Comment on "Reproducibility study of Monte Carlo simulations for nanoparticle dose enhancement and biological modeling of cell survival curves" by Veltenet al[Biomed Phys Eng Express 2023;9:045004]

This comment highlights two methodological issues with the recent article by Velten et al [Biomed Phys Eng Express 2023;9:045004]: First, the approach taken in this work with a local effect model (LEM) in 2D leads to a significant overstimation of the number of radiation-induced lesions. This results in order of magnitude smaller predicted survival rates compared to the conventional LEM. Second, the dose without nanoparticles is used as the 'macroscopic dose' against which cell survival is plotted. However, for the considered gold concentrations, the average absorbed dose under secondary particle equilibrium is between 2 and 20 times higher with nanoparticles than without.

Correction: Belchior et al. Repair Kinetics of DSB-Foci Induced by Proton and α-Particle Microbeams of Different Energies. Life 2022, 12, 2040

In the original publication [...].

Repair Kinetics of DSB-Foci Induced by Proton and α-Particle Microbeams of Different Energies

In this work, the induction and repair of radiation-induced 53BP1 foci were studied in human umbilical vein endothelial cells irradiated at the PTB microbeam with protons and α-particles of different energies. The data were analyzed in terms of the mean number of 53BP1 foci induced by the different ion beams. The number of 53BP1 foci found at different times post-irradiation suggests that the disappearance of foci follows first order kinetics. The mean number of initially produced foci shows the expected increase with LET. The most interesting finding of this work is that the absolute number of persistent foci increases with LET but not their fraction. Furthermore, protons seem to produce more persistent foci as compared to α-particles of even higher LET. This may be seen as experimental evidence that protons may be more effective in producing severe DNA lesions, as was already shown in other work, and that LET may not be the best suited parameter to characterize radiation quality.

Investigation into the foundations of the track-event theory of cell survival and the radiation action model based on nanodosimetry

This work aims at elaborating the basic assumptions behind the "track-event theory" (TET) and its derivate "radiation action model based on nanodosimetry" (RAMN) by clearly distinguishing between effects of tracks at the cellular level and the induction of lesions in subcellular targets. It is demonstrated that the model assumptions of Poisson distribution and statistical independence of the frequency of single and clustered DNA lesions are dispensable for multi-event distributions because they follow from the Poisson distribution of the number of tracks affecting the considered target volume. It is also shown that making these assumptions for the single-event distributions of the number of lethal and sublethal lesions within a cell would lead to an essentially exponential dose dependence of survival for practically relevant values of the absorbed dose. Furthermore, it is elucidated that the model equation used for consideration of repair within the TET is based on the assumption that DNA lesions induced by different tracks are repaired independently. Consequently, the model equation is presumably inconsistent with the model assumptions and requires an additional model parameter. Furthermore, the methodology for deriving model parameters from nanodosimetric properties of particle track structure is critically assessed. Based on data from proton track simulations it is shown that the assumption of statistically independent targets leads to the prediction of negligible frequency of clustered DNA damage. An approach is outlined how track structure could be considered in determining the model parameters, and the implications for TET and RAMN are discussed.

Consistency checks of results from a Monte Carlo code intercomparison for emitted electron spectra and energy deposition around a single gold nanoparticle irradiated by X-rays

Organized by the European Radiation Dosimetry Group (EURADOS), a Monte Carlo code intercomparison exercise was conducted where participants simulated the emitted electron spectra and energy deposition around a single gold nanoparticle (GNP) irradiated by X-rays. In the exercise, the participants scored energy imparted in concentric spherical shells around a spherical volume filled with gold or water as well as the spectral distribution of electrons leaving the GNP. Initially, only the ratio of energy deposition with and without GNP was to be reported. During the evaluation of the exercise, however, the data for energy deposition in the presence and absence of the GNP were also requested. A GNP size of 50 nm and 100 nm diameter was considered as well as two different X-ray spectra (50 kVp and 100kVp). This introduced a redundancy that can be used to cross-validate the internal consistency of the simulation results. In this work, evaluation of the reported results is presented in terms of integral quantities that can be benchmarked against values obtained from physical properties of the radiation spectra and materials involved. The impact of different interaction cross-section datasets and their implementation in the different Monte Carlo codes is also discussed.

