Molecular oxygen has long been recognized as a powerful radiosensitizer that enhances the cell-killing efficiency of ionizing radiation. Radiosensitization by oxygen occurs at very low concentrations with the half-maximum radiosensitization at approximately 3 mmHg. However, robust hypoxia-induced signal transduction can be induced at <15 mmHg and can elicit a wide range of cellular responses that will affect therapy response as well as malignant progression. Great strides have been made, especially since the 1990s, toward identification and characterization of the oxygen-regulated molecular pathways that affect tumor response to ionizing radiation. In this review, we will discuss the current advances in our understanding of oxygen-dependent molecular modification and cellular signal transduction and their impact on tumor response to therapy. We will specifically address mechanistic distinctions between radiobiological hypoxia (0–3 mmHg) and pathological hypoxia (3–15 mmHg). We also propose a paradigm that hypoxia increases radioresistance by maintaining the cancer stem cell phenotype.

Philip Marcus (1927–2013), a prominent and celebrated virus and interferon researcher, was also influential to the field of radiobiology. His work as a graduate student led to the development of the first mammalian cell clonogenic assay. This tribute to Philip Marcus is written to memorialize this inventive scientist and share the stimulating story of how he and his mentors developed the clonogenic assay.

Advancements made over the past decades in both molecular imaging and radiotherapy planning and delivery have enabled studies that explore the efficacy of heterogeneous radiation treatment (“dose painting”) of solid cancers based on biological information provided by different imaging modalities. In addition to clinical trials, preclinical studies may help contribute to identifying promising dose painting strategies. The goal of this current study was twofold: to develop a reproducible positioning and set-up verification protocol for a rat tumor model to be imaged and treated on a clinical platform, and to assess the dosimetric accuracy of dose planning and delivery for both uniform and positron emission tomography-computed tomography (PET-CT) based heterogeneous dose distributions. We employed a syngeneic rat rhabdomyosarcoma model, which was irradiated by volumetric modulated arc therapy (VMAT) with uniform or heterogeneous 6 MV photon dose distributions. Mean dose to the gross tumor volume (GTV) as a whole was kept at 12 Gy for all treatment arms. For the nonuniform plans, the dose was redistributed to treat the 30% of the GTV representing the biological target volume (BTV) with a dose 40% higher than the rest of the GTV (GTV – BTV) (~15 Gy was delivered to the BTV vs. ~10.7 Gy was delivered to the GTV – BTV). Cone beam computed tomography (CBCT) images acquired for each rat prior to irradiation were used to correctly reposition the tumor and calculate the delivered 3D dose. Film quality assurance was performed using a water-equivalent rat phantom. A comparison between CT or CBCT doses and film measurements resulted in passing rates >98% with a gamma criterion of 3%/2 mm using 2D dose images. Moreover, between the CT and CBCT calculated doses for both uniform and heterogeneous plans, we observed maximum differences of <2% for mean dose to the tumor and mean dose to the biological target volumes. In conclusion, we have developed a robust method for dose painting in a rat tumor model on a clinical platform, with a high accuracy achieved in the delivery of complex dose distributions. Our work demonstrates the technical feasibility of this approach and enables future investigations on the therapeutic effect of preclinical dose painting strategies using a state-of-the-art clinical platform.

