Creation of ORNL NURBS-based phantoms Evaluation of the voxel effect on absorbed doses from radiopharmaceuticals

International audienceDoses from radiopharmaceuticals absorbed by organs can be assessed using Monte Carlo simulations and computational phantoms. Patient-based voxel phantoms improve the realism of organ topology but present unrealistic stair-stepped surfaces. The goal of this research was to study the voxel effect on the basis of creation and voxelisation of a series of non-uniform rational B-spline (NURBS) reference phantoms issued from the publication of the Oak Ridge National Laboratory (ORNL). Absorbed doses from various radiopharmaceuticals were calculated and compared with the values obtained for the corresponding analytical phantoms for models of an adult male and a 5-y-old child. Dose differences lower than 12.5% were observed when the critical structure of the skin was excluded. Moreover, the highest differences were noted for small organs and walls. Finally, all NURBS phantoms of the ORNL series, their voxelised version and the corresponding Monte Carlo N-Particle eXtended input files were programmed and are available for further simulations. © The Author 2012. Published by Oxford University Press. All rights reserved

Simple polynomial approximation to modified Bethe formula low-energy electron stopping powers data

International audienceA recently published detailed and exhaustive paper on cross-sections for ionisation induced by keV electrons clearly shows that electron phenomena occurring in parallel with X-ray processes may have been dramatically overlooked for many years, mainly when low atomic number species are involved since, in these cases, the fluorescence coefficient is smaller than the Auger yield. An immediate problem is encountered while attempting to tackle the issue. Accounting for electron phenomena requires the knowledge of the stopping power of electrons within, at least, a reasonably small error. Still, the Bethe formula for stopping powers is known to not be valid for electron energies below 30 keV, and its use leads to values far off experimental ones. Recently, a few authors have addressed this problem and both detailed tables of electron stopping powers for various atomic species and attempts to simplify the calculations, have emerged. Nevertheless, its implementation in software routines to efficiently calculate keV electron effects in materials quickly becomes a bit cumbersome. Following a procedure already used to establish efficient methods to calculate ionisation cross-sections by protons and alpha particles, it became clear that a simple polynomial approximation could be set, which allows retrieving the electronic stopping powers with errors of less than 20% for energies above 500 eV and less than 50% for energies between 50 eV and 500 eV. In this work, we present this approximation which, based on just six parameters, allows to recover electron stopping power values that are less than 20% different from recently published experimentally validated tabulated data. © 2015 Elsevier B.V. All rights reserved

Evaluation of absorbed and effective doses to patients from radiopharmaceuticals using the ICRP 110 reference computational phantoms and ICRP 103 formulation

International audienceIn diagnostic nuclear medicine, mean absorbed doses to patients' organs and effective doses are published for standard stylised anatomic models. To provide more realistic and detailed geometries of the human morphology, the International Commission on Radiological Protection (ICRP) has recently adopted male and female voxel phantoms to represent the reference adult. This work investigates the impact of the use of these new computational phantoms. The absorbed doses were calculated for 11 different radiopharmaceuticals currently used in diagnostic nuclear medicine. They were calculated for the ICRP 110 reference computational phantoms using the OEDIPE software and the MCNP extended Monte Carlo code. The biokinetic models were issued from ICRP Publications 53, 80 and 106. The results were then compared with published values given in these ICRP Publications. To discriminate the effect of anatomical differences on organ doses from the effect of the calculation method, the Monte Carlo calculations were repeated for the reference adult stylised phantom. The voxel effect, the influence of the use of different densities and nuclear decay data were also investigated. Effective doses were determined for the ICRP 110 adult reference computational phantom with the tissue weighting factor of ICRP Publication 60 and the tissue weighting factors of ICRP Publication 103. The calculation method and, in particular, the simulation of the electron transport have a significant influence on the calculated doses, especially, for small and walled organs. Overestimates of >200 % were observed for the urinary bladder wall of the stylised phantom compared with the computational phantoms. The unrealistic organ topology of the stylised phantom leads to important dose differences, sometimes by an order of magnitude. The effective doses calculated using the new computational phantoms and the new tissue weighting factors are globally lower than the published ones, except for some radiopharmaceuticals, where the differences can reach 60 % higher than the published values. This study analyses the first set of absorbed and effective doses with the new ICRP male and female reference computational phantoms for different radiopharmaceuticals. It highlights the importance of taking into account the electron transport and the realism of the shape and inter-organ distances of the anthropomorphic model used. © The Author 2013. Published by Oxford University Press. All rights reserved