EURADOS STRATEGIC RESEARCH AGENDA 2020: VISION FOR THE DOSIMETRY OF IONISING RADIATION

Since 2012, the European Radiation Dosimetry Group (EURADOS) has developed its Strategic Research Agenda (SRA), which contributes to the identification of future research needs in radiation dosimetry in Europe. Continued scientific developments in this field necessitate regular updates and, consequently, this paper summarises the latest revision of the SRA, with input regarding the state of the art and vision for the future contributed by EURADOS Working Groups and through a stakeholder workshop. Five visions define key issues in dosimetry research that are considered important over at least the next decade. They include scientific objectives and developments in (i) updated fundamental dose concepts and quantities, (ii) improved radiation risk estimates deduced from epidemiological cohorts, (iii) efficient dose assessment for radiological emergencies, (iv) integrated personalised dosimetry in medical applications and (v) improved radiation protection of workers and the public. This SRA will be used as a guideline for future activities of EURADOS Working Groups but can also be used as guidance for research in radiation dosimetry by the wider community. It will also be used as input for a general European research roadmap for radiation protection, following similar previous contributions to the European Joint Programme for the Integration of Radiation Protection Research, under the Horizon 2020 programme (CONCERT). The full version of the SRA is available as a EURADOS report (www.eurados.org).

Intercomparison of Monte Carlo calculated dose enhancement ratios for gold nanoparticles irradiated by X-rays: Assessing the uncertainty and correct methodology for extended beams

Results of a Monte Carlo code intercomparison exercise for simulations of the dose enhancement from a gold nanoparticle (GNP) irradiated by X-rays have been recently reported. To highlight potential differences between codes, the dose enhancement ratios (DERs) were shown for the narrow-beam geometry used in the simulations, which leads to values significantly higher than unity over distances in the order of several tens of micrometers from the GNP surface. As it has come to our attention that the figures in our paper have given rise to misinterpretation as showing 'the' DERs of GNPs under diagnostic X-ray irradiation, this article presents estimates of the DERs that would have been obtained with realistic radiation field extensions and presence of secondary particle equilibrium (SPE). These DER values are much smaller than those for a narrow-beam irradiation shown in our paper, and significant dose enhancement is only found within a few hundred nanometers around the GNP. The approach used to obtain these estimates required the development of a methodology to identify and, where possible, correct results from simulations whose implementation deviated from the initial exercise definition. Based on this methodology, literature on Monte Carlo simulated DERs has been critically assessed.

Quality assurance for the use of computational methods in dosimetry: activities of EURADOS Working Group 6 'Computational Dosimetry'

Working Group (WG) 6 'Computational Dosimetry' of the European Radiation Dosimetry Group promotes good practice in the application of computational methods for radiation dosimetry in radiation protection and the medical use of ionising radiation. Its cross-sectional activities within the association cover a large range of current topics in radiation dosimetry, including more fundamental studies of radiation effects in complex systems. In addition, WG 6 also performs scientific research and development as well as knowledge transfer activities, such as training courses. Monte Carlo techniques, including the use of anthropomorphic and other numerical phantoms based on voxelised geometrical models, play a strong part in the activities pursued in WG 6. However, other aspects and techniques, such as neutron spectra unfolding, have an important role as well. A number of intercomparison exercises have been carried out in the past to provide information on the accuracy with which computational methods are applied and whether best practice is being followed. Within the exercises that are still ongoing, the focus has changed towards assessing the uncertainty that can be achieved with these computational methods. Furthermore, the future strategy of WG 6 also includes an extension of the scope toward experimental benchmark activities and evaluation of cross-sections and algorithms, with the vision of establishing a gold standard for Monte Carlo methods used in medical and radiobiological applications.

Stopping power of water for carbon ions with energies in the Bragg peak region

The stopping power of liquid water was measured for carbon ions with energies in the Bragg peak region using the inverted Doppler shift attenuation method. Among the semiempirical data, the results of this work agree best with the data recommended in the Errata and Addendum of ICRU Report No. 73, which is based on an I value of 78 eV for water. The agreement was worse when the present results were compared to the newer recommendation of the ICRU published in ICRU Report No. 90. The srim code seems to slightly overestimate the stopping power of water for carbon ions above 3 MeV. A semiexperimental stopping power of water for α particles was derived from the present results using the theoretical ratio between the stopping powers of water for carbon ions and α particles computed by means of the casp code. These values agree well with the experimental data for α particles within the uncertainties.

