Ionization Cluster Size Distributions Research Articles

Nanodosimetric investigation of the track structure of therapeutic carbon ion radiation part 1: measurement of ionization cluster size distributions

At the Heidelberg Ion-Beam Therapy Center, the track structure of carbon ions of therapeutic energy after penetrating layers of simulated tissue was investigated for the first time. Measurements were conducted with carbon ion beams of different energies and polymethyl methacrylate (PMMA) absorbers of different thicknesses to realize different depths in the phantom along the pristine Bragg peak. Ionization cluster size (ICS) distributions resulting from the mixed radiation field behind the PMMA absorbers were measured using an ion-counting nanodosimeter. Two different measurements were carried out: (i) variation of the PMMA absorber thickness with constant carbon ion beam energy and (ii) combined variation of PMMA absorber thickness and carbon ion beam energy such that the kinetic energy of the carbon ions in the target volume is constant. The data analysis revealed unexpectedly high mean ICS values compared to stopping power calculations and the data measured at lower energies in earlier work. This suggests that in the measurements the carbon ion kinetic energies behind the PMMA absorber may have deviated considerably from the expected values obtained by the calculations. In addition, the results indicate the presence of a marked contribution of nuclear fragments to the measured ICS distributions, especially if the carbon ion does not cross the target volume.

Primary study of the relative and compound biological effectiveness model for boron neutron capture therapy based on nanodosimetry.

The current radiobiological model employed for boron neutron capture therapy (BNCT) treatment planning, which relies on microdosimetry, fails to provide an accurate representation the biological effects of BNCT. The precision in calculating the relative biological effectiveness (RBE) and compound biological effectiveness (CBE) plays a pivotal role in determining the therapeutic efficacy of BNCT. Therefore, this study focuses on how to improve the accuracy of the biological effects of BNCT. The purpose of this study is to propose new radiation biology models based on nanodosimetry to accurately assess RBE and CBE for BNCT. Nanodosimetry, rooted in ionization cluster size distributions (ICSD), introduces a novel approach to characterize radiation quality by effectively delineating RBE through the ion track structure at the nanoscale. In the context of prior research, this study presents a computational model for the nanoscale assessment of RBE and CBE. We establish a simplified model of DNA chromatin fiber using the Monte Carlo code TOPAS-nBio to evaluate the applicability of ICSD to BNCT and compute nanodosimetric parameters. Our investigation reveals that both homogeneous and heterogeneous nanodosimetric parameters, as well as the corresponding biological model coefficients α and β, along with RBE values, exhibit variations in response to varying intracellular 10B concentrations. Notably, the nanodosimetric parameter effectively captures the fluctuations in model coefficients α and RBE. Our model facilitates a nanoscale analysis of BNCT, enabling predictions of nanodosimetric quantities for secondary ions as well as RBE, CBE, and other essential biological metrics related to the distribution of boron. This contribution significantly enhances the precision of RBE calculations and holds substantial promise for future applications in treatment planning.

Intercomparison of nanodosimetric distributions in nitrogen simulated with Geant4 and PTra track structure codes

To facilitate the use of Geant4-DNA for radiation transport simulations in micro- and nanodosimeters, which are physically operated with tissue-equivalent gases such as nitrogen (and propane), this work aims to extend the cross section data available in Geant4-DNA to include those of nitrogen for electron energies ranging from 1 MeV down to the ionisation threshold. To achieve this, interaction cross section data for nitrogen that have been used with the in-house PTB PTra track structure code have been implemented in the current state-of-the-art Geant4-DNA simulation toolkit. An intercomparison has been performed between the two codes to validate this implementation. To quantify the agreement between the cross section models for nitrogen adopted in PTra and those implemented in Geant4-DNA, the simulation results of both codes were analysed using three physical parameters describing the ionisation cluster size distribution (ICSD): mean ionisation cluster size, variance of the cluster size and the probability to obtain a single ionisation within the target. Statistical analysis of the results indicates that the interaction cross section models for nitrogen used in PTra (elastic scattering, impact ionisations and electronic excitations) have been successfully implemented in Geant4-DNA. In addition, simulated ICSDs were compared to those measured with the Jet Counter nanodosimeter for energies between 100 and 2000 eV. For greater energies, the ICRP data for LET and particle range were used as a reference. The modified Geant4-DNA code and data successfully passed all these benchmarks fulfilling the requirement for their public release in the next version of the Geant4 toolkit.