Dosimetry at the sub-cellular scale of Auger-electron emitter 99mTc in a mouse single thyroid follicle

International audienceThe Auger-electrons emitted by 99mTc have been recently associated with the induction of thyroid stunning in in vivo experiments in mice, making the dosimetry at the sub-cellular level of 99mTc a pertinent and pressing subject. The S-values for 99mTc were calculated using MCNP6, which was first validated for studies at the sub-cellular scale and for low energies electrons. The calculation was then performed for 99mTc within different cellular compartments in a single mouse thyroid follicle model, considering the radiative and non-radiative transitions of the 99mTc radiation spectrum. It was shown that the contribution of the 99mTc Auger and low energy electrons to the absorbed dose to the follicular cells' nucleus is important, being at least of the same order of magnitude compared to the emitted photons' contribution and cannot be neglected. The results suggest that Auger-electrons emitted by 99mTc play a significant role in the occurrence of the thyroid stunning effect in mice. © 2015 Elsevier Ltd

Development of a dosimetric model for in vitro labelled cells with β+ emitters in PET tracking studies

International audiencePositron emission tomography (PET) offers an effective method for tracking β+ emitters-labeled cells in vivo. However, in vitro high labelling activities used may cause cell damage or death. Our understanding of the impact of such procedure remains limited by the fact that the biological effects are usually linked to the activity per cell rather than the absorbed dose. To assess the dose delivered to the cells during the radiolabelling, a multi-cellular dosimetry computational tool was developed, allowing the study of two key parameters the cell density and the labelling efficiency. Through a hybrid method based on Monte Carlo simulations (MCNP6 code) and an analytical approach implemented in Python, the mean absorbed dose received by a target cell was calculated for distributions with a very large number of cells - up to hundreds of millions. An advanced investigation of in vitro cell labelling with β-emitting radionuclides was carried out via (i) a systematic study of the effects of the labelling parameters on the cell absorbed dose for18F,64Cu and68Ga, and (ii) a quantitative comparison between cellular and conventional dosimetry. The results provided a thorough analysis of how the dose (self, cross and extracellular medium dose contributions) varies with the initial labelling parameters selected and highlighted the conditions where the cellular dosimetry is required over the conventional dosimetry. The dosimetric model was finally applied to real conditions of 18F-FDG labelling on the basis of eight reported studies. The results showed that similar activity per cell can lead to significantly different absorbed dose and pointed out differences between cellular and conventional dosimetry up to a factor of 5. © 2019 Institute of Physics and Engineering in Medicine

Energetic electron processes fluorescence effects for structured nanoparticles X-ray analysis and nuclear medicine applications

International audienceSuperparamagnetic iron oxide (SPIO) nanoparticles are widely used as contrast agents for nuclear magnetic resonance imaging (MRI), and can be modified for improved imaging or to become tissue-specific or even protein-specific. The knowledge of their detailed elemental composition characterisation and potential use in nuclear medicine applications, is, therefore, an important issue. X-ray fluorescence techniques such as particle induced X-ray emission (PIXE) or X-ray fluorescence spectrometry (XRF), can be used for elemental characterisation even in problematic situations where very little sample volume is available. Still, the fluorescence coefficient of Fe is such that, during the decay of the inner-shell ionised atomic structure, keV Auger electrons are produced in excess to X-rays. Since cross-sections for ionisation induced by keV electrons, for low atomic number atoms, are of the order of 103 barn, care should be taken to account for possible fluorescence effects caused by Auger electrons, which may lead to the wrong quantification of elements having atomic number lower than the atomic number of Fe. Furthermore, the same electron processes will occur in iron oxide nanoparticles containing 57Co, which may be used for nuclear medicine therapy purposes. In the present work, simple approximation algorithms are proposed for the quantitative description of radiative and non-radiative processes associated with Auger electrons cascades. The effects on analytical processes and nuclear medicine applications are quantified for the case of iron oxide nanoparticles, by calculating both electron fluorescence emissions and energy deposition on cell tissues where the nanoparticles may be embedded. © 2016 Elsevier B.V. All rights reserved

Microdosimetry of alpha particles for simple and 3d voxelised geometries using MCNPX and Geant4 monte carlo codes

