This work provides a description of a new interaction, cross-section-based model for radiation-induced cellular inactivation, sublethal damage, DNA repair and cell survival, with the ability to more accurately elucidate different radiation-response phenomena. The principal goal of this work is to describe the damage-induction cross sections, as well as repair and survival, as Poisson processes with two main types of damage: mild damage that can be rapidly handled by the most basic repair processes; and more complex damage requiring longer repair times and the high-fidelity homologous recombination (HR) repair process to ensure accuracy and safety in the survival. This work is unique in its use of Poisson statistics to quantify the main repairable cell compartments that are exposed to simple and more complex sublethal hits, the cross section of which determines what is homologically and non-homologically repairable. The new method is applied to central radiation damage and survival data, such as in vitro cellular repair and survival with key DNA repair genes knocked out, low-dose hypersensitivity (LDHS), change in survival over the cell cycle, and variation with linear energy transfer (LET) for densely ionizing ions, all results supporting our basic assumptions. Among the results, it was shown that less than 1% of the simple DSBs are lethal at approximately 2 Gy and below for sparsely ionizing radiations, but their δ-electron track ends of between 1.5 and 0.5 keV can deliver 0.5 MGy to a few hundred nm³ volumes, mainly due to multiple scatter detours and multiple secondary electrons. They can cause dual double-strand breaks (DSBs) on the periphery of nucleosomes that are the most common multiply damaged sites, with an average of 1–2 δ-electron track ends per cell nucleus at 2 Gy. LDHS is most likely due to the normal lack of fast, efficient repair of sublethal damage below approximately 0.5 Gy, and requires largely intact key DNA repair genes to achieve significant repair recovery at higher doses. The new repair model describes this phenomenon quite accurately. Cells with key non-homologous end joining (NHEJ) genes knocked-out, lose LDHS but provoke HR repair, and cells with HR genes knocked out may lose some LDHS, but provoke NHEJ repair. The DNA duplication during the S phase results in a direct doubling as well of the total and sublethal hit cross sections. For the lowest LET carbon ions, NHEJ is reduced to where it is almost eliminated at maximum relative biological effectiveness (RBE), while HR is induced more than by X rays, due to complex damage and misrepair of DSBs produced by numerous δ electrons. The use of a lower LET such as electrons or photons during the final week of radiation treatment may potentially maximize complication-free cure. Optimally-designed weekly fractionation schedules are proposed to maximize the DNA repair potential in normal tissues. Additionally, the optimal therapeutic ion species, LET, apoptosis and permanent growth arrest/senescence window is identified with helium, lithium and boron ions and LETs at approximately 15–55 eV/nm, to maximize these quantities in the tumor and minimize them in the normal tissues, resulting in a very high probability of complication-free cure.

Autophagy has been reported to play a radioresistance role in high-dose-rate irradiation. However, its mechanisms and roles in continuous low-dose-rate (CLDR) irradiation have not been clearly understood. Iodine-125 (I-125) seed brachytherapy is a modality of CLDR irradiation and has been used in the treatment of various cancers. In this study, we investigated the mechanisms and roles of autophagy induced by I-125 seed radiation in human esophageal squamous cell carcinoma (ESCC) cell lines (Eca-109 and EC-109) and a xenograft mouse model. The results of this work showed that I-125 seed radiation induced a dose-dependent increase in autophagy in both cell lines. In Eca-109 cells, I-125 seed radiation-induced endoplasmic reticulum (ER) stress, manifesting as the increased levels of intracellular Ca²⁺ and Grp78/BiP, and activated PERK-eIF2α, IRE1, and ATF6 pathways of the unfolded protein response. Knockdown of PERK led to the decreased expression of autophagy marker, LC3B-II. Inhibition of autophagy by chloroquine or knockdown of ATG5 enhanced I-125 seed radiation-induced cell proliferation inhibition and apoptosis. Interestingly, chloroquine did not aggravate ER stress but promoted apoptosis via the mitochondrial pathway. The animal experiment showed that inhibition of autophagy by chloroquine improved the efficacy of I-125 seed radiation. In summary, our data demonstrate that I-125 seed CLDR radiation induces ER stress-mediated autophagy in ESCC. Autophagy plays a pro-survival role in I-125 seed CLDR irradiation, and chloroquine is a potential candidate for use in combination therapy with I-125 seed radiation treatment to improve efficacy against ESCC.