Roadmap for metal nanoparticles in radiation therapy: current status, translational challenges, and future directions

This roadmap outlines the potential roles of metallic nanoparticles (MNPs) in the field of radiation therapy. MNPs made up of a wide range of materials (from Titanium, Z = 22, to Bismuth, Z = 83) and a similarly wide spectrum of potential clinical applications, including diagnostic, therapeutic (radiation dose enhancers, hyperthermia inducers, drug delivery vehicles, vaccine adjuvants, photosensitizers, enhancers of immunotherapy) and theranostic (combining both diagnostic and therapeutic), are being fabricated and evaluated. This roadmap covers contributions from experts in these topics summarizing their view of the current status and challenges, as well as expected advancements in technology to address these challenges.

Intercomparison of dose enhancement ratio and secondary electron spectra for gold nanoparticles irradiated by X-rays calculated using multiple Monte Carlo simulation codes

PURPOSE

Targeted radiation therapy has seen an increased interest in the past decade. In vitro and in vivo experiments showed enhanced radiation doses due to gold nanoparticles (GNPs) to tumors in mice and demonstrated a high potential for clinical application. However, finding a functionalized molecular formulation for actively targeting GNPs in tumor cells is challenging. Furthermore, the enhanced energy deposition by secondary electrons around GNPs, particularly by short-ranged Auger electrons is difficult to measure. Computational models, such as Monte Carlo (MC) radiation transport codes, have been used to estimate the physical quantities and effects of GNPs. However, as these codes differ from one to another, the reliability of physical and dosimetric quantities needs to be established at cellular and molecular levels, so that the subsequent biological effects can be assessed quantitatively.

METHODS

In this work, irradiation of single GNPs of 50 nm and 100 nm diameter by X-ray spectra generated by 50 and 100 peak kilovoltages was simulated for a defined geometry setup, by applying multiple MC codes in the EURADOS framework.

RESULTS

The mean dose enhancement ratio of the first 10 nm-thick water shell around a 100 nm GNP ranges from 400 for 100 kVp X-rays to 600 for 50 kVp X-rays with large uncertainty factors up to 2.3.

CONCLUSIONS

It is concluded that the absolute dose enhancement effects have large uncertainties and need an inter-code intercomparison for a high quality assurance; relative properties may be a better measure until more experimental data is available to constrain the models.

PROPOSAL FOR A EUROPEAN METROLOGY NETWORK ON BIOLOGICAL IONISING RADIATION EFFECTS

Progress in the field of ionising radiation (IR) metrology achieved in the BioQuaRT project raised the question to what extent radiobiological investigations would benefit from metrological support of the applied methodologies. A panel of experts from the medical field, fundamental research and radiation protection attended a workshop at Physikalisch-Technische Bundesanstalt to consult on metrology needs related to biological radiation effects. The panel identified a number of metrological needs including the further development of experimental and computational techniques for micro- and nanodosimetry, together with the determination of related fundamental material properties and the establishment of rigorous uncertainty budgets. In addition to this, a call to develop a metrology support for assisting quality assurance of radiobiology experiments was expressed. Conclusions from the workshop were presented at several international conferences for further discussion with the scientific community and stakeholder groups that led to an initiative within the metrology community to establish a European Metrology Network on biological effects of IR.

Determining dose enhancement factors of high-Z nanoparticles from simulations where lateral secondary particle disequilibrium exists

Nanoparticles (NPs) containing high atomic number (high-Z) materials have been shown to enhance the radiobiological effectiveness of ionizing radiation. This effect is often attributed to an enhancement of the absorbed dose in the vicinity of the NPs, based on Monte Carlo simulations that show a significant local enhancement of the energy deposition on the microscopic scale. The results of such simulations may be significantly biased and lead to a severe overestimation of the dose enhancement if the condition of secondary particle equilibrium is not met in the simulation setup. This current work shows an approach to estimate a 'realistic' dose enhancement from the results of such biased simulations which is based on published photon interaction data and provides a way for correcting biased results.

