Objective.To evaluate the uncertainty of physical features of low-energy electron transport in liquid water due to the use of different Monte Carlo track-structure (MCTS) codes.Approach.Five MCTS codes developed specifically for liquid water, namely, Geant4-DNA, PHITS-TS, RITRACKS, NASIC, and PARTRAC are compared and used to calculate the electronic stopping power, pathlength and absorption range, dose-point-kernel, and the frequency-mean (y¯F) and dose-mean (y¯D) lineal energy for primary electron energies from 20 eV to 100 keV. The uncertainty of each calculated quantity is evaluated by the relative standard deviation (RSD) and the maximum relative difference (MRD) among the codes. The medium-to-high-energy (1-100 keV) performance of the codes is benchmarked against the stopping power and range data for liquid water reported in ICRU Report 90.Main Results.For energies above ∼1 keV theRSDis moderate and mostly between 5%-15%, but increases rapidly at lower energies, reaching 20%-100% at sub-100 eV energies. It is noteworthy that theMRDmay well exceed 100% below 100 eV, while remaining sizeable (>20%) even at relatively high energies (10-100 keV). Fairly good agreement between the MCTS codes and the ICRU data for the stopping power (1-100 keV) and range (10-100 keV) in liquid water is found with average deviations between 1%-16%, depending on the code.Significance.The present work reveals significant differences for low-energy electron transport among liquid water MCTS codes, especially below 100 eV. These differences potentially compromise the accuracy of nanoscale simulations where such electrons play a key role. The observed dispersion of results is a consequence of the limitations of the theoretical models used to calculate electron interaction cross sections and the lack of relevant experimental data for their validation and benchmarking. This highlights the need for further development of the physics models used in MCTS codes to reduce the uncertainties associated with low-energy electron transport calculations in liquid water.

Biological effects induced by diverse types of ionizing radiation are known to show important variations. Nanodosimetry is suitable for studying the link between these variations and the patterns of radiation interactions within nanometer-scale volumes, using experimental techniques complemented by Monte Carlo track structure (MCTS) simulations. However, predicted nanodosimetric quantities differ among MCTS codes, primarily because each code employs distinct molecular-scale particle interaction models. This multi-code study examines these variations for low-energy electrons (20-10,000 eV), which play a critical role in energy deposition and biological effects by virtually all types of ionizing radiation. Specifically, the hypothesis tested in this work is that inter-code variability in nanodosimetry results is mainly caused by differences in assumptions regarding total interaction cross sections. Ionization cluster size distributions and derived nanodosimetric parameters were simulated with seven MCTS codes (PARTRAC, PHITS-TS, MCwater, PTra, and three Geant4-DNA options) in liquid water as a surrogate for biological tissue. Significant inter-code differences were observed, especially at the lowest energies. They were substantially reduced upon replacing the original cross sections in each code with a common, averaged dataset, created ad-hoc for this study and not based on theoretical assumptions. For example, for 50 eV electrons in 8 nm spheres, the variability in the predicted mean ionization numbers decreased from 23% to 5%, and in the probability of inducing two or more ionizations from 34% to 7% (relative standard deviations). This quantification demonstrates that total interaction cross sections are the primary source of uncertainty at low electron energies. A sensitivity test using DNA damage simulations with the PARTRAC code revealed that cross section variations notably affect biological outcome predictions. Replacing the code's original cross sections with the averaged ones increased the predicted double-strand break yield by up to 15%. These findings underscore the urgent need for improved characterization of low-energy electron interaction cross sections to reduce uncertainties in MCTS simulations and enhance mechanistic understanding of radiation-induced biological effects.

The number of patients undergoing diagnostic radiology and radiation therapy procedures has increased drastically owing to improvements in cancer diagnosis and treatment, and consequently, patient survival. However, the risk of secondary malignancies owing to radiation exposure remains a matter of concern. We previously published three hybrid computational fetal phantoms, which contained 27 fetal organs, as a starting point for developing the whole hybrid computational pregnant phantom set, which is the final objective of this study. An International Commission on Radiological Protection (ICRP) reference female voxel model was converted to a non-uniform rational B-spline (NURBS) surface model to construct a hybrid computational female phantom as a pregnant mother for each fetal model. Both fetal and maternal organs were matched with the ICRP- 89 reference data. To create a complete standard pregnant computational phantom set at 20, 30, and 35 weeks of pregnancy, the model mother’s reproductive organs were removed, and fetal phantoms with appropriate placental and uterine models were added to the female pelvis using a 3D-modeling software. With the aid of radiological image sets that had originally been used to construct the fetal models, each fetal position and rotation inside the uterus were carefully adjusted to represent the real fetal locations inside the uterus. The major abdominal soft tissue organs below the diaphragm, namely the small intestine, large intestine, liver, gall bladder, stomach, pancreas, uterus, and urinary bladder, were removed from non-pregnant females. The resulting fetal phantom was positioned in the appropriate location, matching the original radiological image sets. An obstetrician-gynecologist reviewed the complete internal anatomy of all fetus phantoms and the pregnant women for accuracy, and suggested changes were implemented as needed. The remaining female anatomical tissues were reshaped and modified to accommodate the location of the fetus inside the uterus. This new series of hybrid computational pregnant phantom models provides realistic anatomical details that can be useful in evaluating fetal radiation doses in pregnant patients undergoing diagnostic imaging or radiotherapy procedures where realistic fetal computational human phantoms are required.

