T. P. Fell

An image-based skeletal tissue model for the ICRP reference newborn

Hybrid phantoms represent a third generation of computational models of human anatomy needed for dose assessment in both external and internal radiation exposures. Recently, we presented the first whole-body hybrid phantom of the ICRP reference newborn with a skeleton constructed from both non-uniform rational B-spline and polygon-mesh surfaces (Lee et al 2007 Phys. Med. Biol. 52 3309-33). The skeleton in that model included regions of cartilage and fibrous connective tissue, with the remainder given as a homogenous mixture of cortical and trabecular bone, active marrow and miscellaneous skeletal tissues. In the present study, we present a comprehensive skeletal tissue model of the ICRP reference newborn to permit a heterogeneous representation of the skeleton in that hybrid phantom set-both male and female-that explicitly includes a delineation of cortical bone so that marrow shielding effects are correctly modeled for low-energy photons incident upon the newborn skeleton. Data sources for the tissue model were threefold. First, skeletal site-dependent volumes of homogeneous bone were obtained from whole-cadaver CT image analyses. Second, selected newborn bone specimens were acquired at autopsy and subjected to micro-CT image analysis to derive model parameters of the marrow cavity and bone trabecular 3D microarchitecture. Third, data given in ICRP Publications 70 and 89 were selected to match reference values on total skeletal tissue mass. Active marrow distributions were found to be in reasonable agreement with those given previously by the ICRP. However, significant differences were seen in total skeletal and site-specific masses of trabecular and cortical bone between the current and ICRP newborn skeletal tissue models. The latter utilizes an age-independent ratio of 80%/20% cortical and trabecular bone for the reference newborn. In the current study, a ratio closer to 40%/60% is used based upon newborn CT and micro-CT skeletal image analyses. These changes in mineral bone composition may have significant dosimetric implications when considering localized marrow dosimetry for radionuclides that target mineral bone in the newborn child.

Dose to red bone marrow of infants, children and adults from radiation of natural origin

Natural radiation sources contribute much the largest part of the radiation exposure of the average person. This paper examines doses from natural radiation to the red bone marrow, the tissue in which leukaemia is considered to originate, with particular emphasis on doses to children. The most significant contributions are from x-rays and gamma rays, radionuclides in food and inhalation of isotopes of radon and their decay products. External radiation sources and radionuclides other than radon dominate marrow doses at all ages. The variation with age of the various components of marrow dose is considered, including doses received in utero and in each year up to the age of 15. Doses in utero include contributions resulting from the ingestion of radionuclides by the mother and placental transfer to the foetus. Postnatal doses include those from radionuclides in breast-milk and from radionuclides ingested in other foods. Doses are somewhat higher in the first year of life and there is a general slow decline from the second year of life onwards. The low linear energy transfer (LET) component of absorbed dose to the red bone marrow is much larger than the high LET component. However, because of the higher radiation weighting factor for the latter it contributes about 40% of the equivalent dose incurred up to the age of 15.

Doses from the consumption of Cardiff Bay flounder containing organically bound tritium

ICRP dose coefficients for the ingestion of organically bound tritium (OBT) by adults and children are intended for general application to unspecified forms in diet and may not be applicable to intakes of a specific form of OBT. To obtain information relating to OBT in fish from Cardiff Bay, the retention of tritium in adult rats was determined after administration as either tritiated water (HTO) or dried flounder flesh containing OBT. Two components of retention were obtained in each case. The first component, attributable to tritium equilibrating with body water, had a half-time of retention of 3 days in each case, and accounted for 97% of the intake as HTO and 70% after intake of OBT in flounder. Results were consistent with rapid catabolism of a large proportion of flounder OBT to HTO. The second component of retention, attributable to OBT in rat tissues, accounted for 3% of tritium intake as HTO and 30% after intake as flounder OBT; the half-times of retention were 10 days and 25 days, respectively. The results obtained after administration as HTO are consistent with published animal data and correlate with the ICRP assumptions for adult man of half-times of 10 days for 97% behaving as HTO in body tissues and 40 days for 3% incorporated into OBT in body tissues. The results obtained after administration of flounder OBT suggest that appropriate assumptions for retention in adult man are 70% with a 10 day half-time and 30% with a 100 day half-time. These assumptions result in an ingestion dose coefficient of 6 x 10(-11) Sv Bq(-1). This compares with the ICRP value for OBT ingestion by adults of 4.2 x 10(-11) Sv Bq(-1), based on half-times of 10 days and 40 days applied to equal proportions of retained tritium. It is proposed that a dose coefficient of 6 x 10(-11) Sv Bq(-1) should be applied to tritium in flounders from Cardiff Bay. It is further proposed that this dose coefficient should be applied to all ingestion intakes by adults relating to this source of exposure, unless specific information is available showing that a significant proportion of the intake is HTO. The same proposals apply to dose coefficients derived here for flounder OBT consumption by children.

