Heavy ions are riveting in radiation biophysics, particularly in the areas of radiotherapy and space radiation protection. Accelerated charged particles can indeed penetrate deeply in the human body to sterilize tumors, exploiting the favorable depth-dose distribution of ions compared to conventional X rays. Conversely, the high biological effectiveness in inducing late effects presents a hazard for manned space exploration. Even after half a century of accelerator-based experiments, clinical applications and flight research, these two topics remain both fascinating and baffling. Heavy-ion therapy is very expensive, and despite the clinical success it remains controversial. Research on late radiation morbidity in spaceflight led to a reduction in uncertainty, but also pointed to new risks previously underestimated, such as possible damage to the central nervous system. Recently, heavy ions have also been used in other, unanticipated biomedical fields, such as treatment of heart arrhythmia or inactivation of viruses for vaccine development. Heavy-ion science nicely merges physics and biology and remains an extraordinary research field for the 21st century.

Immunization with an inactivated virus is one of the strategies currently being tested towards developing a SARS-CoV-2 vaccine. One of the methods used to inactivate viruses is exposure to high doses of ionizing radiation to damage their nucleic acids. While gamma (γ) rays effectively induce lesions in the RNA, envelope proteins are also highly damaged in the process. This in turn may alter their antigenic properties, affecting their capacity to induce an adaptive immune response able to confer effective protection. Here, we modeled the effect of sparsely and densely ionizing radiation on SARS-CoV-2 using the Monte Carlo toolkit Geant4-DNA. With a realistic 3D target virus model, we calculated the expected number of lesions in the spike and membrane proteins, as well as in the viral RNA. Our findings showed that γ rays produced significant spike protein damage, but densely ionizing charged particles induced less membrane damage for the same level of RNA lesions, because a single ion traversal through the nuclear envelope was sufficient to inactivate the virus. We propose that accelerated charged particles produce inactivated viruses with little structural damage to envelope proteins, thereby representing a new and effective tool for developing vaccines against SARS-CoV-2 and other enveloped viruses.

The harmful effects of space radiation pose a serious health risk to astronauts participating in future long-term missions. Such radiation effects must be considered in the design phase of space vessels as well as in mission planning. Crew radioprotection during long periods in deep space (e.g., transit to Mars) represents a major challenge, especially because of the strong restrictions on the passive shielding load allowed on-board the vessel. Novel materials with better shielding performance compared to the “gold standard” high-density polyethylene are therefore greatly needed. Because of the high hydrogen content of hydrides, lithium hydride has been selected as a starting point for further studies of promising candidates to be used as passive shielding materials. In the current experimental campaign, the shielding performance of lithium hydride was assessed by measuring normalized dose, primary beam attenuation and neutron ambient dose equivalent using 430 MeV/u 12 C, 600 MeV/u 12 C and 228 MeV proton beams. The experimental data were then compared to predictions from the Monte Carlo transport codes PHITS and GRAS. The experimental results show an increased shielding effectiveness of lithium hydride compared to reference materials like polyethylene. For instance, the attenuation length for 600 MeV/u 12 C primary particles in lithium hydride is approximately 20% shorter compared to polyethylene. Furthermore, the comparison results between both transport codes indicates that the standard Tripathi-based total reaction cross-section model of PHITS cannot accurately reproduce the presented experimental data, whereas GRAS shows reasonable agreement.

The roadmap for space exploration foresees longer journeys and further excursions outside low-Earth orbit as well as the establishment of permanent outposts on other celestial bodies, such as the Moon or Mars. The design of spacecrafts and habitats depends heavily on the mission scenario and must consider the radiation protection properties of the structural components as well as dedicated shielding. In fact, short- and long-term effects caused by exposure to cosmic radiation are now considered among the main health risks of space travel. One of the current strategies is to find multifunctional materials that combine excellent mechanical properties with a high shielding effectiveness to minimize the overall load. In this work, the shielding effectiveness of a wide variety of single and multilayer materials of interest for different mission scenarios has been characterized. In the experimental campaign, reference and innovative materials, as well as simulants of Moon and Mars in situ resources, were irradiated with 1,000 MeV/u 4 He, 430 MeV/u 12 C and 962–972 MeV/u 56 Fe. The results are presented in terms of Bragg curves and dose reduction per unit area density. To isolate the shielding effectiveness only due to nuclear fragmentation, a correction for the energy loss in the material is also considered. These findings indicate that the best shield is lithium hydride, which performs even better than polyethylene. However, the technical feasibility of shielding needs to be investigated. The classification of all materials in terms of shielding effectiveness is not influenced by the ion species, but the value changes dramatically depending on the beam energy. The output of this investigation represents a useful database for benchmarking Monte Carlo and deterministic transport codes used for space radiation transport calculations. These findings also contribute to recommendations for optimizing the design of space vessels and habitats in different radiation environments.