To better study biological effects of space radiation using ground-based facilities, the NASA Space Radiation Laboratory (NSRL) at the Brookhaven National Laboratory has been upgraded to rapidly switch ions and energies. This has allowed investigators to design irradiation protocols comprising a mixture of ions and energies more indicative of the galactic cosmic ray (GCR) environment. Despite these advancements, beam selection and delivery schemes should be optimized against facility and experimental constraints and validated to ensure such irradiations are a suitable representation of the space environment. Importantly, since experiments are time consuming and expensive, models capable of predicting biological outcomes over a range of irradiation conditions (single ion, sequential multi ion or mixed fields) are needed to support such efforts. In this work, human fibroblasts were placed behind 20 g/cm² aluminum and 10.345 g/cm² polyethylene and irradiated separately by 344 MeV hydrogen, 344 MeV/n helium, 450 MeV/n oxygen and 950 MeV/n iron ions at various doses. The fluorescence in situ hybridization (FISH) whole chromosome painting technique was then used to assess the cells for chromosome aberrations (CAs), notably simple exchanges. A multi-scale modeling approach was also developed to predict the formation of chromosome aberrations in these experiments. The Geant4 simulation toolkit was used to determine the spectra of particles and energies produced by interactions between the incident beams and shielding. The simulated mixed field generated by shielding was then transferred into the track structure code, RITRACKS (relativistic ion tracks), to generate three-dimensional (3D) voxelized dose maps at the nanometer scale. Finally, these voxel dose maps were input into the new damage and repair model, RITCARD (radiation-induced tracks, chromosome aberrations, repair and damage), to predict the formation of various CAs. The multi-scale model described herein is a significant advancement for the computational tools used to predict biological outcomes in cells exposed to highly complex, mixed ion fields related to the GCR environment. Results show that the simulation and experimental data are in good agreement for the complex radiation fields generated by all ions incident on shielding for most data points. The differences between model predictions and measurements are discussed. Although improvements are needed, the model extends current capabilities for evaluating beam selection and delivery schemes at the NSRL ground-based GCR simulator and for informing NASA risk projection models in the future.

Dosimetric measurement error is known to potentially bias the magnitude of the dose response, and can also affect the shape of dose response. In this report, generalized relative and absolute rate models are fitted to the latest Japanese atomic bomb survivor solid cancer, leukemia and circulatory disease mortality data (followed from 1950 through 2003), with the latest (DS02R1) dosimetry, using Bayesian techniques to adjust for errors in dose estimates and assessing other model uncertainties. Linear-quadratic models are fitted and used to assess lifetime mortality risks for contemporary UK, USA, French, Russian, Japanese and Chinese populations. For a test dose of 0.1 Gy absorbed dose weighted by neutron relative biological effectiveness, solid cancer, leukemia and circulatory disease mortality risks for a UK population using a generalized linear-quadratic relative rate model were estimated to be 3.88% Gy^–1 [95% Bayesian credible interval (BCI): 1.17, 6.97], 0.35% Gy^–1 (95% BCI: –0.03, 0.78) and 2.24% Gy^–1 (95% BCI: –0.17, 13.76), respectively. Using a generalized absolute rate linear-quadratic model at 0.1 Gy, the lifetime risks for these three end points were estimated to be 3.56% Gy^–1 (95% BCI: 0.54, 6.78), 0.41% Gy^–1 (95% BCI: 0.01, 0.86) and 1.56% Gy^–1 (95% BCI: –1.10, 7.21), respectively. There was substantial evidence of curvature for solid cancer (in particular, the group of solid cancers excluding lung, breast and stomach cancers) and leukemia, so that for solid cancer and leukemia, estimates of excess risk per unit dose were nearly doubled by increasing the dose from 0.01 to 1.0 Gy, with most of the increase occurring in the interval from 0.1 to 1.0 Gy. For circulatory disease, the dose-response curvature was inverse, so that risk per unit dose was nearly halved by going from 0.01 t o 1.0 Gy weighted absorbed dose, although there were substantial uncertainties. In general, there were higher radiation risks for females compared to males. This was true for solid cancer and circulatory disease overall, as well as for lung, breast, stomach and the group of other solid cancers, and was the case whether relative or absolute rate projection models were employed; however, for leukemia this pattern was reversed. Risk estimates varied somewhat between populations, with lower cancer risks in aggregate for China and Russia, but higher circulatory disease risks for Russia, particularly using the relative rate model. There was more pronounced variation for certain cancer sites and certain types of projection models, so that breast cancer risk was markedly lower in China and Japan using a relative rate model, but the opposite was the case for stomach cancer. There was less variation between countries using the absolute rate models for stomach cancer and breast cancer, but this was not the case for lung cancer and the group of other solid cancers, or for circulatory disease.

