A practical method for assessment of dose conversion coefficients for aquatic biota

Radiological impact assessment for flora and fauna requires adequate dosimetric data. Due to the variability of habitats, shapes, and masses of the non-human biota, assessment of doses is a challenging task. External and internal dose conversion coefficients for photons and electrons have been systematically calculated by Monte Carlo methods for spherical and ellipsoidal shapes in water medium. An interpolation method has been developed to approximate absorbed fractions for elliptical shape organisms from absorbed fractions for spherical shapes with reasonable accuracy. The method allows an evaluation of dose conversion coefficients for arbitrary ellipsoids for photon and electron sources with energies from 10 keV to 5 MeV, and for organism masses in the range from 10(-6) to 10(3) kg. As an example of the application of the method, a set of dose coefficients for aquatic organisms discussed as reference animals and plants in a draft of an up-coming publication of the International Commission on Radiological Protection has been determined.     

Inter-comparison of absorbed dose rates for non-human biota

A number of approaches have been proposed to estimate the exposure of non-human biota to ionizing radiation. This paper reports an inter-comparison of the unweighted absorbed dose rates for the whole organism (compared as dose conversion coefficients, or DCCs) for both internal and external exposure, estimated by 11 of these approaches for selected organisms from the Reference Animals and Plants geometries as proposed by the International Commission on Radiological Protection. Inter-comparison results indicate that DCCs for internal exposure compare well between the different approaches, whereas variation is greater for external exposure DCCs. Where variation among internal DCCs is greatest, it is generally due to different daughter products being included in the DCC of the parent. In the case of external exposures, particularly to low-energy beta-emitters, variations are most likely to be due to different media densities being assumed. On a radionuclide-by-radionuclide basis, the different approaches tend to compare least favourably for (3)H, (14)C and the alpha-emitters. This is consistent with models with different source/target geometry assumptions showing maximum variability in output for the types of radiation having the lowest range across matter. The intercomparison demonstrated that all participating approaches to biota dose calculation are reasonably comparable, despite a range of different assumptions being made.     

Tables of dose conversion coefficients for estimating internal and external radiation exposures to terrestrial and aquatic biota

Dose conversion coefficients (DCCs) for assessment of internal and external radiation exposures to terrestrial and aquatic biota are compiled for 75 radionuclides, for 14 terrestrial and 22 aquatic reference organisms. DCC values for internal exposure are calculated based on a homogeneous distribution of the radionuclides in both types of organisms. DCC values for external exposure of aquatic organisms are calculated for complete immersion in water. For external exposure of terrestrial organisms the soil is considered as a planar and homogenously contaminated volume source with a surface roughness of 3 mm and a thickness of 10 cm, respectively. For in-soil-organisms, DCC values for external exposure are given assuming that these organisms live in the middle of a uniformly contaminated 50 cm-thick soil layer. The tables can be used for assessment of exposures of animals and plants living in various habitats. The list of considered organisms covers the Reference Animals and Plants as adopted by the ICRP.     

A method for calculating the dose absorbed by terrestrial animals to assess the environmental impact of radioactive contamination

A method for calculating the exposures of terrestrial animals in areas contaminated with radionuclides using a point source dose function is presented. To take into account scattered γ-radiation, the Berger formula for dose buildup factor in an infinite air medium has been parameterized. In the dosimetric model proposed, an animal phantom is presented as a parallelepiped to estimate external exposures and as a tissue-quivalent sphere to estimate internal doses. Using analytical expressions, dose rate conversion coefficients for external and internal exposures of animals have been estimated for individual radionuclides. For energies of γ-rays above 50 keV, the results are in good agreement with those estimated by the Monte Carlo method for ellipsoidal phantoms of animals.     