There is experimental evidence that ultrasoft X rays (0.1–5 keV) show a higher biological effectiveness than high-energy photons. Similar to high-LET radiation, this is attributed to a rather localized dose distribution associated with a considerably smaller range of secondary electrons, which results in an increasing yield of double-strand breaks (DSBs) and potentially more complex lesions. We previously reported on the application of the Giant LOop Binary LEsion (GLOBLE) model to ultrasoft X rays, in which experimental values of the relative biological effectiveness (RBE) for DSB induction were used to show that this increasing DSB yield was sufficient to explain the enhanced effectiveness in the cell inactivation potential of ultrasoft X rays. Complementary to GLOBLE, we report here on a modeling approach to predict the increased DSB yield of ultrasoft X rays on the basis of amorphous track structure formed by secondary electrons, which was derived from Monte Carlo track structure simulations. This procedure is associated with increased production of single-strand break (SSB) clusters, which are caused by the highly localized energy deposition pattern induced by low-energy photons. From this, the RBE of ultrasoft X rays can be determined and compared to experimental data, showing that the inhomogeneity of the energy deposition pattern represents the key variable to describe the increased biological effectiveness of ultrasoft X rays. Thus, this work demonstrates an extended applicability of the amorphous track structure concept and tests its limits with respect to its predictive power. The employed model mechanism offers a possible explanation for how the cellular response to ultrasoft X rays is directly linked to the energy deposition properties on the nanometric scale.

We report here on a qualitative and quantitative comparison of four kinetic photon cell survival models. The commonly used linear-quadratic model extended by a dose protraction factor, the lethal potentially lethal model, the repair misrepair model and the recently reported Giant LOop Binary LEsion (GLOBLE) model are discussed with respect to the proposed underlying biological mechanisms explaining the cellular response to radiation. Furthermore, with the use of eight benchmarks, the accuracy, reliability, resolution power and robustness of the models are assessed and compared. This work demonstrates that the linear-quadratic, lethal potentially lethal and GLOBLE models often perform equivalently and that the repair misrepair model appears to have some drawbacks regarding the end points under investigation.

Accurate and mechanistically plausible mathematical models of DNA double-strand break (DSB) rejoining kinetics are needed to correctly estimate the dependence of cell death and transformation on linear energy transfer, radiation dose and time. When integrated into more comprehensive risk estimation approaches, such models are potentially valuable tools in applications such as treatment planning for radiotherapy. In this study, we compared 10 DSB rejoining models based on data collected from 61 mammalian cell lines after high-dose-rate photon or heavy ion irradiation. The set of models included formalisms with: 1. one, two or three discrete first-order rejoining rates; 2. continuously distributed first-order rejoining rates; and 3. second-order rejoining rates. The Akaike information criterion was used to quantify the relative support for each model from the data, accounting for goodness of fit and model complexity. The best performance was exhibited by a bi-exponential model with two discrete rejoining rates and a model with gamma-distribution rejoining rates. Models with more than three free parameters overfitted the data and models with single DSB rejoining rates or with an inflexible distribution of rejoining rates lacked accuracy. Of special note is that the analyzed data provide little support for models that rely on pairwise interactions to describe DSB rejoining kinetics. Consequently, kinetic cell survival models reflecting bi-exponential DSB rejoining might be preferable to models based on the kinetics of intra- and inter-lesion rejoining.

We present an application of the Giant LOop Binary LEsion (GLOBLE) model to the case of cell killing after irradiation with ultrasoft X rays. The model is based on the analysis of DSB clustering on the level of chromatin organization on a megabase pair length scale; it distinguishes between two classes of damage, characterized by either an isolated, single DSB (iDSB) or multiple, clustered DSB (cDSB) within a single giant loop. These corresponding fractions of iDSB and cDSB depend on the total number of DSB and thus on the dose as well as the yield of DSB per Gy per cell. Based on the increased yield of DSB with decreasing photon energy as reported in the literature, we demonstrate that according to the model this increased yield of DSB is sufficient to explain the increased RBE of ultrasoft X rays. Further assumptions as e.g., a higher lethality of individual DSB induced by ultrasoft X rays compared to high-energy photons, which might be a consequence of the more localized energy deposition, seem not to be a prerequisite. Since the model is also suitable to take into account local dose variations within the cell nucleus, we further analyze the impact of attenuation of low-energy photon radiation when penetrating a cell layer. We show that the inhomogenous dose distribution resulting from attenuation further increases the effectiveness and particularly affects the beta-term of the corresponding dose response curve. Finally, we compare and discuss the mechanisms of increased RBE as observed after ultrasoft X-ray irradiation with those observed after high-LET ion beam irradiation.