Strontium biokinetic model for the lactating woman and transfer to breast milk: application to Techa River studies

This paper presents a biokinetic model for strontium metabolism in the lactating woman and transfer to breast milk for members of Techa River communities exposed as a result of discharges of liquid radioactive wastes from the Mayak plutonium production facility (Russia) in the early 1950s. This model was based on that developed for the International Commission for Radiological Protection with modifications to account for population specific features of breastfeeding and maternal bone mineral metabolism. The model is based on a biokinetic model for the adult female with allowances made for changes in mineral metabolism during periods of exclusive and partial breast-feeding. The model for females of all ages was developed earlier from extensive data on (90)Sr-body measurements for Techa Riverside residents. Measurements of (90)Sr concentrations in the maternal skeleton and breast milk obtained in the1960s during monitoring of global fallout in the Southern Urals region were used for evaluation of strontium transfer to breast and breast milk. The model was validated with independent data from studies of global fallout in Canada and measurements of (90)Sr body-burden in women living in the Techa River villages who were breastfeeding during maximum (90)Sr-dietary intakes. The model will be used in evaluations of the intake of strontium radioisotopes in breast milk by children born in Techa River villages during the radioactive releases and quantification of (90)Sr retention in the maternal skeleton.

Strontium biokinetic model for the pregnant woman and fetus: application to Techa River studies.

A biokinetic model for strontium (Sr) for the pregnant woman and fetus (Sr-PWF model) has been developed for use in the quantification of doses from internal radiation exposures following maternal ingestion of Sr radioisotopes before or during pregnancy. The model relates in particular to the population of the Techa River villages exposed to significant amounts of ingested Sr radioisotopes as a result of releases of liquid radioactive wastes from the Mayak plutonium production facility (Russia) in the early 1950s. The biokinetic model for Sr metabolism in the pregnant woman was based on a biokinetic model for the adult female modified to account for changes in mineral metabolism during pregnancy. The model for non-pregnant females of all ages was developed earlier with the use of extensive data on (90)Sr-body measurements in the Techa Riverside residents. To determine changes in model parameter values to take account of changing mineral metabolism during pregnancy, data from longitudinal studies of calcium homeostasis during human pregnancy were analysed and applied. Exchanges between maternal and fetal circulations and retention in fetal skeleton and soft tissues were modelled as adaptations of previously published models, taking account of data on Sr and calcium (Ca) metabolism obtained in Russia (Southern Urals and Moscow) relating to dietary calcium intakes, calcium contents in maternal and fetal skeletons and strontium transfer to the fetus. The model was validated using independent data on (90)Sr in the fetal skeleton from global fallout as well as unique data on (90)Sr-body burden in mothers and their still-born children for Techa River residents. While the Sr-PWF model has been developed specifically for ingestion of Sr isotopes by Techa River residents, it is also more widely applicable to maternal ingestion of Sr radioisotopes at different times before and during pregnancy and different ages of pregnant women in a general population.

Retrospective estimation of Plutonium-239 doses from transfer to the fetus for Mayak PA workers

Abstract Purpose: The estimation of plutonium fetal transfer and the calculation of individual in utero and postnatal doses for the Mayak Production Association (PA) offspring cohort. Materials and methods: The model developed by the International Commission on Radiological Protection (ICRP) for the transfer of plutonium to the fetus following maternal intakes before and during pregnancy has been adjusted for application to analysis of the fetal transfer of 239Pu for Mayak workers. Improved estimates of fetal to maternal concentration ratios (CF:CM) have been obtained based on a correlation observed between adult offsprings’ measured daily urine 239Pu activity and estimates of their mothers’ systemic activity at conception. Data on 239Pu activity in daily urine samples were collected from 13 selected adults whose mothers worked at the Mayak PA facility during the period from 1948–1953, before and/or during pregnancy. Results: A comparison of measured and modeled excretion data enabled a mean value of 0.18 ± 0.02 (n = 21) to be inferred for the Pu CF:CM ratio, with a coefficient of variation of 60%. Conclusions: Point estimates of the individual in utero and postnatal absorbed doses for the red bone marrow and liver were in the range 2…13 mGy in 95% of the cases for the cohort of 1936 offspring.

Collateral contamination concomitant to the polonium-210 poisoning of Mr Alexander Litvinenko

Mr Litvinenko died on 23 November 2006, having been poisoned with polonium-210 on 1 November, with evidence of a previous poisoning attempt during October 2006. Measurements of 210Po in urine samples were made for a large number of people to determine whether they may have been contaminated. In the majority of cases, measured levels were attributable to the presence of 210Po from normal dietary sources. For a small number of cases, elevated levels provided evidence of direct contamination associated with the poisonings. For one individual, while estimated doses were below thresholds for irreversible organ damage, a notably increased risk of cancer can be inferred. The use of the chelating agent, unithiol, to increase 210Po excretion in this case was only moderately effective in reducing doses received.