Geant4 hadronic physics for space radiation environment

Abstract

Purpose: To test and to develop Geant4 (Geometry And Tracking version 4) Monte Carlo hadronic models with focus on applications in a space radiation environment.

Materials and methods: The Monte Carlo simulations have been performed using the Geant4 toolkit. Binary (BIC), its extension for incident light ions (BIC-ion) and Bertini (BERT) cascades were used as main Monte Carlo generators. For comparisons purposes, some other models were tested too. The hadronic testing suite has been used as a primary tool for model development and validation against experimental data.

Results: The Geant4 pre-compound (PRECO) and de-excitation (DEE) models were revised and improved. Proton, neutron, pion, and ion nuclear interactions were simulated with the recent version of Geant4 9.4 and were compared with experimental data from thin and thick target experiments.

Conclusions: The Geant4 toolkit offers a large set of models allowing effective simulation of interactions of particles with matter. We have tested different Monte Carlo generators with our hadronic testing suite and accordingly we can propose an optimal configuration of Geant4 models for the simulation of the space radiation environment.     

The Relevance of Tumour and Surrounding Normal Tissue Vascular Density in Clinical Hyperthermia of Locally Advanced Breast Carcinoma

Abstract

Purpose: To compare dose distributions on the central- and off-axis for ¹²C and ⁷Li ion beams simulated by the codes SHIELD-HIT (Heavy Ion Transport) and FLUKA (FLUKtuierende KAskade), and compare with experimental data for 300 MeV/u ¹²C and 185 MeV/u ⁷Li ion beams.

Materials and methods: The general purpose Monte Carlo codes, SHIELD-HIT10 and FLUKA 2008.3d.1 were used for the ion dose distribution calculations. SHIELD-HIT transports hadrons and atomic nuclei of arbitrary charge and mass number in an energy range from 1 keV/u up to 1 GeV/u. Similarly, FLUKA transports charged hadrons in an energy range from 100 keV up to 20 TeV. Neutrons are transported down to thermal energies in both codes. Inelastic nuclear interactions are modelled in SHIELD-HIT by the Many Stage Dynamical Model (MSDM), whereas in FLUKA the Pre-Equilibrium Approach to Nuclear Thermalisation (PEANUT) package which includes a Generalized Intra-Nuclear Cascade model was used.

Results: The dose distributions in water irradiated with 300 MeV/u ¹²C and 185 MeV/u ⁷Li ion beams were simulated with the two codes. Studies were performed of the energy deposition both on the central axis and at lateral distances up to 10 cm off-axis. The dose distributions calculated by SHIELD-HIT and FLUKA were compared with published experimental data. The dose mean lineal energy , frequency mean lineal energy , dose mean specific energy , and frequency mean specific energy were calculated with the ion track-structure code PITS99 (Positive Ion Track Structure 99), coupled with the electron code KURBUC for the primary and secondary ions average energies at 1 mm before the Bragg peak.

Conclusion: The Monte Carlo codes show good agreement with experimental results for off-axis dose distributions. The disagreements in the Bragg peak region for the central-axis dose distributions imply that further improvements especially in the nuclear interaction models are required to increase the accuracy of the codes.     

Effects of Anaesthetic Agent on [31P]NMR High-energy Phosphates and Regional Distributions of Intravascular Oxyhaemoglobin Saturations

Abstract

Purpose: The biological response of tissue exposed to radiations emitted by internal radioactivity is often correlated with the mean absorbed dose to a tissue element. However, experimental studies show that even when the mean absorbed dose to the tissue element is constant, the response of the cell population within the tissue element can vary significantly depending on the distribution of radioactivity at the cellular and multicellular levels. The present work develops theoretical models to simulate these observations.

Materials and methods: Two theoretical models were created to simulate experimental three-dimensional cell culture models with homogeneous and inhomogeneous tissue environments. The cells were assigned activities according to lognormal distributions of an alpha particle emitter or a monoenergetic electron emitter. Absorbed doses to the cell nuclei were assessed with point-kernel geometric-factor and Electron Gamma Shower version nrc (EGSnrc) Monte Carlo radiation transport simulations, respectively. The self- and cross-dose to individual cell nuclei were calculated and a Monte Carlo method was used to determine their fate. Survival curves were produced after tallying the live and dead cells.