EURADOS education and training activities

This paper provides a summary of the Education and Training (E&T) activities that have been developed and organised by the European Radiation Dosimetry Group (EURADOS) in recent years and in the case of Training Courses over the last decade. These E&T actions include short duration Training Courses on well-established topics organised within the activity of EURADOS Working Groups (WGs), or one-day events integrated in the EURADOS Annual Meeting (workshops, winter schools, the intercomparison participants' sessions and the learning network, among others). Moreover, EURADOS has recently established a Young Scientist Grant and a Young Scientist Award. The Grant supports young scientists by encouraging them to perform research projects at other laboratories of the EURADOS network. The Award is given in recognition of excellent work developed within the WGs' work programme. Additionally, EURADOS supports the dissemination of knowledge in radiation dosimetry by promoting and endorsing conferences such as the individual monitoring (IM) series, the neutron and ion dosimetry symposia (NEUDOS) and contributions to E&T sessions at specific events.

AN ALGORITHM TO DETERMINE THE NANODOSIMETRIC IMPACT OF GOLD NANOPARTICLES ON CELL MODELS

High-Z nanomaterials, e.g. gold nanoparticles (GNPs), are being investigated worldwide for potential application in radiation imaging and therapy. Photon irradiation of cells containing GNP was shown to produce enhanced DNA damage which is believed to be related to the increased secondary electron (SE) yield and ionization density. In this work, an algorithm was developed for simulating the physical radiation damage inside the nucleus of a spherical cell model for the case of uniformly distributed GNPs within the cytoplasm. Previously calculated energy spectra of SE emerging from a single NP irradiated with different photon sources are used as input to obtain the SE energy spectrum at the surface of the cell nucleus. In a second step, the SE transport inside the cell nucleus is simulated with a track structure Monte Carlo code to obtain the spatial distribution of ionizations. The preliminary results presented here show that the developed algorithm allows for a fast calculation of the SE spectra at the cell nucleus surface, thus enabling a more realistic assessment of the ionization density inside the cell nucleus than that obtained by the simulation of a single GNP. Furthermore, the algorithm can be easily adapted to investigate both the effect of GNP clustering and the impact of GNP-GNP interactions on SE spectra.

INVESTIGATION INTO THE PROBABILITY FOR MISCOUNTING IN FOCI-BASED ASSAYS

When early radiation damage to biological systems is studied based on the formation of foci at the location of DNA double-strand breaks, the foci observed in irradiated cells either may be induced by ionizing radiation (IR) interactions or they may be due to other causes that lead to observation of foci also in unirradiated cells. Generally, to take account of the latter, additional samples are taken where the exposure to IR is skipped in the protocol. The data analysis relies on statistical independence of the frequency distributions of background and radiation-induced foci. In microscopy, however, the observed spatial patterns of foci are 2D projections of the spatial distributions of foci in the observed cell nuclei. This may lead to missing foci when scoring their number, particularly if projections of foci overlap or coincide. This paper investigates to what extent the statistical independence of the frequency distribution of the number of foci coming from IR interaction or other causes is compromised by foci overlapping.

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.

CONSIDERATIONS ON THE TARGET SIZE IN A WALL-LESS NANODOSIMETER

In nanodosimetry, the ionization component of charged particle track structure is characterized by measuring the frequency distribution of ionizations in target volumes that simulate nanometric sites in liquid water. For the Ion Counter nanodosimeter at PTB, the sensitive volume is defined by the electrical field and the extraction aperture. In this paper, a procedure is presented to define a cylindrical effective measurement target based on the second moments of the detection efficiency map. An analytical model of the efficiency map is developed to investigate the dependence of the simulated site size on the nanodosimeter's operating parameters. Within the limits of the simplifying assumptions, the model gives a reasonable approximation of the efficiency map.

