intakes in the infant's diet after weaning. It is shown that doses to the fetus following maternal ingestion and subsequent transfer to the fetus via the placenta dominate the doses received for this particular individual for the first two years of life. Doses to the infant from intakes in breast milk are substantially lower but do make significant contributions to total doses in the first two years after birth. By about the age of two years residual 90Sr from placental transfer still contributes about the same dose as do intakes by the infant, but in later years doses from intakes by the infant dominate.

Biokinetics and dosimetry of chromium, cobalt, hydrogen, iron and zinc radionuclides in male reproductive tissues of the rat.

Following intravenous administration to male rats, the uptake and retention by reproductive tissues of chromium-51, cobalt-57, iron-59, zinc-65 and tritium has been studied for up to 28 days. Chromium-51, 57Co, 59Fe and 3H were not or only transiently accumulated in gonads or accessory sex glands at concentrations greater than whole body concentrations. However, 65Zn was concentrated in the dorsolateral region of the prostate gland and autoradiography showed preferential uptake by epithelial cells and lumen of glands. When combined with other information available from the literature, this data would suggest that current models adequately describe the biokinetics of chromium, cobalt, iron and tritium in the prostate and testes and zinc in the testes. Uptake of zinc by the prostate would appear to be best described by an average value of 0.1% and a conservative value of 0.5%. Allowing for greater uptake of zinc (0.5%) by the prostate, after inhalation of 65Zn in a soluble form increases prostate dose by about 3 fold compared to current models. The pessimistic assumptions of a higher relative biological effectiveness (20) for all Auger emissions from 65Zn in cell nuclei and a heterogeneous distribution of 65Zn to sensitive cells in the prostate increases prostate dose by a further factor of 9. Even on the basis of these cautions estimates, occupational exposures to radioisotopes of these elements do not explain the excess of prostate cancer reported amongst some nuclear workers.

The placental transfer of cerium: experimental studies and estimates of doses to the human fetus from 141Ce and 144Ce.

PURPOSE: To measure the transfer of cerium from mother to fetus in experimental animals and estimate doses to the human fetus following intakes of radioisotopes of Ce. MATERIALS AND METHODS: Cerium-141 in chloride solution was administered intravenously to rats at different stages of pregnancy (days 9.5, 12.5 or 18.5), and retention in the embryo/fetus and associated tissues was measured 3 days later in each case. Retention in rat fetal tissues on day 21.5 (shortly before birth) was also measured after administration of 141Ce chloride 1 month prior to conception or 141Ce citrate on day 18.5. Cerium-141 chloride was administered to guinea pigs on day 50 for measurements of fetal retention on day 57 (shortly before birth). RESULTS: Retention of 141Ce in the rat embryo/fetus, measured at 3 days after administration to the mother, increased from about 0.00002% of injected activity per embryo/fetus on day 12.5 to about 0.014% on day 21.5 of gestation. However, the relative concentrations of 141Ce in the embryo/fetus and mother (CF:CM ratio) were between 0.005 and 0.01 in each case. After 141Ce administration prior to conception, retention by the rat fetus on day 21.5 was substantially lower than after short-term administration. Comparison of retention of 141Ce on day 21.5 after administration on day 18.5 as either chloride or citrate showed similar levels in maternal tissues but greater transfer to the fetus (CF:CM ratio of 0.03). Retention in the guinea pig fetus in late gestation at 7 days after administration of (141)Ce chloride was about 0.05% injected activity per fetus, corresponding to a CF:CM ratio of about 0.02. CONCLUSION: These results and other published animal data have been used to specify CF:CM ratios for use in the calculation of doses to the human fetus. The values used were 0.05 for intakes during pregnancy and 0.01 for intakes prior to conception. Doses to the offspring after maternal ingestion of 141Ce or 144Ce are largely due to irradiation from activity in the maternal colon and are insensitive to CF:CM. After inhalation, however, absorption of Ce to blood is much greater and doses to the offspring are dominated by the contribution from activity in the fetus, and therefore dependent on the CF:CM ratio used.

Transfer of polonium to the embryo and foetus of rat and guinea pig.