Electrostatic field simulations and dynamic Monte Carlo simulations of a nanodosimetric detector

Nanodosimetry uses physical quantities based on ionization cluster size distributions to model the biological effectiveness of different radiation qualities. A nanodosimetric detector in operation is the FIRE (Frequency of Ion Registration) detector, but further design optimization of the detector is required. The simulations presented in this work will help guide future developments of the detector.Within the FIRE detector, the produced ions drift towards a dielectric plate hole, within which they are strongly accelerated towards the cathode and produce an electron avalanche through impact ionization. The electron avalanche is measured at the signal readout. COMSOL Multiphysics was used to calculate the electrostatic fields and Garfield＋＋ was used to simulate ion drift trajectories and electron avalanche formation employing Monte Carlo methods.The dependency of electron avalanche formation on experimental parameters, such as pressure or electric field configuration, was studied and compared to experimental results. Based on the funneling effect ions experience within the drift region, a calculation method for obtaining an optimized hole pitch for future multi-hole detector designs was presented. The results presented in this work offer valuable insights into the inner workings of the FIRE detector, as well as make clarify how different experimental parameters influence signal creation.

Geant4-DNA modeling of nanodosimetric quantities in the Jet Counter for alpha particles

The purpose of this work was to validate the calculation accuracy of nanodosimetric quantities in Geant4-DNA track structure simulation code. We implemented the Jet Counter (JC) nanodosimeter geometry in the simulation platform and quantified the impact of the Geant4-DNA physics models and JC detector performance on the ionization cluster size distributions (ICSD). ICSD parameters characterize the quality of radiation field and are supposed to be correlated to the complexity of the initial DNA damage in nanoscale and eventually the response of biological systems to radiation. We compared Monte Carlo simulations of ICSD in JC geometry performed using Geant4-DNA and PTra codes with experimental data collected for alpha particles at 3.8 MeV. We investigated the impact of simulation and experimental settings, i.e., three Geant4-DNA physics models, three sizes of a nanometer sensitive volume, gas to water density scaling procedure, JC ion extraction efficiency and the presence of passive components of the detector on the ICSD and their parameters. We found that ICSD in JC geometry obtained from Geant4-DNA simulations in water correspond well to ICSD measurements in nitrogen gas for all investigated settings, while the best agreement is for Geant4-DNA physics option 4. This work also discusses the accuracy and robustness of ICSD parameters in the context of the application of track structure simulation methods for treatment planning in particle therapy.

Characterisation of the PTB ion counter nanodosimeter's target volume and its equivalent size in terms of liquid H2O