Nuclear accidents and acts of terrorism have the potential to expose thousands of people to high-dose total-body iradiation (TBI). Those who survive the acute radiation syndrome are at risk of developing chronic, degenerative radiation-induced injuries [delayed effects of acute radiation (DEARE)] that may negatively affect quality of life. A growing body of literature suggests that the brain may be vulnerable to radiation injury at survivable doses, yet the long-term consequences of high-dose TBI on the adult brain are unclear. Herein we report the occurrence of lesions consistent with cerebrovascular injury, detected by susceptibility-weighted magnetic resonance imaging (MRI), in a cohort of non-human primate [(NHP); rhesus macaque, Macaca mulatta] long-term survivors of high-dose TBI (1.1–8.5 Gy). Animals were monitored longitudinally with brain MRI (approximately once every three years). Susceptibility-weighted images (SWI) were reviewed for hypointensities (cerebral microbleeds and/or focal necrosis). SWI hypointensities were noted in 13% of irradiated NHP; lesions were not observed in control animals. A prior history of exposure was correlated with an increased risk of developing a lesion detectable by MRI (P = 0.003). Twelve of 16 animals had at least one brain lesion present at the time of the first MRI evaluation; a subset of animals (n = 7) developed new lesions during the surveillance period (3.7–11.3 years postirradiation). Lesions occurred with a predilection for white matter and the gray–white matter junction. The majority of animals with lesions had one to three SWI hypointensities, but some animals had multifocal disease (n = 2). Histopathologic evaluation of deceased animals within the cohort (n = 3) revealed malformation of the cerebral vasculature and remodeling of the blood vessel walls. There was no association between comorbid diabetes mellitus or hypertension with SWI lesion status. These data suggest that long-term TBI survivors may be at risk of developing cerebrovascular injury years after irradiation.

It has been reported that in cells exposed to low-dose radiation, radio-adaptive responses can be induced which make irradiated cells refractory to subsequent high-dose irradiation. However, whether adaptive responses are possible when only the cytoplasm, not the nucleus, of the cell is exposed to radiation is still unclear. In this study, using the proton microbeam facility at the National Institute of Radiological Sciences (Japan), we found that cytoplasmic irradiation activates radio-adaptive responses in normal human lung fibroblast WI-38 cells. Our results showed that when cells received cytoplasmic irradiation with 500 protons prior to 2 Gy or 6 Gy X-ray broad-beam irradiation, the DNA double-strand break levels were significantly reduced. In contrast, at cytoplasmic irradiation with less than 100 protons, the radio-adaptive response was not detected. Moreover, the time interval between cytoplasmic irradiation and whole-cell X-ray irradiation should be longer than 6 h for the induction of adaptive responses. In addition, cytoplasmic irradiation elevated the level of cellular mitochondrial superoxide, which enhanced the phosphorylation of extracellular signal-regulated kinases 1/2 (ERK 1/2) and its mediated nuclear accumulation of nuclear factor (erythroid-derived 2)-like 2 (NRF2). This signaling pathway contributed to cytoplasmic irradiation-induced adaptive response as supported by the observations that treatment with the mitochondrial superoxide scavenger mito-tempol, ERK 1/2 inhibitor U0126 or NRF2 inhibitor ML385 could repress the adaptive response. Overall, we showed that cytoplasmic irradiation induces radio-adaptive responses and that mitochondrial superoxide/ERK 1/2/NRF2 signaling is a mechanism. Our results provide new information on the biological effects induced by cytoplasm-targeted irradiation.