Transfer of 210Po to the foetus measured 3 days after administration in rat and 7 days later in guinea pig increased with increasing gestational age to about 0.1% injected activity per rat foetus at birth and 0.6% per guinea pig foetus on day 57, corresponding to whole-body foetus:mother concentration ratios of about 0.1:1 in both species. The greatest concentrations of 210Po were measured in the rat yolk sac during its haemopoietic stage, an order of magnitude greater than concentrations in the placenta and two orders of magnitude greater than foetal concentrations. The results obtained have been used to estimate in utero doses to haemopoietic tissues, taking account of transfer to the blastocyst/egg cylinder, yolk sac, liver and bone marrow. The concentration ratios relative to maternal liver for these tissues were taken to be 1, 3, 0.1 and 0.05 respectively and were applied to periods of human gestation of 0-2.5, 2.5-6, 6-12 and 12-38 weeks respectively. For chronic maternal intake by ingestion of 210Po during the year of pregnancy giving a committed effective dose (CED) to the mother of 1 mSv, the total in utero dose to haemopoietic tissue was about 340 microSv compared with a maternal red bone marrow dose of 2.2 mSv. The yolk sac and bone marrow accounted for 66 and 27% of the in utero dose respectively. In addition, the total CED to the offspring was calculated assuming a whole-body foetus:mother concentration ratio of 0.1:1 and that the distribution of 210Po between tissues was the same in the foetus as in adults and children. For chronic intake of 210Po during the year of pregnancy as assumed above, the CED to the offspring was estimated to be 8% of that to the mother.

Impact on internal doses of photon SAFs derived with the GSF adult male voxel phantom.

This paper describes the effect on over 3,000 sets of internal dose estimates of using photon Specific Absorbed Fractions (SAFs) calculated using two different types of phantoms, specifically the MIRD-type anthropomorphic phantom originally developed by Snyder and the new adult male voxel phantom, GOLEM, developed at GSF. The SAFs based on the MIRD-type phantom are currently used by the International Commission on Radiological Protection (ICRP) in internal dose calculations, but there are suggestions of moving towards SAFs generated from voxel phantoms, thus there is an interest in the potential differences in internal doses. Overall, it is found that some tissue doses calculated using the voxel phantoms can differ significantly from those of the MIRD-type phantom; however, the effective dose appears to be quite robust to changes in photon SAFs.

Simplified organ retention functions for physiologically based recycling biokinetic models.

Recent ICRP Publications on biokinetic models show a movement from simple schematic models to more complex, physiologically-based models. Such models require considerably greater computing resources to implement than their predecessors, effectively making them inaccessible to many users. Fortunately, retention in most of the compartments of these large recycling models can be adequately approximated by the sum of a few exponential functions compatible with the simple catenary models used almost exclusively in ICRP Publication 30. An eigenvalue method is used to solve the plutonium model of ICRP Publication 67 for intakes by inhalation, ingestion, and injection. The organ retention expressions so obtained are reduced by least squares minimization to functions consisting of the sum of a few exponential terms only. These simplified functions give committed doses accurate to within 5% and activities to within 10%. A similar treatment is used to obtain simplified expressions for daily excretion rates of plutonium.

Infant doses from the transfer of radionuclides in mothers' milk.

Assessments of potential internal exposures of the child following radionuclide intakes by the mother require consideration of transfers during lactation as well as during pregnancy. Current ICRP work on internal dosimetry includes the estimation of radiation doses to newborn infants from radionuclides ingested in mothers' milk. Infant doses will be calculated for maternal intakes by ingestion or inhalation of the radionuclides, radioisotopes of 31 elements, for which fetal dose coefficients have been published. In this paper, modelling approaches are examined, concentrating on models developed for iodine, caesium, polonium, alkaline earth elements and the actinides. Comparisons of model predictions show maximum overall transfer to milk following maternal ingestion during lactation of about 30% of ingested activity for 131I, 20% for 45Ca and 137Cs, 10% for 90Sr, 1% for 210Po and low values of less than 0.01% for 239Pu and 241Am. The corresponding infant doses from milk consumption are estimated in preliminary calculations to be about two to three times the adult dose for 45Ca and 131I, 70-80% of the adult dose for 90Sr, about 40% for 137Cs, 20% for 210Po, and <0.1% for 239Pu and 241Am. Infant doses from radionuclides in breast milk are compared with doses to the offspring resulting from in utero exposures during pregnancy.

Dosimetry of radioiodine for embryo and fetus.

