Low-dose binary behavior of bystander cell killing after microbeam irradiation of a single cell with focused c(k) x rays

Factors Influencing Effects of Low-dose Radiation Exposure

ABSTRACT

It is now well accepted that the mechanisms induced by low-dose exposures to ionizing radiation (LDR) are different from those occurring after high-dose exposures. However, the downstream effects of these mechanisms are unclear as are the quantitative relationships between exposure, effect, harm, and risk. In this paper, we will discuss the mechanisms known to be important with an overall emphasis on how so-called "non-targeted effects" (NTE) communicate and coordinate responses to LDR. Targeted deposition of ionizing radiation energy in cells causing DNA damage is still regarded as the dominant trigger leading to all downstream events whether targeted or non-targeted. We regard this as an over-simplification dating back to formal target theory. It ignores that last 100 y of biological research into stress responses and signaling mechanisms in organisms exposed to toxic substances, including ionizing radiation. We will provide evidence for situations where energy deposition in cellular targets alone cannot be plausible as a mechanism for LDR effects. An example is where the energy deposition takes place in an organism not receiving the radiation dose. We will also discuss how effects after LDR depend more on dose rate and radiation quality rather than actual dose, which appears rather irrelevant. Finally, we will use recent evidence from studies of cataract and melanoma induction to suggest that after LDR, post-translational effects, such as protein misfolding or defects in energy metabolism or mitochondrial function, may dominate the etiology and progression of the disease. A focus on such novel pathways may open the way to successful prophylaxis and development of new biomarkers for better risk assessment after low dose exposures.

Radiation-induced bystander effect and its clinical implications

For many years, targeted DNA damage caused by radiation has been considered the main cause of various biological effects. Based on this paradigm, any small amount of radiation is harmful to the organism. Epidemiological studies of Japanese atomic bomb survivors have proposed the linear-non-threshold model as the dominant standard in the field of radiation protection. However, there is increasing evidence that the linear-non-threshold model is not fully applicable to the biological effects caused by low dose radiation, and theories related to low dose radiation require further investigation. In addition to the cell damage caused by direct exposure, non-targeted effects, which are sometimes referred to as bystander effects, abscopal effects, genetic instability, etc., are another kind of significant effect related to low dose radiation. An understanding of this phenomenon is crucial for both basic biomedical research and clinical application. This article reviews recent studies on the bystander effect and summarizes the key findings in the field. Additionally, it offers a cross-sectional comparison of bystander effects caused by various radiation sources in different cell types, as well as an in-depth analysis of studies on the potential biological mechanisms of bystander effects. This review aims to present valuable information and provide new insights on the bystander effect to enlighten both radiobiologists and clinical radiologists searching for new ways to improve clinical treatments.

Low dose radiation mechanisms: The certainty of uncertainty

This paper reviews the current understanding of low dose radiobiology, and how it has evolved from classical target theory. It highlights the uncertainty around low dose effects, which is due in part to the complexity of "context" surrounding the ultimate expression of biological effects following low dose exposure. The paper makes special reference to low dose non-targeted effects which, are currently ignored in radiation protection and population level risk assessment, because it is unclear what they mean for risk. The view of the authors is that this "lack of clarity" about what the effects mean is precisely the point. It indicates the uncertainty of outcomes after a given exposure. The uncertainty stems from multiple outcome options resulting from the intrinsic uncertainty of the stochastic interaction of low dose radiation with matter. This uncertainty should be embraced rather than eschewed. The impacts of the uncertainties identified in this paper is explored and an approach to quantifying mutation probability in relation to dose is presented.

Non-targeted effects of radiation: a personal perspective on the role of exosomes in an evolving paradigm

PURPOSE

Radiation-induced non-targeted effects (NTE) have implications in a variety of areas relevant to radiation biology. Here we evaluate the various cargo associated with exosomal signalling and how they work synergistically to initiate and propagate the non-targeted effects including Genomic Instability and Bystander Effects.

CONCLUSIONS

Extra cellular vesicles, in particular exosomes, have been shown to carry bystander signals. Exosome cargo may contain nucleic acids, both DNA and RNA, as well as proteins, lipids and metabolites. These cargo molecules have all been considered as potential mediators of NTE. A review of current literature shows mounting evidence of a role for ionizing radiation in modulating both the numbers of exosomes released from affected cells as well as the content of their cargo, and that these exosomes can instigate functional changes in recipient cells. However, there are significant gaps in our understanding, particularly regarding modified exosome cargo after radiation exposure and the functional changes induced in recipient cells.

Radiobiological and social considerations following a radiological terrorist attack; mechanisms, detection and mitigation: review of new research developments

PURPOSE

This review focuses on recent research in understanding the different aspects of what society should expect from a radiological attack. Although some scenarios of a radiologic event can be impossible to be prepared for, the effort put toward educating and better preparing for these types of events can help minimize some of the issues. The different areas discussed in this review include radioisotopes of concern, detection of radiation dose, biological effects of ionizing radiation exposures, low dose effects, targeted and non-targeted effects (NTE), psychological effects, mitigations, with a brief mention of other considerations such as medical preparedness, communication, policy implications and ethical issues. This review also discusses solutions to rectify the issues faced at hand that may come up in the event of a radiologic terrorist attack.