Results: Both percent cells labeled and breadth of lognormal distribution affected the dose distribution at the cellular level, which in turn, influenced the shape of the cell survival curves.

Conclusions: Multicellular Monte Carlo dosimetry-models offer improved capacity to predict response to radiopharmaceuticals compared to approaches based on mean absorbed dose to the tissue.     

Track structure,radiation quality and initial radiobiological events: Considerations based on the PARTRAC code experience

Abstract

Purpose: The role of track structures for understanding the biological effects of radiation has been the subject of research activities for decades. The physics that describes such processes is the core Monte Carlo codes, such as the biophysical PARTRAC (PARticle TRACks) code described in this review, which follow the mechanisms of radiation-matter interaction from the early stage. In this paper a review of the track structure theory (and of its possible extension concerning non-DNA targets) is presented.

Materials and methods: The role of radiation quality and track structure is analyzed starting from the heavy ions results obtained with the biophysical Monte Carlo code PARTRAC (PARticles TRACks). PARTRAC calculates DNA damage in human cells based on the superposition of simulated track structures in liquid water to an ‘atom-by-atom’ model of human DNA. Results: Calculations for DNA fragmentation compared with experimental data for different radiation qualities are illustrated. As an example, the strong dependence of the complexity of DNA damage on radiation track structure, and the very large production of very small DNA fragments (lower than 1 kbp (kilo base pairs) usually not detected experimentally) after high LET (high-Linear Energy Transfer) irradiation is shown. Furthermore the possible importance of non-nuclear/non-DNA targets is discussed in the particular case of cellular membrane and mitochondria.

Conclusions: The importance of the track structure is underlined, in particular the dependence of a given late cellular effect on the spatial distribution of DNA double-strand breaks (DSB) along the radiation track. These results show that the relative biological effectiveness (RBE) for DSB production can be significantly larger than 1. Moreover the cluster properties of high LET radiation may determine specific initial targets and damage evolution.     

An implementation to read and write IAEA phase-space files in GEANT4-based simulations

Abstract

Purpose: To develop a stand-alone code to make any application coded with the GEANT4 (GEometry ANd Tracking, version 4) toolkit capable of reading and writing phase-space (phsp) files in the format created by the IAEA (International Atomic Energy Agency), so that the exchange of phsp files between other validated Monte Carlo (MC) codes and GEANT4 is possible. Methods: We present a stand-alone code, written in C++ object-oriented language, developed in a way that ensures the compatibility with future versions of the IAEA phsp format. The aim of the reader part is to get the information from a given IAEA phsp file and create the primary particles in a GEANT4 user application. On the other hand, the writer part of the code is the responsible for writing the IAEA phsp files during a run of the GEANT4 application. Results: A testing simulation was written with GEANT4 to verify the performance of this code, with satisfactory results. An example of use in a GEANT4 application which simulates the treatment head of a radiotherapy linear electron accelerator (linac) is also shown, comparing dose calculations with experimental data. Conclusions: This stand-alone package, which can be used in any GEANT4 application, allows the exchange of validated phsp files between different MC codes and the use of phsp data from many different accelerators and fields in dosimetry studies. Furthermore, it also offers additional utilities of interest in medical applications.     

The Auger effect in physical and biological research

Purpose: The paper reports on progress in physics of radiationless transitions and new Auger spectra of ¹²⁵I and ¹²⁴I. We report progress in Monte Carlo track structure simulation of low energy electrons comprising majority electrons released in decay most Auger emitters.

Materials and methods: The input data for electron capture (EC) and internal conversion(IC) were obtained from various physics data libraries. Monte Carlo technique was used for the simulation of Auger electron spectra. Similarly, electron tracks were generated using Monte Carlo track structure methods.

Results: Data are presented for the EC, IC and binding energy (BE) of radionuclides ¹²⁴I and ¹²⁵I. For each of the radionuclides ¹²⁵I and ¹²⁴I some examples of electron spectra of individual decays are given. Because most Auger electrons are low energy and short range, data and a short discussion are presented on recent Monte Carlo track structure development in condensed media and their accuracy.