Radiological Benchmarks for Effects on Aquatic Biota at the Oak Ridge Reservation

Radiological benchmarks for aquatic biota were developed for use at the U.S. Department of Energy's Oak Ridge Reservation as screening values to determine the spatial extent of potential ecological effects and to identify the need for additional site-specific investigation. The Point Source Dose Distribution approach was used to calculate water and sediment activities for selected radionuclides that result in a total dose rate to small and large fish of 1 Rad d^?1, which is the National Council on Radiation Protection and Measurements recommended acceptable dose rate to natural populations of aquatic biota. These screening values incorporate internal and external exposures from parent isotopes and all short-lived daughter products. They also include exposures from all major alpha, beta, and gamma emissions for each isotope. Unlike exposures to chemicals, exposures to radionuclides are expressed as the dose rate received by the organism. Dose rates that account for the biological effects to the organism are additive. If the total dose rate from all radionuclides and pathways exceeds a recommended acceptable dose rate, further analysis is needed to determine the hazards posed by radionuclides. If, however, the total dose rate falls below an acceptable dose rate, radionuclides may be eliminated from further study.     

Calculating the radiological parameters used in non-human biota dose assessment tools using ERICA Tool and site-specific data

The substantial complexity in ecosystem–radionuclide interactions is difficult to be represented in terms of radiological doses. Thus, radiological dose assessment tools use typical exposure situations for generalized organisms and ecosystems. In the present study, site-specific data and radioactivity measurements of terrestrial organisms (grass and herbivore mammals) and abiotic components (soil) are provided. The retrieved data are used in combination with the ERICA Assessment Tool for calculation of radiological parameters. The process of radionuclide transfer within ecosystem components is represented using concentration ratios (CRs), while for the calculation of dose rates the dose conversion coefficient (DCC) methodology is applied. Comparative assessments are performed between the generic and assessment-specific radiological parameters and between the resulting dose rates. Significant differences were observed between CRs calculated in this study and those reported in the literature for cesium and thorium, which can easily be explained. On the other hand, CRs calculated for radium are in very good agreement with those reported in the literature. The DCCs exhibited some small differences between the reference and the assessment-specific organism due to mass differences. The differences were observed for internal and external dose rates, but they were less pronounced for total dose rates which are typically used in the assessment of radiological impact. The results of the current work can serve as a basis for further studies of the radiological parameters in environments that have not been studied yet.     

Absorbed doses in tissue-equivalent spheres above radioactive sources in soil

Doses due to external exposure of terrestrial biota are assessed using differential air kerma from radioactive sources in soil and energy-dependent ‘absorbed dose-per-air kerma’ conversion factors computed for spherical tissue-equivalent bodies. The presented approach allows computing average whole body absorbed dose for terrestrial organisms with body masses from 1 mg to 1,000 kg located at heights from 10 cm to 500 m above ground. Radioactive sources in soil emitting photons with energies from 10 keV to 10 MeV have been considered. Interpolation of the computed quantities over source energy, body mass, and height above ground results in plausible estimates of whole body average absorbed doses for non-human terrestrial biota from gamma-radiation emitted by any radionuclides in contaminated terrain.     

Monte Carlo derived absorbed fractions for a voxelized model of Oncorhynchus mykiss,a rainbow trout

Simple, ellipsoidal geometries have long been the standard for estimating radiation dose rates in non-human biota (NHB). With the introduction of a regulatory protection standard that emphasizes protection of NHB as its own end point, there has been interest in improved models for the calculation of dose rates in NHB. Here, we describe the creation of a voxelized model for a rainbow trout (Oncorhynchus mykiss), a freshwater aquatic salmonid. Absorbed fractions (AFs) for both photon and electron sources were tabulated at electron energies of 0.1, 0.2, 0.4, 0.5, 0.7, 1.0, 1.5, 2.0, and 4.0 MeV and photon energies of 0.01, 0.015, 0.02, 0.03, 0.05, 0.1, 0.2, 0.5, 1.0, 1.5, 2.0, and 4.0 MeV. A representative set of the data is made available in this publication; the entire set of absorbed fractions is available as electronic supplementary materials. These results are consistent with previous voxelized models and reinforce the well-understood relationship between the AF and the target’s mass and location, as well as the energy of the incident radiation.     