PROSPECTS FOR METROLOGY RELATED TO BIOLOGICAL RADIATION EFFECTS OF ION BEAMS

In recent years, several approaches have been proposed to provide an understanding of the enhanced relative biological effectiveness of ion beams based on multi-scale models of their radiation effects. Among these, the BioQuaRT project was the only one which focused on developing metrology for a multi-scale characterization of particle track structure. The progress made within the BioQuaRT project has motivated the formation of a department 'Radiation Effects' at PTB dedicated to metrological research on ionizing radiation effects. This paper gives an overview of the department's present research directions and shortly discusses ideas for the future development of metrology related to biological effects of ion beams that are based on a stakeholder consultation.

WHAT ROLES FOR TRACK-STRUCTURE AND MICRODOSIMETRY IN THE ERA OF -omics AND SYSTEMS BIOLOGY?

Ionizing radiation is a peculiar perturbation when it comes to damage to biological systems: it proceeds through discrete energy depositions, over a short temporal scale and a spatial scale critical for subcellular targets as DNA, whose damage complexity determines the outcome of the exposure. This lies at the basis of the success of track structure (and nanodosimetry) and microdosimetry in radiation biology. However, such reductionist approaches cannot account for the complex network of interactions regulating the overall response of the system to radiation, particularly when effects are manifest at the supracellular level and involve long times. Systems radiation biology is increasingly gaining ground, but the gap between reductionist and holistic approaches is becoming larger. This paper presents considerations on what roles track structure and microdosimetry can have in the attempt to fill this gap, and on how they can be further exploited to interpret radiobiological data and inform systemic approaches.

From Energy Deposition of Ionizing Radiation to Cell Damage Signaling: Benchmarking Simulations by Measured Yields of Initial DNA Damage after Ion Microbeam Irradiation

Advances in accelerator technology, which have enabled conforming radiotherapy with charged hadronic species, have brought benefits as well as potential new risks to patients. To better understand the effects of ionizing radiation on tumor and surrounding tissue, it is important to investigate and quantify the relationship between energy deposition at the nanometric scale and the initial biological events. Monte Carlo track structure simulation codes provide a powerful tool for investigating this relationship; however, their success and reliability are dependent on their improvement and development accordingly to the dedicated biological data to which they are challenged. For this aim, a microbeam facility that allows for fluence control, down to one ion per cell nucleus, was used to evaluate relative frequencies of DNA damage after interaction between the incoming ion and DNA according to radiation quality. Primary human cells were exposed to alpha particles of three different energies with respective linear energy transfers (LETs) of approximately 36, 85 or 170 keV·µm^-1 at the cells' center position, or to protons (19 keV·µm^-1). Statistical evaluation of nuclear foci formation (53BP1/γ-H2AX), observed using immunofluorescence and related to a particle traversal, was undertaken in a large population of cell nuclei. The biological results were adjusted to consider the factors that drive the experimental uncertainties, then challenged with results using Geant4-DNA code modeling of the ionizing particle interactions on a virtual phantom of the cell nucleus with the same mean geometry and DNA density as the cells used in our experiments. Both results showed an increase of relative frequencies of foci (or simulated DNA damage) in cell nuclei as a function of increasing LET of the traversing particles, reaching a quasi-plateau when the LET exceeded 80-90 keV·µm^-1. For the LET of an alpha particle ranging from 80-90 to 170 keV·µm^-1, 10-30% of the particle hits did not lead to DNA damage inducing 53BP1 or γ-H2AX foci formation.

A New Standard DNA Damage (SDD) Data Format

Our understanding of radiation-induced cellular damage has greatly improved over the past few decades. Despite this progress, there are still many obstacles to fully understand how radiation interacts with biologically relevant cellular components, such as DNA, to cause observable end points such as cell killing. Damage in DNA is identified as a major route of cell killing. One hurdle when modeling biological effects is the difficulty in directly comparing results generated by members of different research groups. Multiple Monte Carlo codes have been developed to simulate damage induction at the DNA scale, while at the same time various groups have developed models that describe DNA repair processes with varying levels of detail. These repair models are intrinsically linked to the damage model employed in their development, making it difficult to disentangle systematic effects in either part of the modeling chain. These modeling chains typically consist of track-structure Monte Carlo simulations of the physical interactions creating direct damages to DNA, followed by simulations of the production and initial reactions of chemical species causing so-called "indirect" damages. After the induction of DNA damage, DNA repair models combine the simulated damage patterns with biological models to determine the biological consequences of the damage. To date, the effect of the environment, such as molecular oxygen (normoxic vs. hypoxic), has been poorly considered. We propose a new standard DNA damage (SDD) data format to unify the interface between the simulation of damage induction in DNA and the biological modeling of DNA repair processes, and introduce the effect of the environment (molecular oxygen or other compounds) as a flexible parameter. Such a standard greatly facilitates inter-model comparisons, providing an ideal environment to tease out model assumptions and identify persistent, underlying mechanisms. Through inter-model comparisons, this unified standard has the potential to greatly advance our understanding of the underlying mechanisms of radiation-induced DNA damage and the resulting observable biological effects when radiation parameters and/or environmental conditions change.