Radiation-induced bystander effects (RIBE) in vivo in the higher plant Arabidopsis thaliana (A. thaliana) have been well demonstrated in terms of effects on development and genetics. However, there is not yet robust evidence regarding RIBE-mediated epigenetic changes in plants. To address this, in the current work the roots of A. thaliana seedlings were locally irradiated with 10 Gy of α particles, after which DNA methylation in bystander aerial plants were detected using the methylation-sensitive amplification polymorphism (MSAP) and bisulfite sequencing PCR (BSP). Results showed that irradiation of the roots led to long-distance changes in DNA methylation patterns at some CCGG sites over the whole genome, specifically from hemi-methylation to non-methylation, and the methylation ratios, mainly at CG sites, strongly indicating the existence of RIBE-mediated epigenetic changes in higher plants. Root irradiation also influenced expressions of DNA methylation-related MET1, DRM2 and SUVH4 genes and demethylation-related DML3 gene in bystander aerial plants, suggesting a modulation of RIBE to the methylation machinery in plants. In addition, the multicopy P35S:GUS in A. thaliana line L5-1, which is silenced epigenetically by DNA methylation and histone modification, was transcriptionally activated through the RIBE. The transcriptional activation could be significantly inhibited by the treatment with reactive oxygen species (ROS) scavenger dimethyl sulfoxide (DMSO), indicative of a pivotal role of ROS in RIBE-mediated epigenetic changes. Time course analyses showed that the bystander signaling molecule(s) for transcriptional activation of multicopy P35S:GUS, although of unknown chemical nature, were generated in the root cells within 24 h postirradiation.

The aim of this report is to present the spectrum of initial radiation-induced cellular DNA damage [with particular focus on non-double-strand break (DSB) damage] generated by computer simulations. The radiation types modeled in this study were monoenergetic electrons (100 eV–1.5 keV), ultrasoft X-ray photons C_k, Al_K and Ti_K, as well as some selected ions including 3.2 MeV/u proton; 0.74 and 2.4 MeV/u helium ions; 29 MeV/u nitrogen ions and 950 MeV/u iron ions. Monte Carlo track structure methods were used to simulate damage induction by these radiation types in a cell-mimetic condition from a single-track action. The simulations took into account the action of direct energy deposition events and the reaction of hydroxyl radicals on atomistic linear B-DNA segments of a few helical turns including the water of hydration. Our results permitted the following conclusions: a. The absolute levels of different types of damage [base damage, simple and complex single-strand breaks (SSBs) and DSBs] vary depending on the radiation type; b. Within each damage class, the relative proportions of simple and complex damage vary with radiation type, the latter being higher with high-LET radiations; c. Overall, for both low- and high-LET radiations, the ratios of the yields of base damage to SSBs are similar, being about 3.0 ± 0.2; d. Base damage contributes more to the complexity of both SSBs and DSBs, than additional SSB damage and this is true for both low- and high-LET radiations; and e. The average SSB/DSB ratio for low-LET radiations is about 18, which is about 5 times higher than that for high-LET radiations. The hypothesis that clustered DNA damage is more difficult for cells to repair has gained currency among radiobiologists. However, as yet, there is no direct in vivo experimental method to validate the dependence of kinetics of DNA repair on DNA damage complexity (both DSB and non-DSB types). The data on the detailed spectrum of DNA damage presented here, in particular the non-DSB type, provide a good basis for testing mechanistic models of DNA repair kinetics such as base excision repair.

Pediatric cranial radiation therapy can induce long-term neurocognitive deficits, the risk and severity of these deficits are amplified in females and in those individuals exposed at a younger age and/or those irradiated at higher doses. To investigate the developmental consequences of these factors in greater detail, male and female C57Bl/6J mice between infancy and late childhood (16 and 36 days) were irradiated at a single time point with a whole-brain dose of 0, 3, 5 or 7 Gy. In vivo and ex vivo magnetic resonance imaging (MRI) and deformation-based morphometry was used to identify radiation-induced volume differences. As expected, exposure to 7 Gy of radiation at 16 days of age induced widespread volume deficits that were largely mitigated by increasing treatment age or decreasing dose. Notable exceptions were regions in the olfactory bulbs and hippocampus that displayed both a detectable difference in volume and a loss in neurogenesis for most doses and ages. Furthermore, white matter regions located at the front of the brain remained sensitive to radiation at later treatment ages, compared to regions at the back. Differences due to sex were subtle, with increased radiosensitivity in females detectable only in the mammillary bodies and fornix. Our results reveal anatomical alterations in brain development consistent with expectations based on pediatric patient neurocognitive outcomes. This data demonstrates that neuroimaging of the mouse is an effective tool for investigating radiation-induced late effects.