Visualization of biodistribution of Zn complex with antidiabetic activity using semiconductor Compton camera GREI

Various types of zinc (Zn) complexes have been developed as promising antidiabetic agents in recent years. However, the pharmacological action of Zn complex is not elucidated because the biodistribution of the complex in a living organism has not been studied. Nuclear medicine imaging is superior technology for the noninvasive analysis of the temporal distribution of drug candidates in living organisms. Gamma-ray emission imaging (GREI), which was developed by our laboratory as a novel molecular imaging modality, was adopted to visualize various γ-ray–emitting radionuclides that are not detected by conventional imaging techniques such as positron emission tomography and single-photon emission computed tomography. Therefore, we applied GREI to a biodistribution assay of Zn complexes. In the present study, ⁶⁵Zn was produced in the ^natCu(p,n) reaction in an azimuthal varying field cyclotron for the GREI experiment. The distribution was then noninvasively visualized using GREI after the intravenous administration of a ⁶⁵Zn-labeled di(1-oxy-2-pyridinethiolato)zinc [Zn(opt)₂], ZnCl₂, and di(l-histidinato)zinc. The GREI images were validated using conventional invasive assays. This novel study showed that GREI is a powerful tool for the biodistribution analysis of antidiabetic Zn complexes in a living organism. In addition, accumulation of ⁶⁵Zn in the cardiac blood pool was observed for [Zn(opt)₂], which exhibits potent antidiabetic activity. These results suggest that the slow elimination of Zn from the blood is correlated to the antidiabetic activity of [Zn(opt)₂].     

Dose estimation for astronauts using dose conversion coefficients calculated with the PHITS code and the ICRP/ICRU adult reference computational phantoms

Absorbed-dose and dose-equivalent rates for astronauts were estimated by multiplying fluence-to-dose conversion coefficients in the units of Gy.cm² and Sv.cm², respectively, and cosmic-ray fluxes around spacecrafts in the unit of cm⁻² s⁻¹. The dose conversion coefficients employed in the calculation were evaluated using the general-purpose particle and heavy ion transport code system PHITS coupled to the male and female adult reference computational phantoms, which were released as a common ICRP/ICRU publication. The cosmic-ray fluxes inside and near to spacecrafts were also calculated by PHITS, using simplified geometries. The accuracy of the obtained absorbed-dose and dose-equivalent rates was verified by various experimental data measured both inside and outside spacecrafts. The calculations quantitatively show that the effective doses for astronauts are significantly greater than their corresponding effective dose equivalents, because of the numerical incompatibility between the radiation quality factors and the radiation weighting factors. These results demonstrate the usefulness of dose conversion coefficients in space dosimetry.     

Effective dose conversion coefficients for radionuclides exponentially distributed in the ground

In order to provide fundamental data required for dose evaluation due to environmental exposures, effective dose conversion coefficients, that is, the effective dose rate per unit activity per unit area, were calculated for a number of potentially important radionuclides, assuming an exponential distribution in ground, over a wide range of relaxation depths. The conversion coefficients were calculated for adults and a new-born baby on the basis of dosimetric methods that the authors and related researchers have previously developed, using Monte Carlo simulations and anthropomorphic computational phantoms. The differences in effective dose conversion coefficients due to body size between the adult and baby phantoms were found to lie within 50?%, for most cases; however, for some low energies, differences could amount to a factor of 3. The effective dose per unit source intensity per area was found to decrease by a factor of 2–5, for increasing relaxation depths from 0 to 5?g/cm², above a source energy of 50?keV. It is also shown that implementation of the calculated coefficients into the computation of the tissue weighting factors and the adult reference computational phantoms of ICRP Publication 103 does not significantly influence the effective dose conversion coefficients of the environment. Consequently, the coefficients shown in this paper could be applied for the evaluation of effective doses, as defined according to both recommendations of ICRP Publications 103 and 60.     