The pattern of inelastic interactions in subcellular targets, especially in DNA, is largely responsible for radiation-induced damage to tissue. The track structure of ionising particles is therefore of particular importance in view of the biological effectiveness of ionising radiation.In approaches to model radiation effects to DNA, track structure details on the nanometre scale and the specifics of the target geometry play an important role. Hence, reliable experimental benchmark data for these models require detailed knowledge on the size and shape of the target volume of the devices used to measure quantities related to track structure in simulated nanometric target volumes.A dedicated investigation of the target size of a nanodosimeter device has been carried out in order to investigate to what extent measured ionisation cluster size distributions can serve as benchmark data for modelling approaches, for the first time with particular focus on the equivalent target size in terms of liquid H2O. To this end, measurements with alpha particles from a241Am source were carried out using three different target gases, H2O, C3H8 and C4H8O. For each of the three target gases, three different drift-time windows were applied to realise three different target sizes.A method has been developed to determine the dimensions of the simulated nanometric target volume in liquid H2O for cylindrical and spherical shape, as often used in approaches to model radiation effects to DNA. Simulations with nanometric targets of dimensions determined with this method agree very well with the corresponding measurements.Scaling of the spatial distribution of the extraction efficiency for different target gases and drift-time windows, which corresponds to the nanodosimeter's target volume, in terms of liquid H2O using (ρλion)-ratios has also been investigated and proved to yield an estimate of the target volume in liquid H2O.Thus, it can be concluded that ionisation cluster size distributions measured with a nanodosimeter device are suited as benchmark data for approaches that model radiation induced damage to DNA in nanometric volumes of liquid H2O in simple geometries such as cylinders or spheres, provided that the nanodosimeter's target volume is characterised accordingly. By proper selection of drift-time window length as well as target gas and density a wide range of target volume dimensions in terms of liquid H2O can be realised with the PTB Ion Counter nanodosimeter according to specific requirements of modelling approaches.

Experimental nanodosimetry: Track structure of 80 MeV boron ions

The radiobiological effects of ionising radiations are strongly correlated to the local density of lesions produced in micrometre- and nanometre-sized subcellular structures, and thus to the track structure properties of interacting particles. These properties have been studied mainly by means of Monte Carlo simulations, however experimental methodologies and instruments have also been developed that allow to measure the stochastics of ionisations produced in a nanometric sensitive volume (SV) by charged particles crossing it or passing nearby at specified distances. The number ν of ionisations produced in SV by the passage of a single primary ionising particle is measured, and the measurement is repeated many times to produce a probability distribution P(ν), called ionisation cluster size distribution. Measurable quantities can be derived that correlate with the biological effectiveness of the specific radiation quality (ion type and velocity). In view of the emerging interest of carbon ions in radiotherapy, it is important to characterize not only the track structure of carbon ions but also that of other light ions that are produced in nuclear interactions. This work presents first experimental results for boron ions at 80 MeV. Ionisation cluster size distributions were measured with the Startrack-Counter installed at the TANDEM-ALPI accelerator complex at the Legnaro National Laboratories (LNL) of the Italian Istituto Nazionale di Fisica Nucleare (INFN) and simulated by means of a dedicated Monte Carlo code. Measurements and simulations were performed for particles that traverse directly the sensitive volume or pass nearby at specified impact parameters.

Feasibility study of macroscopic simulations of nanodosimetric parameters for proton therapy.

In view of the potential of treatment plan optimization based on nanodosimetric quantities, fast Monte Carlo methods for obtaining nanodosimetric quantities in macroscopic volumes are important. In this work, a "fast" method for obtaining nanodosimetric parameters from a clinical proton pencil beam in a macroscopic volume is compared with a slow and detailed method. Furthermore, the variations of these parameters, when obtained with the Monte Carlo codes TOPAS and NOREC, are investigated. Monte Carlo track structure simulations of 1keV-100MeV protons and 12eV-1MeV electrons in a volume of 8nm liquid water provided us with an atlas of cluster size distributions. Two kinds of ionization cluster size distributions were recorded, counting all ionizations or only ionizations directly produced by the primary particle. The simulations of the proton pencil beam were performed in two different ways. A "fast" method where only the protons were simulated and a "slow and detailed" method where protons and electrons were simulated in order to obtain spectra at different depths. The obtained spectra were then convoluted with cluster size distributions. It was shown that the nanodosimetric quantity from the "fast" method is, depending on the location, between 43.6% and 63.6% smaller than the obtained by the "slow and detailed" method. However, it was also shown that variations of nanodosimetric quantities are even larger when the cluster size distributions of the electrons are simulated with the Monte Carlo code NOREC, that is, the cumulative probabilities obtained with NOREC were between 50.8% and 75.5% smaller than the probabilities obtained with TOPAS. As long as the uncertainties of different Monte Carlo codes are not improved, it is feasible to only simulate protons in a macroscopic volume. It must be noted, however, that the uncertainty is in the order of 100%.