We irradiated normal human lymphocytes and fibroblasts with 137 Cs γ rays, 3.5 MeV α particles and 1 GeV/amu 56 Fe ions and measured the subsequent formation of chromosome-type aberrations by mFISH at the first mitosis following irradiation. This was done for the purposes of characterizing the shape of dose-response relationships and determining the frequency distribution of various aberration types with respect to the parameters of dose, radiation quality and cell type. Salient results and conclusions include the following. For low-LET γ rays, lymphocytes showed a more robust dose response for overall damage and a higher degree of upward curvature compared to fibroblasts. For both sources of high-LET radiation, and for both cell types, the response for simple and complex exchanges was linear with dose. Independent of all three parameters considered, the most likely damage outcome was the formation of a simple exchange event involving two breaks. However, in terms of the breakpoints making up exchange events, the majority of damage registered following HZE particle irradiation was due to complex aberrations involving multiple chromosomes. This adds a decidedly nonlinear component to the overall breakpoint response, giving it a significant degree of positive curvature, which we interpret as being due to interaction between ionizations of the primary HZE particle track and long-range δ rays produced by other nearby tracks. While such track interaction had been previously theorized, to the best of our knowledge, it has never been demonstrated experimentally.

A new, simple mechanistic dose-response model for cell survival after photon irradiation is presented. Its ingredients are motivated by the concept of giant loops, which constitute a level of chromatin organization on a megabase pair length scale. Double-strand breaks (DSBs) that are induced within different loop domains of the DNA are assumed to be processed independently by the cell's repair mechanism. The model distinguishes between two classes of damage, characterized by either a single DSB or multiple DSBs within a single loop. Different repair fidelities are associated with these two damage classes from which lethality of damages and consequently the survival probability of cells is derived. Given the giant loop chromatin organization and the assumption of two damage classes represent the main pillars of this new approach, we propose to call it the Giant LOop Binary LEsion (GLOBLE) approach. In this paper, we discuss the motivation and the formulation of the model as well as some basic implications. First applications to experimental data obtained with 250 kV X-rays exhibit that the model is able to reveal important features of the dose-response curves describing cell survival. These comprise a linear-quadratic behavior at lower doses and a transition to a straight dose-response relationship at high doses. We establish relationships to the parameters α and β of the linear-quadratic model and discuss possible generalizations. When expressed in terms of the linear-quadratic model, we demonstrate that our new model predicts an intrinsic anticorrelation between β and α, in line with an analysis of a large set of experimental data that is based on survival curves for more than 150 cell lines.

Durante, M. Biomarkers of Space Radiation Risk. Radiat. Res. 164, 467–473 (2005). Radiation risk estimates are based on epidemiological data obtained on Earth for cohorts exposed predominantly to acute doses of γ rays, and the extrapolation to the space environment is highly problematic and error-prone. The uncertainty can be reduced if risk estimates are compared directly to space radiation-induced biological alterations, i.e. by detecting biomarkers in astronauts. Chromosomal aberrations in peripheral blood lymphocytes are the only biomarker that can provide simultaneous information on dose, dose equivalent and risk, and they have been measured extensively in astronauts during the past 10 years. Individual relative risks calculated from chromosomal aberration measurements in crew members after single space missions in low-Earth orbit fall in the same range as the estimates derived from physical dosimetry, suggesting that the current system for radiogenic risk evaluation is essentially sound. However, the output of the biomarker test is dependent upon the sampling time. Recent results show a fast time-dependent decay of chromosomal aberrations in blood lymphocytes after space flight and a lack of correlation between translocations and cumulative dose in astronauts involved in two to five space missions. This “time factor” may reflect individual variability and time dependence in the risk produced by exposure to cosmic radiation during the flight. Biomarkers may be superior to dose in predicting space radiation risk, pending technical improvements in sensitivity, and validation by epidemiological studies.