External radiation doses from <Superscript>137</Superscript>Cs to frog phantoms in a wetland area: in situ measurements and dose model calculations

For assessment of external radiation doses to frogs in a wetland area contaminated with ¹³⁷Cs, frog phantoms were constructed from polymethyl methacrylate (PMMA). The frog phantoms contained thermoluminescence (TL) chips and were used in situ at two study sites to measure doses. To test if higher doses are received by the sensitive skin of frogs, extra-thin TL chips were applied close to the surface of the frog phantoms. In addition, the measured doses were compared with those calculated on the basis of soil sample data from the wetland multiplied with dose-conversion coefficients from the US Department of Energy’s RESRAD-BIOTA code and from the ERICA assessment tool. Measured doses were generally lower than those calculated to ellipsoids used to model frogs. Higher doses were measured at the frog phantoms’ surfaces in comparison to inner parts at one of the two sites indicating that the frogs’ thin skin could receive a higher radiation dose than expected. In the efforts to assure protection of non-human biota, in situ measurements with phantoms provide valuable dose information and input to dose models in site-specific risk assessments of areas contaminated with radionuclides.     

Distribution of 226Ra body burden of workers in an underground uranium mine in India

Uranium mine workers are exposed to ore dust containing uranium and its daughter products during different mining operations. These radionuclides may pose inhalation hazards to workers during the course of their occupation. The most significant among these radionuclides is ²²⁶Ra. The measurement of radium body burden of uranium mine workers is important to assess their internal exposure. For this purpose, the radon-in-breath measurement technique has been used in the present paper. Workers at the Jaduguda mine, India, associated with different categories of mining operations were monitored between 2001 and 2007. The measurement results indicate that workers—depending on mining operation category—show ²²⁶Ra body burdens ranging from 0.15 to 2.85 kBq. The maximum body burden was found for workers associated with timbering operations, with an average ²²⁶Ra body burden of 0.85 ± 0.54 kBq. Overall, the average value observed for 800 workers was 0.76 ± 0.51 kBq, which gives rise to an average effective dose of 1.67 mSv per year for inhalation and 0.21 mSv per year for ingestion.     

Global searches for microalgae and aquatic plants that can eliminate radioactive cesium, iodine and strontium from the radio-polluted aquatic environment: a bioremediation strategy

The Fukushima 1 Nuclear Power Plant accident in March 2011 released an enormously high level of radionuclides into the environment, a total estimation of 6.3 × 10¹⁷ Bq represented by mainly radioactive Cs, Sr, and I. Because these radionuclides are biophilic, an urgent risk has arisen due to biological intake and subsequent food web contamination in the ecosystem. Thus, urgent elimination of radionuclides from the environment is necessary to prevent substantial radiopollution of organisms. In this study, we selected microalgae and aquatic plants that can efficiently eliminate these radionuclides from the environment. The ability of aquatic plants and algae was assessed by determining the elimination rate of radioactive Cs, Sr and I from culture medium and the accumulation capacity of radionuclides into single cells or whole bodies. Among 188 strains examined from microalgae, aquatic plants and unidentified algal species, we identified six, three and eight strains that can accumulate high levels of radioactive Cs, Sr and I from the medium, respectively. Notably, a novel eustigmatophycean unicellular algal strain, nak 9, showed the highest ability to eliminate radioactive Cs from the medium by cellular accumulation. Our results provide an important strategy for decreasing radiopollution in Fukushima area.     

Detection of intracellular neutral lipid content in the marine microalgae Prorocentrum micans and Phaeodactylum tricornutum using Nile red and BODIPY 505/515

Diatoms and dinoflagellates not only have extensive distribution and a huge biomass in marine ecosystems, but also have high lipid accumulation in nature or after physiological and genetic modification, which indicates that these organisms may be optimal candidate algal strains for biodiesel production. In this study, we determined the content of intracellular neutral lipids (triacylglycerol [TAG]) in the dinoflagellate Prorocentrum micans and in the diatom Phaeodactylum tricornutum using NR and BODIPY 505/515 staining. The freshwater green alga Scenedesmus obliquus was used as a control. Optimum concentrations of 1.000 and 1.500 μg mL^?1 were determined for neutral lipid Nile red (NR) staining in P. micans and P. tricornutum. Unlike NR staining, the optimal concentrations of BODIPY 505/515 staining in P. micans and P. tricornutum were lower, at 0.100 and 0.075 μg mL^?1, respectively. High correlation coefficients of R ²?=?0.990 and R ²?=?0.989 were obtained for P. micans and P. tricornutum intracellular neutral lipid content and the relative fluorescence intensity with NR staining, while the reference alga, S. obliquus, had a relatively low correlation coefficient of R ²?=?0.908 when stained with NR. The neutral lipid content determined by thin-layer chromatography-flame ionization detector matched the analytical data from fluorescence measurements. These results indicated that NR and BODIPY 505/515 staining can be used as an excellent high-throughput approach to screen marine diatoms and dinoflagellates.     