Monte Carlo track-structure for the radionuclide Copper-64: characterization of S-values, nanodosimetry and quantification of direct damage to DNA

TOPAS-nBio was used to simulate, collision-to-collision, the complete trajectories of electrons in water generated during the explicit simulation of 64Cu decay. S-values and direct damage to the DNA were calculated representing the cell (C) and the cell nucleus (N) with concentric spheres of 5 μm and 4 μm in radius, respectively. The considered ‘target’←‘source’ configurations, including the cell surface (Cs) and cytoplasm (Cy), were: C←C, C←Cs, N←N, N←Cy and N←Cs. Ionization cluster size distributions were also calculated in a cylinder immersed in water corresponding to a DNA segment of 10 base-pairs in length (diameter 2.3 nm, length 3.4 nm), modeling a radioactive point source moving from the central axis to the edge of the cylinder. For that, the first moment (M1) and cumulative probability of having a cluster size of 2 or more ionizations in the cylindrical volume (F2) were obtained. Finally, the direct damage to the DNA was estimated by quantifying double-strand breaks (DSBs) using the clustering algorithm DBSCAN.The S-values obtained with TOPAS-nBio for 64Cu were 7.879 × 10−4 ± 5 × 10−7, 4.351 × 10−4 ± 6 × 10−7, 1.442 × 10−3 ± 1 × 10−6, 2.596 × 10−4 ± 8 × 10−7, 1.127 × 10−4 ± 4 × 10−7 Gy Bq-s−1 for the configurations C←C, C←Cs, N←N, N←Cy and N←Cs, respectively. The difference of these values, compared with previously reported S-values for 64Cu with the code MNCP and software MIRDCell, ranged from −4% to −25% for the configurations N←N and N←Cs, respectively. On the other hand, F2 was maximum with the source at the center of the cylinder 0.373 ± 0.001, and monotonically decreased until reaching a value of 0.058 ± 0.001 at 2.3 nm. The same behavior was observed for M1 with values ranging from 2.188 ± 0.004 to 0.242 ± 0.002. Finally, the DBSCAN algorithm showed that the mean number of DNA DSBs per decay were 0.187 ± 0.001, 0.0317 ± 0.0005, and 0.0125 ± 0.0002 DSB-(Bq-s)−1 for the configurations N←N, N←Cs, and N←Cy, respectively.In conclusion, the results of the S-values show that the absorbed dose strongly depends on the distribution of the radionuclide in the cell, the dose being higher when 64Cu is internalized in the cell nucleus, which is reinforced by the nanodosimetric study by the presence of DNA DSBs attributable to the Auger electrons emitted during the decay of 64Cu.

Three-dimensional nanodosimetric characterisation of proton track structure

This work presents a detailed investigation into the nanodosimetric properties of the track structure of protons in water at energies relevant for proton radiotherapy. The ionization component of the tracks had been simulated in previous work using Geant4-DNA for proton start energies between 1 MeV and 100 MeV. From the simulation results, the frequency distribution of ionization clusters formed in nanometric target volumes was obtained in dependence of the impact parameter of the proton trajectory with respect to the target center. In the track core, targets of cylindrical shape and a size comparable to a short segment of DNA were used for scoring ionization cluster size distributions. For the penumbra region, three different options for defining the cylinder shell sectors were investigated, with each cylinder shell sector volume the same as the cylinder target volume. The radial distributions were numerically integrated to obtain the effective track cross sections with respect to different nanodosimetric parameters. Graphically displaying the radial dependence in a similar way as microdosimetric distributions allowed elucidating the contribution of different radial distances to the overall radial integral of the quantities under consideration. Furthermore, it was tested how well the radial dependence of the nanodosimetric parameters could be fitted with a model function derived in literature for the radial energy deposition in proton tracks. It was found that this model function allows describing only the radial dependence of the total frequency of nanometric targets where ionizations occur. The deviation from a 1/r2-dependence of the radial dependence of the frequency of targets receiving more than a minimum number of ionizations (exceeding one) includes a second peak centred around 10 nm radial distance from the proton trajectory.