Recent publications have suggested that oxidative DNA damage mediated by hydroxyl radical (˙OH) is unimportant in vivo, and that carbonate anion radical (CO3˙-) plays the key role. We examine these claims and summarize the evidence that ˙OH does play a key role as an important member of the reactive oxygen species (ROS) in vivo.

Radiation exposure and associated radiation risks are major concerns for fetal development for pregnant patients who undergo radiation therapy or diagnostic imaging procedures. In order to accurately estimate the radiation dose to the fetus and assess the uncertainty of fetal position and rotation, three hybrid computational fetus phantoms were constructed using magnetic resonance imaging (MRI) for each fetus model as a starting point to construct a complete anatomically accurate fetus, gravid uterus, and placenta. A total of 27 fetal organs were outlined from radiological images via the Velocity Treatment Planning System. The DICOM-Structure set was imported to Rhinoceros software for further reconstruction of 3D fetus phantom model sets. All fetal organ masses were compared with ICRP-89 reference data. Our fetal model series corresponds to 20, 31, and 35 weeks of pregnancy, thus covering the second and third trimester. Fetal positions and locations were carefully adapted to represent the real fetus locations inside the uterus for each trimester of pregnancy. The new series of hybrid computational fetus models together with pregnant female models can be used in evaluating fetal radiation doses in diagnostic imaging and radiotherapy procedures.

Most accelerator-based space radiation experiments have been performed with single ion beams at fixed energies. However, the space radiation environment consists of a wide variety of ion species with a continuous range of energies. Due to recent developments in beam switching technology implemented at the NASA Space Radiation Laboratory (NSRL) at Brookhaven National Laboratory (BNL), it is now possible to rapidly switch ion species and energies, allowing for the possibility to more realistically simulate the actual radiation environment found in space. The present paper discusses a variety of issues related to implementation of galactic cosmic ray (GCR) simulation at NSRL, especially for experiments in radiobiology. Advantages and disadvantages of different approaches to developing a GCR simulator are presented. In addition, issues common to both GCR simulation and single beam experiments are compared to issues unique to GCR simulation studies. A set of conclusions is presented as well as a discussion of the technical implementation of GCR simulation.

Monte Carlo track structure simulations follow the primary as well as all produced secondary particles event-by-event, from starting or ejection energy down to total stopping. Simulation codes require both transport models and reliable interaction cross sections for the primary and secondary particles with the material under consideration. In general, charged particles can scatter elastically (they change direction, but do not lose energy) and can ionize or excite the target material. In addition, if the primary particle is an ion, i.e., an atomic system, it can change its charge state by electron capture and electron loss at low energy as it slows down. Different charge states of the same atom will have different ionization and excitation probabilities.

A modified and updated version of the model of the dielectric response function of liquid water as currently implemented in the PARTRAC code is presented. The updated version takes advantage of the newer experimental information from the Sendai group and implements some improvements in modeling and usability.

Monte Carlo track-structure simulations provide a detailed and accurate picture of radiation transport of charged particles through condensed matter of biological interest. Liquid water serves as a surrogate for soft tissue and is used in most Monte Carlo track-structure codes. Basic theories of radiation transport and track-structure simulations are discussed and differences compared to condensed history codes highlighted. Interaction cross sections for electrons, protons, alpha particles, and light and heavy ions are required input data for track-structure simulations. Different calculation methods, including the plane-wave Born approximation, the dielectric theory, and semi-empirical approaches are presented using liquid water as a target. Low-energy electron transport and light ion transport are discussed as areas of special interest.

We study the emission of L alpha, L beta, and L gamma characteristic x rays by the impact of electrons on Hf, Ta, W, Re, Os, Au, Pb, and Bi atoms. To this end, ionization cross sections of the L-1, L-2, and L-3 subshells of these atoms are calculated within the distorted-wave Born approximation. The considered energy interval spans from the ionization threshold up to 50 keV. Atomic relaxation parameters (i.e., Coster-Kronig and radiative transition probabilities, fluorescence yields, and emission rates) taken from the literature are then used to evaluate x-ray production cross sections. The theoretical predictions are compared with published experimental information. Good agreement is found for Ta, W, Os, Au, Pb, and Bi. In the case of Hf and Re, the measured cross sections are lower than the theoretical estimates by around 30%. The observed discrepancies might be attributed to the methods employed to correct the raw experimental data for the excess of detected characteristic x rays caused by the finite thickness of the sample's active layer and the presence of the thick substrate.