Wu, H., Durante, M., Furusawa, Y., George, K., Kawata, T. and Cucinotta, F. A. Truly Incomplete and Complex Exchanges in Prematurely Condensed Chromosomes of Human Fibroblasts Exposed In Vitro to Energetic Heavy Ions. Radiat. Res. 160, 418–424 (2003). Confluent human fibroblast cells (AG1522) were irradiated with γ rays, 490 MeV/nucleon silicon ions, or iron ions at either 200 or 500 MeV/nucleon. The cells were allowed to repair at 37°C for 24 h after exposure, and a chemically induced premature chromosome condensation (PCC) technique was used to condense chromosomes in the G 2 phase of the cell cycle. Incomplete and complex exchanges were analyzed in the irradiated samples. To verify that chromosomal breaks were truly unrejoined, chromosome aberrations were analyzed using a combination of whole-chromosome specific probes and probes specific for the telomere region of the chromosome. Results showed that the frequency of unrejoined chromosome breaks was higher after irradiation with the heavy ions of high LET, and consequently the ratio of incomplete to complete exchanges increased steadily with LET up to 440 keV/μm, the highest LET included in the present study. For samples exposed to 200 MeV/nucleon iron ions, chromosome aberrations were analyzed using the multicolor FISH (mFISH) technique, which allows identification of both complex and truly incomplete exchanges. Results of the mFISH study showed that 0.7 and 3 Gy iron ions produced similar ratios of complex to simple exchanges and incomplete to complete exchanges; these ratios were higher than those obtained after exposure to 6 Gy γ rays. After 0.7 Gy of iron ions, most complex aberrations were found to involve three or four chromosomes, which is a likely indication of the maximum number of chromosome domains traversed by a single iron-ion track.

George, K., Durante, M., Willingham, V., Wu, H., Yang, T. C. and Cucinotta, F. A. Biological Effectiveness of Accelerated Particles for the Induction of Chromosome Damage Measured in Metaphase and Interphase Human Lymphocytes. Radiat. Res. 160, 425–435 (2003). Chromosome aberrations were investigated in human lymphocytes after in vitro exposure to 1 H-, 3 He-, 12 C-, 40 Ar-, 28 Si-, 56 Fe-, or 197 Au-ion beams, with LET ranging from approximately 0.4-1393 keV/μm in the dose range of 0.075–3 Gy. Dose–response curves for chromosome exchanges, measured at the first mitosis postirradiation using fluorescence in situ hybridization (FISH) with whole-chromosome probes, were fitted with linear or linear-quadratic functions. The relative biological effectiveness (RBE) was estimated from the initial slope of the dose–response curve for chromosomal damage with respect to low- or high-dose-rate γ rays. Estimates of RBE max values for mitotic spreads, which ranged from near 0.7 to 11.1 for total exchanges, increased with LET, reaching a maximum at about 150 keV/μm, and decreased with further increase in LET. RBEs for complex aberrations are undefined due to the lack of an initial slope for γ rays. Additionally, the effect of mitotic delay on RBE values was investigated by measuring chromosome aberrations in interphase after chemically induced premature chromosome condensation (PCC), and values were up to threefold higher than for metaphase analysis.

We have studied the induction of chromosomal aberrations in human lymphocytes exposed in G 0 to X rays or carbon ions. Aberrations were analyzed in G 0 , G 1 , G 2 or M phase. Analysis during the interphase was performed by chemically induced premature chromosome condensation, which allows scoring of aberrations in G 1 , G 2 and M phase; fusion-induced premature chromosome condensation was used to analyze the damage in G 0 cells after incubation for repair; M-phase cells were obtained by conventional Colcemid block. Aberrations were scored by Giemsa staining or fluorescence in situ hybridization (chromosomes 2 and 4). Similar yields of fragments were observed in G 1 and G 2 phase, but lower yields were scored in metaphase. The frequency of chromosomal exchanges was similar in G 0 (after repair), G 2 and M phase for cells exposed to X rays, while a lower frequency of exchanges was observed in M phase when lymphocytes were irradiated with high-LET carbon ions. The results suggest that radiation-induced G 2 -phase block is associated with unrejoined chromosome fragments induced by radiation exposure during G 0 .

Chromosome aberrations induced by high-energy charged particles in normal human lymphocytes and human fibroblasts have been investigated. The charged particles included 250 MeV/nucleon protons, 290 MeV/nucleon carbon ions and 1 GeV/nucleon iron ions. The energies of the charged particles were higher than in most of the studies reported in the literature. Lymphocytes were stimulated to grow immediately after irradiation, while fibroblasts were incubated at 37°C for 24 h for repair. Chromosomes were collected at the first mitosis after irradiation and chromosome aberrations were scored using the fluorescence in situ hybridization (FISH) technique with a whole-chromosome 4 probe. Chromosome aberrations were classified as reciprocal exchanges, incomplete exchanges, deletions and complex exchanges. The relative biological effectiveness (RBE) for each type of aberration was calculated by dividing a dose of 4 Gy by the dose of the charged particles producing the same effect as 4 Gy of γ rays. Results of this study showed that complex aberrations have the highest RBE for radiation of high linear energy transfer (LET) for human lymphocytes, but for fibroblasts, the greatest effect was for incomplete exchanges. For both lymphocytes and fibroblasts, iron ions induced a similar fraction of aberrant cells.