This paper discusses the biokinetic and dosimetric models adopted in ICRP Publication 88 for the evaluation of fetal doses resulting from maternal intakes of radioiodine. The biokinetic model is used to simulate the behaviour of iodine in both the mother and the fetus. Such simulations provide the basis for the estimation of the dose to the embryo and determine the distribution of maternal iodine at the beginning of the fetal period. The model considers iodine to accumulate in the fetal thyroid from the 11th week. The dose to the fetus delivered following birth is evaluated with the biokinetic and dosimetric models described in ICRP Publication 67. Although a substantial fraction of the emitted energy of electrons and photons is less than 10 keV, conventionally assumed to be non-penetrating radiation, these emissions can escape the small fetal thyroid. Absorbed fractions for both self-dose and crossfire were evaluated for the requirements of radioiodine dosimetry in ICRP Publication 88.

Transfer of alkaline earth elements in mothers' milk and doses from 45Ca, 90Sr and 226Ra.

An international programme of work is currently under way to develop methods for calculating doses to infants from ingestion of radionuclides present in mothers' milk. This paper considers the special case of the alkaline earth elements. Models have been developed for 45Ca, 90Sr and 226Ra and the sensitivity of results to various changes in parameter values is discussed. A complication when calculating doses from intakes of radium is that the International Commission on Radiological Protection has previously recommended that doses from decay products of radium should be calculated using element-specific biokinetic models (so-called independent biokinetics). An extension of this method to the models for breastfeeding is proposed. Preliminary estimates of the doses received by the infant for a number of maternal intake scenarios show that doses to the infant can exceed the corresponding adult dose, such as for 45Ca (ratio = 3.1) while, in other cases such as 90Sr, the infant dose can be a significant fraction of the adult dose.

Some aspects of the fetal doses given in ICRP Publication 88.

The International Commission on Radiological Protection (ICRP) has recently published dose coefficients (dose per unit intake, Sv Bq(-1)) for the offspring of women exposed to radionuclides during or before pregnancy. These dose estimates include in utero doses to the embryo and fetus and doses delivered postnatally to the newborn child from radionuclides retained at birth. This paper considers the effect on doses of the time of radionuclide intake and examines the proportion of dose delivered in utero and postnatally for different radionuclides. Methods used to calculate doses to the fetal skeleton are compared. For many radionuclides, doses are greatest for intakes early in pregnancy but important exceptions, for which doses are greatest for intakes later in pregnancy, are iodine isotopes and isotopes of the alkaline earth elements, including strontium. While radionuclides such as 131I deliver dose largely in utero, even for intakes late in pregnancy, others such as 239Pu deliver dose largely postnatally, even for intakes early during pregnancy. For alpha emitters deposited in the skeleton, the assumption made is of uniform distribution of the radionuclide and of target cells for leukaemia and bone cancer in utero; that is, the developing bone structure is not considered. However, for beta emitters, the bone structure was considered. Both approaches can be regarded as reasonably conservative, given uncertainties in particular in the location of the target cells and the rapid growth and remodelling of the skeleton at this stage of development.

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.

Effective and organ doses from thoron decay products at different ages.

The dosimetry of radon-220, often known as thoron, and its decay products has received less attention than has that of radon-222. Dose coefficients used by international bodies such as UNSCEAR and ICRP and by the UK's former National Radiological Protection Board are based on calculations from the 1980s. We present calculations for thoron decay products using the most recent ICRP models. These indicate that the effective dose is dominated by the doses to lung and that, under the present models, these doses are somewhat higher than under the previous consensus. Conversely, the present models give doses to organs outside the respiratory tract that are somewhat lower than those previously calculated. Dose coefficients for children are somewhat higher than those for adults. However, breathing rates for children are lower than those for adults and there are no great differences in annual doses.

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

Committee 2 of the International Commission on Radiological Protection (ICRP) has the responsibility for calculating radiation doses from intakes of radionuclides for all age groups in the population. Publication 88 of the ICRP, which has recently been published, describes the development of models used for calculating radiation doses to the embryo and foetus following intakes of radionuclides by the mother. It also gives radiation doses to the offspring for intakes of radionuclides by the mother either before or during pregnancy. The approaches used in the development of the biokinetic and dosimetric models are summarised here together with a comparison of the doses to the offspring with those to the reference adult.