Finding sensitive parameters in internal dose calculations for radiopharmaceuticals commonly used in clinical nuclear medicine

Internal dosimetry after incorporation of radionuclides requires standardized biokinetic and dosimetric models. The aim of the present work was to identify the parameters and the components of the models which contribute most to dosimetric uncertainty. For this a method was developed allowing for the calculation of the uncertainties of the absorbed dose coefficients. More specifically, the sampling-based regression method and the variance-based method were used to develop and apply a global method of sensitivity analysis. This method was then used to quantify the impact of various biokinetic and dosimetric parameters on the uncertainty of internal doses associated with the incorporation of seven common radiopharmaceuticals. It turned out that the correlation between biokinetic parameters and time-integrated activity or calculated absorbed dose is strongest when the source and target organ are identical, in accordance with the ICRP and the MIRD approach. According to the ICRP approach, the parameter F_s which describes the fractional distribution of any incorporated radioactivity to organ S, has the greatest correlation with the time-integrated activity in the corresponding source organ or with the calculated dose in the corresponding target organ. In contrast, the MIRD approach suggested several biokinetic parameters with similar correlation. The dosimetric parameters usually contribute more to uncertainty in the calculated dose coefficients than the biokinetic parameters, in both approaches. The results obtained are helpful for the revision of biokinetic models for radiopharmaceuticals, because the most important parameters in clinical applications can now be identified and investigated in future studies.     

A cost-effective method to quantify biological surface sediment reworking

We propose a simple and inexpensive method to determine the rate and pattern of surface sediment reworking by benthic organisms. Unlike many existing methods commonly used in bioturbation studies, which usually require sediment sampling, our approach is fully non-destructive and is well suited for investigating non-cohesive fine sediments in streams and rivers. Optical tracer (e.g. luminophores or coloured sand) disappearance or appearance is assessed through time based on optical quantification of surfaces occupied by tracers. Data are used to calculate surface sediment reworking (SSR) coefficients depicting bioturbation intensities. Using this method, we evaluated reworking activity of stream organisms (three benthic invertebrates and a fish) in laboratory microcosms mimicking pool habitats or directly in the field within arenas set in depositional zones. Our method was sensitive enough to measure SSR as low as 0.2 cm² day^?1, such as triggered by intermediate density (774 m^?2) of Gammarus fossarum (Amphipoda) in microcosms. In contrast, complex invertebrate community in the field and a fish (Barbatula barabatula) in laboratory microcosms were found to yield to excessively high SSR (>60 cm² day^?1). Lastly, we suggest that images acquired during experiments can be used for qualitative evaluation of species-specific effects on sediment distribution.     

Dosimetric comparison between realistic ocular model and other models for COMS plaque brachytherapy with <Superscript>103</Superscript>Pd, <Superscript>131</Superscript>Cs,and <Superscript>125</Superscript>I radioisotopes

Nowadays, Monte Carlo calculations are commonly used for the evaluation of dose distributions and dose volume histograms in eye brachytherapy. However, currently available eye models have simple geometries, and main substructures of the eye are either not defined in details or not distinguished at all. In this work absorbed doses of eye substructures have been estimated for eye plaque brachytherapy using the most realistic eye model available, and compared with absorbed doses obtained with other available eye models. For this, a medium-sized tumour on the left sides of the right eye was considered. Dosimetry calculations were performed for four different eye models developed based on a literature review, and using a 12 mm Collaborative Ocular Melanoma Study plaque containing ¹³¹Cs, ¹⁰³Pd, and ¹²⁵I sources. Obtained results illustrate that the estimated doses received by different eye substructures strongly depend on the model used to represent the eye. It is shown here that using a non-realistic eye model leads to a wrong estimation of doses for some eye substructures. For example, dose differences of up to 35% were observed between the models proposed by Nogueira and co-workers and Yoriyaz and co-workers, while doses obtained by use of the models proposed by Lesperance and co-workers, and Behrens and co-workers differed up to 100 and 63% as compared to the situation when a realistic model was used, respectively. Moreover, comparing different radionuclides showed that the most uniform dose distribution in the considered tumour region was that from ¹³¹Cs, with a coefficient of variation of 33%. In addition, considering the realistic eye model, it was found that the radiosensitive region of the lens received more than the threshold dose of cataract induction (0.5 Gy), for all investigated radionuclides.     