Gadolinium is a commonly used contrast agent for magnetic resonance imaging (MRI). The goal of this work was to determine how MRI contrast agents affect radiosensitivity for tumour cells. Using a 225kVp X-ray cabinet source, immunofluorescence and clonogenic assays were performed on six cancer cell lines: lung (H460), pancreas (MiaPaCa2), prostate (DU145), breast (MCF7), brain (U87) and liver (HEPG2). Dotarem^® contrast agent, at concentrations of 0.2, 2 and 20 mM, was used to determine its effect on DNA damage and cell survival. Measurements were performed using inductively coupled plasma mass spectrometry (ICP-MS) to determine the amount of gadolinium taken up by each cell line for each concentration. A statistically significant increase in DNA damage was seen for all cell lines at a dose of 1 Gy for concentrations of 2 and 20 mM, at 1 h postirradiation. At 24 h postirradiation, most of the DNA damage had been repaired, with approximately 90% repair for almost all doses of radiation and concentrations of Dotarem. Clonogenic results showed no statistically significant decrease in cell survival for any cell line or concentration. Uptake measurements showed cell line-specific variations in uptake, with MCF7 and HEPG2 cells having a high percentage uptake compared to other cell lines, with 151.4 ± 0.3 × 10^–15 g and 194.8 ± 0.4 × 10^–15 g per cell, respectively, at 2 mM Dotarem concentration. In this work, a variability in gadolinium uptake was observed between cell lines. A significant increase was seen in initial levels of DNA damage after 1 Gy irradiation for all six cancer cell lines; however, no significant decrease in cell survival was seen with the clonogenic assay. The observation of high levels of repair suggest that while initial levels of DNA damage are increased, this damage is almost entirely repaired within 24 h, and does not affect the ability of cells to survive and produce colonies.

Many cases of human exposures to high-dose radiation have been documented, including individuals exposed during the detonation of atomic bombs in Hiroshima and Nagasaki, nuclear power plant disasters (e.g., Chernobyl), as well as industrial and medical accidents. For many of these exposures, injuries to the skin have been present and have played a significant role in the progression of the injuries and survivability from the radiation exposure. There are also instances of radiation-induced skin complications in routine clinical radiotherapy and radiation diagnostic imaging procedures. In response to the threat of a radiological or nuclear mass casualty incident, the U.S. Department of Health and Human Services tasked the National Institute of Allergy and Infectious Diseases (NIAID) with identifying and funding early- to mid-stage medical countermeasure (MCM) development to treat radiation-induced injuries, including those to the skin. To appropriately assess the severity of radiation-induced skin injuries and determine efficacy of different approaches to mitigate/treat them, it is necessary to develop animal models that appropriately simulate what is seen in humans who have been exposed. In addition, it is important to understand the techniques that are used in other clinical indications (e.g., thermal burns, diabetic ulcers, etc.) to accurately assess the extent of skin injury and progression of healing. For these reasons, the NIAID partnered with two other U.S. Government funding and regulatory agencies, the Biomedical Advanced Research and Development Authority (BARDA) and the Food and Drug Administration (FDA), to identify state-of-the-art methods in assessment of skin injuries, explore animal models to better understand radiation-induced cutaneous damage and investigate treatment approaches. A two-day workshop was convened in May 2019 highlighting talks from 28 subject matter experts across five scientific sessions. This report provides an overview of information that was presented and the subsequent guided discussions.