A significant association has been observed between radon exposure and cerebrovascular disease (CeVD) mortality among French uranium miners, but risk factors for circulatory system diseases (CSD) have not been previously considered. We conducted new analyses in the recently updated (through 2007) French cohort of uranium miners (n = 5,086), which included 442 deaths from CSD, 167 of them from ischemic heart disease (IHD) and 105 from CeVD. A nested case-control study was then set up to collect and investigate the influence of these risk factors on the relationships between mortality from CSD and occupational external gamma ray and internal ionizing radiation exposure (radon and long-lived radionuclides) in this updated cohort. The nested case-control study included miners first employed after 1955, still employed in 1976 and followed up through 2007. Individual information about CSD risk factors was collected from medical files for the 76 deaths from CSD (including 26 from IHD and 16 from CeVD) and 237 miners who had not died of CSD by the end of follow-up. The exposure-risk relationships were assessed with a Cox proportional hazard model weighted by the inverse sampling probability. A significant increase in all CSD and CeVD mortality risks associated with radon exposure was observed in the total cohort [hazard ratios: HR_{CSD/100 working level months (WLM)} = 1.11, 95% confidence interval (1.01; 1.22) and HR_{CeVD/100 WLM} = 1.25 (1.09; 1.43), respectively]. A nonsignificant exposure-risk relationship was observed for every type of cumulative ionizing radiation exposure and every end point [e.g., HR_CSD/100WLM = 1.43 (0.71; 2.87)]. The adjustment for each CSD risk factor did not substantially change the exposure-risk relationships. When the model was adjusted for overweight, hypertension, diabetes, hypercholesterolemia and smoking status, the HR_/100WLM for CSD, for example, was equal to 1.21 (0.54; 2.75); and when it was adjusted for risk factors selected with the Akaike information criterion, it was equal to 1.44 (0.66; 3.14). To our knowledge, this is the first study to use a uranium miner cohort to consider the major standard CSD risk factors in assessing the relationships between ionizing radiation exposure and the risk of death from these diseases. These results suggest that the significant relationship between CeVD risk and radon exposure observed in the total French cohort is probably not affected by the CSD risk factors. Extending the collection of information about CSD risk factors to a larger subsample would be useful to confirm this result.

Radiation-induced liver injury remains a clinical problem and data suggest that sinusoidal endothelial cells (SECs) are an important target. The purpose of this study was to determine whether the inhibition of Kupffer cells before exposure would protect SECs from radiation-induced injury. Sprague-Dawley rats were intravenously injected 24 h before irradiation with Kupffer cell inhibitor gadolinium chloride (GdCl3) (10 mg/kg body weight). Three groups of animals were treated: 1. control group (saline and sham irradiation); 2. GdCl3 30 Gy radiation group and 3. 30 Gy radiation only group. Specimens were collected at 2, 6, 12, 24 and 48 h after completion of each treatment. Liver tissue was assessed for inflammatory cytokine expression and radiation-induced SEC injury based on serum hyaluronic acid (HA) level, apoptosis and ultrastructural and histological analyses. The results showed that radiation exposure caused apoptosis of SECs, but not hepatocytes. Inflammatory cytokine expression, including tumor necrosis factor-α (TNF-α) and interleukin-1β (IL-1β) expression, was significantly attenuated in the GdCl₃ 30 Gy radiation group, compared with the 30 Gy radiation-only group (P < 0.05). The GdCl₃ radiation-treated rats exhibited significantly lower levels of HA and SEC apoptosis than the radiation-treated only rats at early time points, and radiation-induced liver injury was also attenuated. In conclusion, we hypothesize that selective Kupffer cell inhibition by gadolinium chloride was shown to reduce apoptosis in SECs caused by irradiation of the live and protected the liver against radiation-induced injury.