State of The Art of Instrumentation in Experimental Nanodosimetry

Nanodosimetry is a branch of dosimetry for investigation and modeling of the interaction pattern of ionizing radiation in nanometre site-sizes (at unit density), which dates back to the 1970's (Pszona S. A track ion counter. Proceedings of Fifth Symposium on Microdosimetry EUR 5452 d-e-f, Published by the Commission of the European Communities, Luxemburg, pp. 1107-1122 (1976)). To date, the different experimental approaches have lead to developing of three fully functional nanodosimeters: the Jet Counter operated at NCBJ, the Ion Counter operated at PTB and Startrack Counter operated at INFN-LNL. Descriptions of each nanodosimeter as well as of the techniques used to investigate the track structure of ionizing particles are presented.

NANODOSIMETRY: TOWARDS A NEW CONCEPT OF RADIATION QUALITY

The biological action of ionizing charged particles is initiated at the DNA level, and the effectiveness with which the initial physical effect changes into measurable biological damage is likely ruled by the stochastics of ionizations produced by the incident ions in subcellular nanometric volumes. Based on this hypothesis, experimental nanodosimetry aims at establishing a new concept of radiation quality that builds on measurable characteristics of the particle track structure at the nanometer scale. Three different nanodosimetric detection systems have been developed to date that allow measurements of the number of ionizations produced by the passage of a primary particle in a nanometer-size gas volume (in unit density scale). Within the Italian project MITRA (MIcrodosimetry and TRAck structure), funded by the Italian Istituto Nazionale di Fisica Nucleare (INFN) and the EMRP Joint Research Project 'BioQuaRT' (Biologically Weighted Quantities in Radiotherapy), experiments have been carried out, in which the frequency distribution of ionizations produced by proton and carbon ion beams of given energy was measured with the three nanodosimetric detectors. Descriptors of the track structure can be derived from these distributions. In particular, the first moment M1, representing the mean number of ionizations produced in the target volume, and the cumulative probability Fk of measuring a number ν ≥ k of ionizations. The correlation between measured nanodosimetric quantities and experimental radiobiological data available in the literature is here presented and discussed.

Analysis of Radiation-Induced Chromosomal Aberrations on a Cell-by-Cell Basis after Alpha-Particle Microbeam Irradiation: Experimental Data and Simulations

There is a continued need for further clarification of various aspects of radiation-induced chromosomal aberration, including its correlation with radiation track structure. As part of the EMRP joint research project, Biologically Weighted Quantities in Radiotherapy (BioQuaRT), we performed experimental and theoretical analyses on chromosomal aberrations in Chinese hamster ovary cells (CHO-K1) exposed to α particles with final energies of 5.5 and 17.8 MeV (absorbed doses: ∼2.3 Gy and ∼1.9 Gy, respectively), which were generated by the microbeam at the Physikalisch-Technische Bundesanstalt (PTB) in Braunschweig, Germany. In line with the differences in linear energy transfer (approximately 85 keV/μm for 5.5 MeV and 36 keV/μm for 17.8 MeV α particles), the 5.5 MeV α particles were more effective than the 17.8 MeV α particles, both in terms of the percentage of aberrant cells (57% vs. 33%) and aberration frequency. The yield of total aberrations increased by a factor of ∼2, although the increase in dicentrics plus centric rings was less pronounced than in acentric fragments. The experimental data were compared with Monte Carlo simulations based on the BIophysical ANalysis of Cell death and chromosomal Aberrations model (BIANCA). This comparison allowed interpretation of the results in terms of critical DNA damage [cluster lesions (CLs)]. More specifically, the higher aberration yields observed for the 5.5 MeV α particles were explained by taking into account that, although the nucleus was traversed by fewer particles (nominally, 11 vs. 25), each particle was much more effective (by a factor of ∼3) at inducing CLs. This led to an increased yield of CLs per cell (by a factor of ∼1.4), consistent with the increased yield of total aberrations observed in the experiments.