Fish otoliths as biological dosimeter: internal dose calculation

Otoliths are organs used by fish for hearing and keeping balance. They consist of biogenic crystals of hydroxyapatite and do not contain any living cells. Upon exposure to ionizing radiation, otolith hydroxyapatite accumulates radiation-induced stable CO₂^? radicals whose amount is proportional to absorbed dose. In electron paramagnetic resonance (EPR) dosimetry, carbonate ions are registered and, hence, the total accumulated dose in the fish otolith can be quantified. Therefore, otoliths can be used as individual fish dosimeters to support radiobiological and radioecological studies. An important aspect of otolith-based EPR dosimetry on fish from contaminated water bodies is the potential presence of bone-seeking ⁹⁰Sr. Consequently, cumulative absorbed doses measured with EPR in otoliths may reflect the superposition of internal exposure to ⁹⁰Sr/⁹⁰Y and external exposure due to radionuclides circulating in soft tissue of the fish as well as due to environmental contamination. The objective of the present study was to develop a method that allows for an assessment of the contribution of ⁹⁰Sr to the total dose in otolith. The method has been tested using otoliths from seven fish taken from reservoirs located in the Southern Urals contaminated with radionuclides including ⁹⁰Sr. It has been shown that dose to otoliths is largely determined by ⁹⁰Sr in the hydroxyapatite. The internal dose component can be calculated using activity concentration-to-dose conversion factors, which vary slightly in the range of 2.0–2.8?×?10^–3 Gy year^?1 per Bq g^?1 depending on fish species and age. Internal doses to fish from water bodies with different levels of ⁹⁰Sr contamination were calculated in the range from 2 mGy to?~?200 Gy. External dose contribution was derived for two fish only to be about 100 and 40 Gy. It is concluded that EPR dosimetry on fish otoliths is a promising tool when external exposure prevails or is comparable to internal exposure due to ⁹⁰Sr.

     

A Monte Carlo approach to small-scale dosimetry of solid tumour microvasculature for nuclear medicine therapies with 223Ra-, 131I-, 177Lu- and 111In-labelled radiopharmaceuticals

The small-scale dosimetry of radionuclides in solid-tumours is directly related to the intra-tumoral distribution of the administered radiopharmaceutical, which is affected by its egress from the vasculature and dispersion within the tumour. The aim of the present study was to evaluate the combined dosimetric effects of radiopharmaceutical distribution and range of the emitted radiation in a model of tumour microvasculature.We developed a computational model of solid-tumour microenvironment around a blood capillary vessel, and we simulated the transport of radiation emitted by ²²³Ra, ¹¹¹In, ¹³¹I and ¹⁷⁷Lu using the GEANT4 Monte Carlo. For each nuclide, several models of radiopharmaceutical dispersion throughout the capillary vessel were considered.Radial dose profiles around the capillary vessel, the Initial Radioactivity (IR) necessary to deposit 100 Gy of dose at the edge of the viable tumour-cell region, the Endothelial Cell Mean Dose (ECMD) and the Tumour Edge Mean Dose (TEMD), i.e. the mean dose imparted at the 250-μm layer of tissue, were computed. The results for beta and Auger emitters demonstrate that the photon dose is about three to four orders of magnitude lower than that deposited by electrons. For ²²³Ra, the beta emissions of its progeny deliver a dose about three orders of magnitude lower than that delivered by the alpha emissions.Such results may help to characterize the dose inhomogeneities in solid tumour therapies with radiopharmaceuticals, taking into account the interplay between drug distribution from vasculature and range of ionizing radiations.