Particle track structure measurements from 0.5 to 18 nm in nitrogen using the Jet Counter nanodosemeter

Experimental nanodosimetry aims at establishing a new concept of radiation quality that builds on measurable characteristics of the particle track structure at the nanometre scale. This article presents the capabilities and limits of the Jet Counter nanodosemeter for the whole range of experimentally available targets’ sizes. The Jet Counter device can be used for reliable measurements of ionisation cluster-size distributions for target sizes ranging from 1 to 16 nm in nitrogen. Measurements outside this range have also been performed, but their reliability has yet to be proven.

Nanodosimetry of light ions in targets of different size

Nanodosimetry measures the frequency distribution of the number of ionizations produced by primary particles in target volumes of nanometric size. At the Legnaro National Laboratories of INFN, a large measurement campaign was carried out with the Startrack nanodosimetric counter, which showed a consistent characterization of the ionization yield produced by different primary particles when the counter is filled at the design gas pressure of 300 Pa, corresponding to a mass thickness of 2.0 μg/cm2, i.e. about 20 nm at unit density. In order to investigate the operative limits of the Startrack counter and analyse the stochastics of the ionization yield also in targets of other sizes, new measurements were carried out at lower gas pressure. A first set of measurement was carried out with different primary ions at pressures of 200 Pa and 170 Pa, corresponding to a site size in mass per area of 1.4 μg/cm2 and 1.1 μg/cm2, respectively. A second set of measurements was carried out with low-energy lithium ions, varying the pressure in the range 150–300 Pa, i.e. down to a site size of 1.0 μg/cm2 (10 nm at unit density). The measured ionization cluster-size distributions were compared with Monte Carlo simulations performed with the MC-Startrack track structure code, in order to obtain a first estimation of the decrease in detection efficiency with decreasing gas pressure. The new results and their consistency with previous measurements are presented and discussed in this paper.

Track structure of carbon ions: New measurements and simulations

Abstract: It is recognized today that the observable radiobiological effects of ionizing radiations are strongly correlated to the local density of lesions produced in micrometer- and nanometer-sized subcellular structures, and thus to the track structure properties of interacting particles. In view of the emerging interest of carbon ions in radiotherapy, an experimental study was done for characterizing the track structure of carbon ions at different energies close to the Bragg peak. Ionization cluster size distributions for nanometer-sized target volumes were measured with the Startrack-Counter installed at the TANDEM-ALPI accelerator complex at LNL, and calculated by means of a dedicated Monte Carlo code. Measurements and simulations were performed for particle tracks crossing directly the target volume or passing nearby at specified impact parameters. Results will be presented and discussed for 12C-ions at 6, 8, 12.5 and 20 MeV per nucleon.

Reducing the background of secondary ions in an ion-counting nanodosimeter

In previous investigations using the PTB Ion Counter nanodosimeter, significant deviations for large cluster sizes were found in the comparison between measured and simulated data of ionisation cluster size distributions. These deviations could be explained quantitatively using a simple parametric model for a background of secondary ions, which are produced within the transport system of the target gas ions to the ion detector. To correct for these secondary ions, unfolding procedures were applied to the measured data to remove the effects of this background. However, the parameters of the model describing the background of secondary ions strongly depend on operational parameters of the nanodosimeter, and any change in these operational parameters requires a new characterisation of the model's parameters. In this work, the influence of the nanodosimeter's operational parameters on the background of secondary ions and the collection efficiency of the detected ions was therefore systematically investigated. At the end of this process operational conditions were found where the background of secondary ions is reduced substantially while the effect on the spatial distribution of the collection efficiency is still acceptable. For some beam geometries of particular relevance for radiation biology, cluster size distributions measured with these new operational parameters can be used directly so that the unfolding procedures, and consequently the characterisation of the secondary ion background with respect to the parameters required by the corresponding model used in the unfolding, can be omitted.