Conclusions: Accuracy of electron spectra calculated in the decay of electron shower by Auger emitting radionuclides depends on availability of accurate physics data. There are many gaps in these libraries and there is a need for detailed comparison between analytical method and Monte Carlo calculations to refine the method of calculations. On simulation of electron tracks, although improved models for sub-keV electron interaction cross sections for liquid water are now available, more experimental data are needed for benchmarking. In addition, it is desirable to make data and programs for calculations of Auger spectra available online for use by students and researchers.     

Track structure calculations on intracellular targets responsible for signal release in bystander experiments with transfer of irradiated cell-conditioned medium

Abstract

Purpose: To investigate alternative scenarios for the dose-dependent emission of bystander signals by irradiated cells in medium transfer experiments.

Methods: Energy deposition patterns to hypothetical intracellular targets whose hit by radiation initiates the emission of bystander signals have been simulated by Monte Carlo code PARTRAC, evaluating the effects of target size, multiplicity and threshold energy for activation. Scenarios in which individual irradiated cells release signals independently as well as those with signal amplification by neighbour cells have been analyzed. The non-linear response of unirradiated cells to signals in the transferred medium has been considered.

Results: The experimentally observed dose dependence of bystander effects is consistent with cell-autonomous signal release with a wide distribution of characteristic doses, covering the range of 3 mGy to 3 Gy. Alternatively, the data can be explained by assuming that only cells receiving a high specific energy (3 Gy to 0.5 μm targets) release primary signals, which are then amplified by secondary signalling by neighbour cells within about a millimetre distance.

Conclusion: Alternative signal emission scenarios are consistent with the observed dose dependence of bystander effects in medium transfer experiments. Thus, further experimental research is needed to identify the actual mechanism of bystander signal emission.     

Monte Carlo simulations of therapeutic proton beams for relative biological effectiveness of double-strand break

Abstract

Purpose: The relative biological effectiveness (RBE) values relative to ⁶⁰Co for the induction of double-strand breaks (DSB) were calculated for therapeutic proton beams. RBE-weighted absorbed doses were determined at different depths in a water phantom for proton beams.

Materials and methods: The depth-dose distributions and the fluence spectra for primary protons and secondary particles were calculated using the FLUKA (FLUktuierende KAskade) MC (Monte Carlo) transport code. These spectra were combined with the MCDS (Monte Carlo damage simulation) code to simulate the spectrum-averaged yields of clustered DNA lesions. RBE for the induction of DSB were then determined at different depths in a water phantom for the unmodulated and modulated proton beams.

Results: The maximum RBE for the induction of DSB at 1 Gy absorbed dose was found about 1.5 at 0.5 cm distal to the Bragg peak maximum for an unmodulated 160 MeV proton beam. The RBE-weighted absorbed dose extended the biologically effective range of the proton beam by 1.9 mm. The corresponding maximum RBE value was inversely proportional to the proton beam energy, reaching a value of about 1.9 for 70 MeV proton beam. For a modulated 160 MeV proton beam, the RBE weightings were more pronounced near the spread-out Bragg peak (SOBP) distal edge.

Conclusions: It was demonstrated that a fast MCDS code could be used to simulate the DNA damage yield for therapeutic proton beams. Simulated RBE for the induction of DSB were comparable to RBE measured in vitro and in vivo. Depth dependent RBE values in the SOBP region might have to be considered in certain treatment situations.     

Light transport feature for SCINFUL.

An extended version of the scintillator response function prediction code SCINFUL has been developed by incorporating PHOTRACK, a Monte Carlo light transport code. Comparisons of calculated and experimental results for organic scintillators exposed to neutrons show that the extended code improves the predictive capability of SCINFUL.     

Cellular- and micro-dosimetry of heterogeneously distributed tritium

Abstract

Purpose: The assessment of radiotoxicity for heterogeneously distributed tritium should be based on the subcellular dose and relative biological effectiveness (RBE) for cell nucleus. In the present work, geometry-dependent absorbed dose and RBE were calculated using Monte Carlo codes for tritium in the cell, cell surface, cytoplasm, or cell nucleus.