Double differential cross sections for proton induced electron emission from molecular analogues of DNA constituents for energies in the Bragg peak region

For track structure simulations in the Bragg peak region, measured electron emission cross sections of DNA constituents are required as input for developing parameterized model functions representing the scattering probabilities. In the present work, double differential cross sections were measured for the electron emission from vapor-phase pyrimidine, tetrahydrofuran, and trimethyl phosphate that are structural analogues to the base, the sugar, and the phosphate residue of the DNA, respectively. The range of proton energies was from 75 keV to 135 keV, the angles ranged from 15° to 135°, and the electron energies were measured from 10 eV to 200 eV. Single differential and total electron emission cross sections are derived by integration over angle and electron energy and compared to the semi-empirical Hansen-Kocbach-Stolterfoht (HKS) model and a quantum mechanical calculation employing the first Born approximation with corrected boundary conditions (CB1). The CB1 provides the best prediction of double and single differential cross section, while total cross sections can be fitted with semi-empirical models. The cross sections of the three samples are proportional to their total number of valence electrons.

'BioQuaRT' project: design of a novel in situ protocol for the simultaneous visualisation of chromosomal aberrations and micronuclei after irradiation at microbeam facilities

The aim of the 'BioQuaRT' (Biologically weighted Quantities in RadioTherapy) project is to develop measurement techniques for characterising charged particle track structure on different length scales, and to correlate at the cellular level the track structure properties with the biological effects of radiation. This multi-scale approach will allow characterisation of the radiation qualities used in radiotherapy and the related biological effects. Charged-particle microbeam facilities were chosen as the platforms for all radiobiology experiments in the 'BioQuaRT' project, because they allow targeting single cells (or compartments of a cell) with a predefined number of ionising particles and correlating the cell-by-cell induced damage with type and energy of the radiation and with the number of ions per cell. Within this project, a novel in situ protocol was developed for the analysis of the misrepaired and/or unrepaired chromosome damage induced by charged-particle irradiations at the Physikalisch-Technische Bundesanstalt (PTB) ion microbeam facility. Among the cytogenetic biomarkers to detect and estimate radiation-induced DNA damage in radiobiology, chromosomal aberrations and micronuclei were chosen. The characteristics of the PTB irradiation system required the design of a special in situ assay: specific irradiation dishes with a base made from a biofoil 25-µm thick and only 3000-4000 cells seeded and irradiated per dish. This method was developed on Chinese hamster ovary (CHO) cells, one of the most commonly used cell lines in radiobiology in vitro experiments. The present protocol allows the simultaneous scoring of chromosome aberrations and micronuclei on the same irradiated dish. Thanks to its versatility, this method could also be extended to other radiobiological applications besides the single-ion microbeam irradiations.

Experimental investigation of ionisation track structure of carbon ions at HIL Warsaw

In view of the upcoming radiation therapy with carbon ions, the ionisation structure of the carbon ion track at the nanometre scale is of particular interest. Two different nanodosimeters capable of measuring track structure of ionising particles in a gas target equivalent to a nanometric site in condensed matter were involved in the presented experimental investigation, namely the NCBJ Jet Counter and the PTB Ion Counter. At the accelerator facility of the HIL in Warsaw, simulated nanometric volumes were irradiated with carbon ions of 45 and 76 MeV of kinetic energy, corresponding to a range in the tissue of ∼85 µm and ∼190 µm, respectively. The filling gas of both nanodosimeters' ionisation volume was molecular nitrogen N2, and the ionisation cluster size distributions, i.e. the statistical distribution of the number of ionizations produced by one single primary carbon ion in the filling gas, were measured for the two primary particle energies.