Unfolding the background of secondary ions in measured nanodosimetric ionisation cluster size distributions

Nanodosimetry is a methodology for quantifying the effects of ionising radiation on matter by determining the frequency distributions of the cluster size of ionisations in nanometric target volumes. In previous investigations with the Ion Counter nanodosimeter operated at PTB, significant deviations for large cluster sizes were found in the comparison between measured and simulated data of ionisation cluster size distributions. These deviations could be explained by a background of secondary ions, which are produced within the transport system of the ionised target molecules. In this paper, two different approaches were investigated to correct for the background of secondary ions in the measured data to obtain the “true” cluster size distribution to be used, e. g., in predictions of biological effectiveness. In the first approach, the correction of the background was treated as a minimising problem. In the second approach, an iterative unfolding algorithm using Bayes statistics was employed. In all cases where the convolution of the background-corrected results with the secondary ion background agrees well with the corresponding measured cluster size distribution, the background-correction led to an improved agreement between measurement and simulation. For the removal of a background of secondary ions from measured cluster size distributions, the unfolding algorithm using Bayes statistics is the preferred method as it proved to be the most effective and the least sensitive to boundary conditions. Moreover, it was considerably less time consuming.

ON THE TWO MODES OF NANODOSIMETRIC EXPERIMENT.

A method of estimation of the ionisation cluster-size distribution for single primary particle from experimental data obtained in setup without primary particle detector is presented. The Jet Counter nanodosemeter was used in two kinds of experiments with alpha particles to validate the method. Possible application of the method is to perform measurements with previously precluded types of radiation like photons, neutrons and electrons. Details and limitations of the proposed approach are discussed.

ASSESSING THE CONTRIBUTION OF CROSS-SECTIONS TO THE UNCERTAINTY OF MONTE CARLO CALCULATIONS IN MICRO- AND NANODOSIMETRY.

Within EURADOS Working Group 6 'Computational Dosimetry', the micro and nanodosimetry task group 6.2 has recently conducted a Monte Carlo (MC) exercise open to participants around the world. The aim of this exercise is to quantify the contribution to the uncertainty of micro and nanodosimetric simulation results arising from the use of different electron-impact cross-sections, and hence physical models, employed by different MC codes (GEANT4-DNA, PENELOPE, MCNP6, FLUKA, NASIC and PHITS). Comparison of the participants' simulation results for both micro and nanodosimetric quantities using different MC codes was the first step of the exercise. The deviation between results is due to different cross-sections but also different tracking methods and particle transport cut-off energies. The second step of the exercise will involve using identical cross-section datasets to account only for the other variations in the first step, thus enabling the determination of the uncertainty contribution due to different cross-sections. This paper presents a comparison of the MC simulation results obtained in the first part of the exercise. For the microdosimetric simulations, particularly in the configuration where the electron source is contained within the micrometric target, the choice of MC code has a small influence on the results. For the nanodosimetric results, on the other hand, the mean ionisation cluster size distribution (ICSD) was sensitive to the physical models used in the MC codes. The ICSD was therefore chosen to study the influence of different cross-section data on the uncertainty of simulation results.