Materials and methods: Penelope (PENetration and Energy LOss of Positrins and Electrons) code was used to calculate the geometry-dependent absorbed dose, lineal energy, and electron fluence spectrum. RBE for the intestinal crypt regeneration was calculated using a lineal energy-dependent biological weighting function. RBE for the induction of DNA double strand breaks was estimated using a nucleotide-level map for clustered DNA lesions of the Monte Carlo damage simulation (MCDS) code.

Results: For a typical cell of 10 μm radius and 5 μm nuclear radius, tritium in the cell nucleus resulted in much higher RBE-weighted absorbed dose than tritium distributed uniformly. Conversely, tritium distributed on the cell surface led to trivial RBE-weighted absorbed dose due to irradiation geometry and great attenuation of beta particles in the cytoplasm. For tritium uniformly distributed in the cell, the RBE-weighted absorbed dose was larger compared to tritium uniformly distributed in the tissue.

Conclusions: Cellular- and micro-dosimetry models were developed for the assessment of heterogeneously distributed tritium.     

Development of the specific purpose Monte Carlo code CEARXRF for the design and use of in vivo X-ray fluorescence analysis systems for lead in bone

X-ray fluorescence (XRF) systems have been increasingly used for in vivo toxic trace-element analysis in the human body, such as lead in the tibia. Monte Carlo simulation can provide an efficient and flexible method for designing and using in vivo XRF systems. The Monte Carlo code CEARXRF has been developed specifically to simulate the complete pulse height spectrum of energy dispersive XRF systems. This code is capable of tracking photons in a general geometry and modelling all of the physics of photon interactions in the energy range 1–150 keV for elements Z = 1–94, including primary and higher degree excitations of K and L XRF, the Doppler broadening of Compton-scattered photon energies, and the polarization effects in low-energy photon scatterings. The scattering background for minimum detectable concentration (MDC) analysis may be simulated more accurately by taking into account Doppler broadening in the distribution of the Compton-scattered photon energy due to electron-binding effects. The use of polarized excitation photons has been shown to be important in producing a low scattering background and good measurement sensitivity. The code has two very unique and important features: (1) complete composition and density correlated sampling that is extremely useful for studying measurement sensitivity to small changes in sample composition and density; and (2) Monte Carlo library spectra calculation for the determination of elemental amounts by the Monte Carlo-Library Least-Squares (MCLLS) method. The capability of CEARXRF to aid the design and optimization of in vivo XRF analysis has been verified by modelling hypothesized lead K and L XRF measurement systems.     

Stochastic modelling of DSB repair after photon and ion irradiation

Abstract

Purpose: To test the stochastic model for DNA double-strand break (DSB) repair via non-homologous end joining (NHEJ) implemented in the Monte Carlo code PARTRAC (PARticle TRACks) against measured repair kinetics after nitrogen ion and ⁶⁰Co γ reference irradiation.

Material and methods: By combining Monte Carlo track structure calculations with multi-scale models of cellular DNA, yields of DSB are calculated for N ion and ⁶⁰Co γ-irradiation. The NHEJ model in PARTRAC is used to determine rejoining kinetics of the DNA ends and DNA fragment distributions after certain repair times. Model parameters are adapted to the measured rejoining kinetics for the different radiation types.

Results: DSB rejoining kinetics after low- and high-linear energy transfer (LET) irradiation have been reproduced after refinements of the DNA repair model, in particular by considering an ongoing production of detectable DSB in the initial phase, e.g., by enzymatic processing of labile sites, and by assuming a limited availability of repair enzymes needed for processing complex lesions during the slow repair phase.

Conclusions: The need for certain model refinements suggests mechanisms that may significantly contribute to the DSB rejoining kinetics during both initial and later phases of NHEJ.     