For the last two decades radiation-induced bystander effects (RIBEs) have attracted significant attention due to their possible implications for radiotherapy. However, despite extensive research, the molecular pathways associated with RIBEs are still not completely known. In the current study we investigated the role of senescence in the bystander response. Irradiated (2, 4, 6 and 8 Gy) human colorectal carcinoma cells (HCT116) with p53 ^/ (wild-type) or p53^–/– (knockout) gene were co-incubated with nonirradiated cells of the same type. Clonogenic and senescence assays were used for both irradiated and co-incubated bystander cell populations. We also performed additional measurements on the number of remaining cells after the whole co-incubation period. For radiation doses larger than 2 Gy we observed much larger fractions of senescent cells in p53-positive populations compared to their p53-negative counterparts (15.81% vs. 3.63% in the irradiated population; 2.89% vs. 1.05% in the bystander population; 8 Gy; P < 0.05). Statistically significant differences between cell lines in the clonogenic cell surviving fraction were observed for doses higher than 4 Gy (1.61% for p53 ^/ vs. 0.19% for p53^–/– in irradiated population; 3.57% for / vs. 50.39% for –/– in bystander population; 8 Gy; P < 0.05). Our main finding was that the number of senescent cells in the irradiated population correlated strongly with the clonogenic cell surviving fraction (R = −0.98, P < 0.001) and the number of senescent cells (R = 0.97, P < 0.001) in the bystander population. We also extended the standard linear-quadratic radiation response model by incorporating the influence of the signals released by the senescent cells, which accurately described the radiation response in the bystander population. Our findings suggest that radiation-induced senescence might be a key player in RIBE, i.e., the strength of RIBE depends on the amount of radiation-induced senescence.

Exposure to high-dose radiation results in deleterious effects on skeletal tissue. However, the effects of combined trauma such as radiation and hemorrhage on skeletal properties have yet to be elucidated. The purpose of this study was to evaluate the effects of radiation injury combined with hemorrhage on trabecular bone properties and biomarkers of bone metabolism, and to determine whether hemorrhage enhances radiation-associated bone loss. Male CD2F1 mice (10 weeks old) were exposed to one single dose of gamma radiation (⁶⁰Co): 0 or 7.25 Gy. Two hours after irradiation, animals were bled 0% (n = 8) or 20% (n = 8) of total blood volume via the submandibular vein. Mice were euthanized 30 days after irradiation, and distal femora were analyzed using standard histomorphometry to determine changes in trabecular bone volume (BV/TV), thickness (Tb.Th), spacing (Tb.Sp), number (Tb.N) and marrow adipocyte density. Femurs from mice euthanized 1, 7 and 15 days post injury were flushed and total bone marrow cells were counted. Radiation exposure resulted in deleterious effects on distal femur BV/TV (−63%), Tb.Th (−34%), Tb.N (−45%), Tb.Sp ( 125%) and adipocyte density ( 286%) compared with the sham-irradiated mice (0 Gy; P < 0.05). Hemorrhage after irradiation resulted in greater deleterious effects on the distal femur with BV/TV (−13%), Tb.Th (−44%), Tb.N (−26%), Tb.Sp ( 29%) and marrow adipocyte density ( 33%) compared with radiation exposure only (P < 0.05). Analysis of the biomarkers of bone metabolism in serum from irradiated and hemorrhaged mice revealed significantly lower levels of osteocalcin (−60%) and procollagen type 1 amino-terminal propeptide (−36%; P1NP, biomarkers of bone formation activity), as well as elevations in sclerostin ( 56%; SOST, an inhibitor of bone formation) compared with serum from irradiated only mice (P < 0.05). Additionally, the onset of bone marrow cell depletion in irradiated and hemorrhaged mice occurred earlier and to a greater extent compared to that in irradiated only mice. This study provides definitive, preliminary evidence that hemorrhage further exacerbates trabecular bone loss associated with nonlethal high-dose gamma radiation.