Fast calculation of nanodosimetric quantities in treatment planning of proton and ion therapy

Details of the pattern of ionization formed by particle tracks extends knowledge of dose effects on the nanometer scale. Ionization detail (ID), frequently characterized by ionization cluster size distributions (ICSD), is obtained through time-consuming Monte Carlo (MC) track-structure simulations. In this work, TOPAS-nBio was used to generate a highly precise database of biologically significant ID quantities, sampled with randomly oriented 2.3 nm diameter cylinders, 3.4 nm (10 base pairs) long, inside a chromatin-size cylinder, irradiated by 1–1000 MeV/u ions of Z = 1–8. A macroscopic method developed to utilize the database using condensed-history MC was used to calculate distributions of the ICSD first moment and cumulative probability in a 20 × 20 × 40 cm3 water phantom irradiated with proton and carbon spread-out Bragg peak (SOBP) of 10.5 cm range, 2 cm width. Results were verified against detailed MC track-structure simulations using phase space scored at several depths. ID distributions were then obtained for intensity modulated proton and carbon radiotherapy plans in a digitized anthropomorphic phantom of a base of skull tumor to demonstrate clinical application of this approach.The database statistical uncertainties were 0.5% (3 standard deviations). Fluence-averaged ID as implemented proved unsuitable for macroscopic calculation. Edep-averaged ID agreed with track-structure results within 0.8% for protons. For carbon, maximum absolute differences of 2.9% ± 1.6% and 5.6% ± 1.9% for , 1.7% ± 0.8% and 1.9% ± 0.4% (1 standard deviation) for , were found in the plateau and SOBP, respectively, up to 11.5% ± 5.6% in the tail region. Macroscopic ID calculation was demonstrated for a realistic treatment plan. Computation times with or without ID calculation were comparable in all cases.Pre-calculated nanodosimetric data may be used for condensed-history MC for nanodosimetric ID-based treatment planning in ion radiotherapy in the future. The macroscopic approach developed has the calculation speed of condensed-history MC while approaching the accuracy of full track structure simulations.

Measurement of track structure parameters of low and medium energy helium and carbon ions in nanometric volumes

Ionization cluster size distributions produced in the sensitive volume of an ion-counting wall-less nanodosimeter by monoenergetic carbon ions with energies between 45 MeV and 150 MeV were measured at the TANDEM-ALPI ion accelerator facility complex of the LNL-INFN in Legnaro. Those produced by monoenergetic helium ions with energies between 2 MeV and 20 MeV were measured at the accelerator facilities of PTB and with a 241Am alpha particle source. C3H8 was used as the target gas. The ionization cluster size distributions were measured in narrow beam geometry with the primary beam passing the target volume at specified distances from its centre, and in broad beam geometry with a fan-like primary beam. By applying a suitable drift time window, the effective size of the target volume was adjusted to match the size of a DNA segment. The measured data were compared with the results of simulations obtained with the PTB Monte Carlo code PTra. Before the comparison, the simulated cluster size distributions were corrected with respect to the background of additional ionizations produced in the transport system of the ionized target gas molecules. Measured and simulated characteristics of the particle track structure are in good agreement for both types of primary particles and for both types of the irradiation geometry. As the range in tissue of the ions investigated is within the typical extension of a spread-out Bragg peak, these data are useful for benchmarking not only ‘general purpose’ track structure simulation codes, but also treatment planning codes used in hadron therapy. Additionally, these data sets may serve as a data base for codes modelling the induction of radiation damages at the DNA-level as they almost completely characterize the ionization component of the nanometric track structure.

Distributions of deposited energy and ionization clusters around ion tracks studied with Geant4 toolkit

The Geant4-based Monte Carlo model for Heavy-Ion Therapy (MCHIT) was extended to study the patterns of energy deposition at sub-micrometer distance from individual ion tracks. Dose distributions for low-energy 1H, 4He, 12C and 16O ions measured in several experiments are well described by the model in a broad range of radial distances, from 0.5 to 3000 nm. Despite the fact that such distributions are characterized by long tails, a dominant fraction of deposited energy (∼80%) is confined within a radius of about 10 nm. The probability distributions of clustered ionization events in nanoscale volumes of water traversed by 1H, 2H, 4He, 6Li, 7Li, and 12C ions are also calculated. A good agreement of calculated ionization cluster-size distributions with the corresponding experimental data suggests that the extended MCHIT can be used to characterize stochastic processes of energy deposition to sensitive cellular structures.