Monte Carlo-simulated Auger electron spectra for nuclides of radiobiological and medical interest – a validation with noble gas ionization data

Abstract

Purpose: To further validate Monte Carlo calculation codes simulating cascades of Auger electron transitions in radionuclides that decay by electron capture or internal conversion. In particular, the need for an appropriate kinetic energy determination of the Auger electrons emitted from multiple-ionized atoms as well as the consideration of shake-off electrons would be investigated implicitly.

Methods: Charge distributions of noble gases after photoionization for different photon energies were calculated and compared with experimental data from the literature. In addition, new electron emission spectra were generated for ^99mTc and ¹²³I.

Results: By including strict energy book-keeping and allowing shake-off electrons, the agreement between experimentally detected charge distributions and Monte Carlo simulations was very good. On this basis, the number of emitted electrons per decay was found to be between 1 and 17 with a mean of 4.0 for ^99mTc and between 1 and 26 with a mean of 7.4 for ¹²³I.

Conclusions: Because of the good agreement with the experimental findings, the validation can be considered to be successful.     

Comparison of experimental 3D dose curves in a heterogeneous phantom with results obtained by MCNP5 simulation and those extracted from a commercial treatment planning system

Commercial planning systems used in radiotherapy treatments use determinist correlations to evaluate dose distribution around regions of interest. Estimated dose with this type of planners can be problematic, especially when analyzing heterogeneous zones. The present work is focused in quantifying the dose distribution in a heterogeneous medium irradiated by a 6 MeV photon beam emitted by an Elekta Precise Radiotherapy Unit head. Dose mapping inside the heterogeneous water phantom has been simulated with the photon and electron transport with Monte Carlo computer code MCNP5 and also, using a commercial treatment planning software in the same irradiation conditions. The calculated results were compared with experimental relative dose curves. This comparison shows that inside the heterogeneity region, the commercial algorithms are not able to predict the variation of dose in the heterogeneous zones with the same precision as MCNP5.     

Abstracts

Abstract

Purpose: To model interaction cross sections and energy loss for carbon projectiles C⁰–C⁶⁺ of 1–10⁴ keV/u (u: atomic mass unit) in water.

Materials and methods: The classical trajectory Monte Carlo method was used to calculate the ionisation and charge-transfer cross sections. The excitation cross sections were scaled from proton data using equilibrium charges determined from the charge-transfer cross sections. Energy loss was obtained from the singly differential cross sections, and ionisation potentials of the target and projectile.

Results: The calculated total ionisation cross sections are consistent with measured data, while the calculated electron-capture cross sections are larger than experimental data by a factor of 3. By scaling the latter to the measured data, the cross sections were made consistent with these data for 1–10 keV/u energies. The present stopping cross sections agree well with experimental data below 10 keV/u, and with other model calculations above 2 MeV/u. Deviation from the latter is found where electron capture is competitive with ionisation, and also arises from different energy-transfer calculations.

Conclusions: In this paper we report our efforts in the developments of full slowing-down Monte Carlo track structure calculations for carbon ions. Further development and refinement of the model are currently underway.     

Applications of the microdosimetric function implemented in the macroscopic particle transport simulation code PHITS

Abstract

Purpose: Microdosimetric quantities such as lineal energy are generally considered to be better indices than linear energy transfer (LET) for expressing the relative biological effectiveness (RBE) of high charge and energy particles. To calculate their probability densities (PD) in macroscopic matter, it is necessary to integrate microdosimetric tools such as track-structure simulation codes with macroscopic particle transport simulation codes.

Methods: As an integration approach, the mathematical model for calculating the PD of microdosimetric quantities developed based on track-structure simulations was incorporated into the macroscopic particle transport simulation code PHITS (Particle and Heavy Ion Transport code System). The improved PHITS enables the PD in macroscopic matter to be calculated within a reasonable computation time, while taking their stochastic nature into account.

Applications: The microdosimetric function of PHITS was applied to biological dose estimation for charged-particle therapy and risk estimation for astronauts. The former application was performed in combination with the microdosimetric kinetic model, while the latter employed the radiation quality factor expressed as a function of lineal energy.

Conclusion: Owing to the unique features of the microdosimetric function, the improved PHITS has the potential to establish more sophisticated systems for radiological protection in space as well as for the treatment planning of charged-particle therapy.