This review describes the PARTRAC suite of comprehensive Monte Carlo simulation tools for calculations of track structures of a variety of ionizing radiation qualities and their biological effects. A multi-scale target model characterizes essential structures of the whole genomic DNA within human fibroblasts and lymphocytes in atomic resolution. Calculation methods and essential results are recapitulated regarding the physical, physico-chemical and chemical stage of track structure development of radiation damage induction. Recent model extension towards DNA repair processes extends the time dimension by about 12 orders of magnitude and paves the way for superior predictions of radiation risks.

Data from Monte Carlo transport codes are used to model radiobiological effects. We previously reported the Fourier analysis of ionization data generated by simulating a 500-keV proton traversing water. Here, we extend Fourier analysis to energy transfer data of another radiation type, a 14-MeV α-particle. A radiobiological model based on this frequency-domain analysis views cell as an information processing system [8]. It lends itself naturally to traditional engineering analyses. One engineering principle—the output response of a linear system to random signal—is applied here to explain the fact that there is measurable difference in the magnitude of the biological effectiveness when a given biological system is irradiated with two different radiation types of the same Linear Energy Transfer (LET).

We have derived inelastic interaction cross sections for electrons and protons with Al, Cu, and Au within the framework of a plane-wave Born approximation and an optical data model. We have implemented a transport model for protons and electrons in Cu into the Monte Carlo track structure code PARTRAC. We have simulated secondary electron yields in Cu and compared them with measurements from the literature. We find that simulated yields in general follow the measurements but clearly overestimate them at energies below 50 eV.

We have determined the optical and dielectric response functions of metallic calcium. We have extensively analyzed available published data from the literature as well as theoretical calculations and constraints. The adopted functions fulfil all sum rules. The determined mean excitation value for calcium is I = 171.8 eV.

We have calculated Lα, Lβ, and Lγ x-ray production cross sections by the impact of electrons on Hf, W, Re, Os, Au, and Pb. We used the distorted-wave Born approximation to calculate ionization cross sections of the L subshells of these atoms. Atomic relaxation parameters were taken from the literature. The theoretical predictions are compared with published experimental information. Good agreement is generally found except for Re, where the measured cross sections are systematically lower than the theoretical estimates by around 20 %. The observed discrepancy may be due to the relatively large uncertainty in the determination of the sample thickness.

We study the ionization cross sections of the K shell of Cu and the L shells of Ag, In, and Sn by positron impact using the distorted-wave Born approximation, focusing on the near-threshold energy range (below 40 keV) to compare with recent measurements. The distorted-wave formalism proves to be an appropriate approach, especially to describe the shape of the cross-section curves for positrons accurately. Furthermore, we calculate x-ray production cross sections of the considered L shells having recourse to two sets of atomic relaxation parameters involved in the conversion from ionization to x-ray production cross sections. The 5%-10% differences in the theoretical values calculated with these parameter sets are apparently not large enough to explain the discrepancy between the distorted-wave curves and the experimental data, particularly for Ag. Other sources of uncertainty that could be affecting these comparisons are discussed.

Electron emission spectra from thin metal foils with thin layers of water frozen on them (amorphous solid water) after fast proton impact have been measured and have been simulated in liquid water using the event-by-event track structure code PARTRAC. The electron transport model of PARTRAC has been extended to simulate electron transport down to 1eV by including low-energy phonon, vibrational and electronic excitations as measured by Michaud et al. [Michaud, M., Wen, A., Sanche, L., 2003. Cross sections for low-energy (1–100eV) electron elastic and inelastic scattering in amorphous ice. Radiat. Res. 159, 3–22] for amorphous ice. Simulated liquid water yields follow in general the amorphous solid water measurements at higher energies, but overestimate them significantly at energies below 50eV.

Space and cosmic radiation is characterized by energetic heavy ions of high linear energy transfer (LET). Although both low- and high-LET radiations can create oxidative clustered DNA lesions and double-strand breaks (DSBs), the local complexity of oxidative clustered DNA lesions tends to increase with increasing LET. We irradiated 28SC human monocytes with doses from 0-10 Gy of Fe-56 ions (1.046 GeV/nucleon, LET = 148 keV/mu m) and determined the induction and processing of prompt DSBs and oxidative clustered DNA lesions using pulsed-field gel electrophoresis (PFGE) and Number Average Length Analysis (NALA). The Fe-56 ions produced decreased yields of DSBs (10.9 DSB Gy(-1) Gbp(-1)) and clusters (1 DSB: similar to 0.8 Fpg clusters: similar to 0.7 Endo III clusters: similar to 0.5 Endo IV clusters) compared to previous results with Cs-137 gamma rays. The difference in the relative biological effectiveness (RBE) of the measured and predicted DSB yields may be due to the formation of spatially correlated DSBs (regionally multiply damaged sites) which result in small DNA fragments that are difficult to detect with the PFGE assay. The processing data suggest enhanced difficulty compared with gamma rays in the processing of DSBs but not clusters. At the same time, apoptosis is increased compared to that seen with gamma rays. The enhanced levels of apoptosis observed after exposure to Fe-56 ions may be due to the elimination of cells carrying high levels of persistent DNA clusters that are removed only by cell death and/or "splitting" during DNA replication. (c) 2007 by Radiation Research Society.