International audienceMicrodosimetry using Monte Carlo simulation is a suitable technique to describe the stochastic nature of energy deposition by alpha particle at cellular level. Because of its short range, the energy imparted by this particle to the targets is highly non-uniform. Thus, to achieve accurate dosimetric results, the modelling of the geometry should be as realistic as possible. The objectives of the present study were to validate the use of the MCNPX and Geant4 Monte Carlo codes for microdosimetric studies using simple and three-dimensional voxelised geometry and to study their limit of validity in this last case. To that aim, the specific energy (z) deposited in the cell nucleus, the single-hit density of specific energy f 1(z) and the mean-specific energy 〈z 1〉 were calculated. Results show a good agreement when compared with the literature using simple geometry. The maximum percentage difference found is andlt;6 %. For voxelised phantom, the study of the voxel size highlighted that the shape of the curve f 1(z) obtained with MCNPX for andlt;1 μ m voxel size presents a significant difference with the shape of non-voxelised geometry. When using Geant4, little differences are observed whatever the voxel size is. Below 1 μ m, the use of Geant4 is required. However, the calculation time is 10 times higher with Geant4 than MCNPX code in the same conditions. © The Author 2011. Published by Oxford University Press. All rights reserved

Application of the new ICRP reference phantoms to internal dosimetry: Calculation of specific absorbed fractions of energy for photons and electrons.

Introduction: Connecting the emission of radiation from a contaminated body region with the dose received by a radio-sensitive tissue, the specific absorbed fraction (SAF) of energy is an essential element of internal dose assessment. Here is reported a set of specific absorbed fractions calculated using the male and female reference computational phantoms recently published by the International Commission on Radiological Protection (ICRP). This work was performed simultaneously at the Helmholtz Zentrum München (HMGU, Germany) and IRSN (France) for quality assurance purpose. The results were then compared to the SAF values for mathematic phantoms. At IRSN, the Monte Carlo transport code MCNPX version 2.6f was used to simulate monoenergetic photons and electrons with energies ranging from 15 keV to 10 MeV. The OEDIPE software, developed by IRSN, was used to create the MCNPX input file describing the two voxel phantoms. The particles were emitted from three source organs: lungs, thyroid and liver. SAFs were calculated for several target regions in the body (lungs, colon wall, breast, stomach wall) and compared with the results obtained at HMGU using the EGSnrc Monte Carlo code. The results show general agreement for photons and high-energy electrons with discrepancies less than 6%. Nevertheless, significant differences were found for electrons of lower energy due to statistical uncertainties larger than 10%. The comparison of the SAF values between the new ICRP voxel phantoms and the mathematic ones shows significant differences. The present SAFs calculation for the new ICRP reference phantoms is validated by the intercomparison of results obtained by HGMU and IRSN and gives an insight into the evolution from the former SAFs derived from stylized phantoms

Application of the ICRP/ICRU reference computational phantoms to internal dosimetry: Calculation of specific absorbed fractions of energy for photons and electrons

The emission of radiation from a contaminated body region is connected with the dose received by radiosensitive tissue through the specific absorbed fractions (SAFs) of emitted energy, which is therefore an essential quantity for internal dose assessment. A set of SAFs were calculated using the new adult reference computational phantoms, released by the International Commission on Radiological Protection (ICRP) together with the International Commission on Radiation Units and Measurements (ICRU). Part of these results has been recently published in ICRP Publication 110 (2009 Adult reference computational phantoms (Oxford: Elsevier)). In this paper, we mainly discuss the results and also present them in numeric form. The emission of monoenergetic photons and electrons with energies ranging from 10 keV to 10 MeV was simulated for three source organs: lungs, thyroid and liver. SAFs were calculated for four target regions in the body: lungs, colon wall, breasts and stomach wall. For quality assurance purposes, the simulations were performed simultaneously at the Helmholtz Zentrum München (HMGU, Germany) and at the Institute for Radiological Protection and Nuclear Safety (IRSN, France), using the Monte Carlo transport codes EGSnrc and MCNPX, respectively. The comparison of results shows overall agreement for photons and high-energy electrons with differences lower than 8%. Nevertheless, significant differences were found for electrons at lower energy for distant source/target organ pairs. Finally, the results for photons were compared to the SAF values derived using mathematical phantoms. Significant variations that can amount to 200% were found. The main reason for these differences is the change of geometry in the more realistic voxel body models. For electrons, no SAFs have been computed with the mathematical phantoms; instead, approximate formulae have been used by both the Medical Internal Radiation Dose committee (MIRD) and the ICRP due to the limitations imposed by the computing power available at this time. These approximations are mainly based on the assumption that electrons are absorbed locally in the source organ itself. When electron SAFs are calculated explicitly, discrepancies with this simplifying assumption are notable, especially at high energies and for neighboring organs where the differences can reach the same order of magnitude as for photon SAFs. © 2010 Institute of Physics and Engineering in Medicine