CONCLUSIONS

A review of recent work in the area shows that a multi-layered and interdisciplinary approach is needed to prepare for a radiological terrorist attack. As well as medical preparedness, the approach needs to include sociological and psychological planning as well as an understanding of ethical issues. Since the likely 'dirty bomb' scenarios may involve low dose exposures to high numbers of people, a much better theoretical and practical understanding of low dose radiobiology and the development of robust low dose exposure biomarkers is needed as part of an integrated plan.

Targeted and Non-Targeted Mechanisms for Killing Hypoxic Tumour Cells-Are There New Avenues for Treatment?

PURPOSE

A major issue in radiotherapy is the relative resistance of hypoxic cells to radiation. Historic approaches to this problem include the use of oxygen mimetic compounds to sensitize tumour cells, which were unsuccessful. This review looks at modern approaches aimed at increasing the efficacy of targeting and radiosensitizing hypoxic tumour microenvironments relative to normal tissues and asks the question of whether non-targeted effects in radiobiology may provide a new "target". Novel techniques involve the integration of recent technological advancements such as nanotechnology, cell manipulation, and medical imaging. Particularly, the major areas of research discussed in this review include tumour hypoxia imaging through PET imaging to guide carbogen breathing, gold nanoparticles, macrophage-mediated drug delivery systems used for hypoxia-activate prodrugs, and autophagy inhibitors. Furthermore, this review outlines several features of these methods, including the mechanisms of action to induce radiosensitization, the increased accuracy in targeting hypoxic tumour microenvironments relative to normal tissue, preclinical/clinical trials, and future considerations.

CONCLUSIONS

This review suggests that the four novel tumour hypoxia therapeutics demonstrate compelling evidence that these techniques can serve as powerful tools to increase targeting efficacy and radiosensitizing hypoxic tumour microenvironments relative to normal tissue. Each technique uses a different way to manipulate the therapeutic ratio, which we have labelled "oxygenate, target, use, and digest". In addition, by focusing on emerging non-targeted and out-of-field effects, new umbrella targets are identified, which instead of sensitizing hypoxic cells, seek to reduce the radiosensitivity of normal tissues.

The γH2AX DSB marker may not be a suitable biodosimeter to measure the biological MRT valley dose

PURPOSE

γH2AX biodosimetry has been proposed as an alternative dosimetry method for microbeam radiation therapy (MRT) because conventional dosimeters, such as ionization chambers, lack the spatial resolution required to accurately measure the MRT valley dose. Here we investigated whether γH2AX biodosimetry should be used to measure the biological valley dose of MRT-irradiated mammalian cells.

MATERIALS AND METHODS

We irradiated human skin fibroblasts and mouse skin flaps with synchrotron MRT and broad beam (BB) radiation. BB doses of 1-5 Gy were used to generate a calibration curve in order to estimate the biological MRT valley dose using the γH2AX assay.

RESULTS

Our key finding was that MRT induced a non-linear dose response compared to BB, where doses 2-3 times greater showed the same level of DNA DSB damage in the valley in cell and tissue studies. This indicates that γH2AX may not be an appropriate biodosimeter to estimate the biological valley doses of MRT-irradiated samples. We also established foci yields of 5.9 ± 0 . 04 and 27.4 ± 2 . 5   foci/cell/Gy in mouse skin tissue and human fibroblasts respectively, induced by BB. Using Monte Carlo simulations, a linear dose response was seen in cell and tissue studies and produced predicted peak-to-valley dose ratios (PVDRs) of ∼30 and ∼107 for human fibroblasts and mouse skin tissue respectively.

CONCLUSIONS

Our report highlights novel MRT radiobiology, attempts to explain why γH2AX may not be an appropriate biodosimeter and suggests further studies aimed at revealing the biological and cellular communication mechanisms that drive the normal tissue sparing effect, which is characteristic of MRT.

Isolation of the effects of alpha-related components from total effects of radium at low doses

PURPOSE

Radium is the most common source of alpha radiation exposure to humans and non-human species in the environment but the dosimetry is complicated by the decay chain which involves gamma exposure due to radon daughters. This paper seeks to determine the separate contributions of alpha and gamma doses to the total dose and total direct and non-targeted effect in a fish and a human cell line.

MATERIALS AND METHODS

This study aimed to isolate the effect of alpha particles following exposure to low doses of radium in cells, and their progeny which received no further exposure. This was initially done by comparing the survival values of a human keratinocyte cell line (HaCaT) and an embryonic Chinook salmon cell line (CHSE-214) exposed to gamma radiation, from survival of the same cell lines exposed to mixed alpha and gamma radiation through exposure to Ra-226 and its decay products. A Monte Carlo simulation was later performed to determine the contributions of radium decay products including radon daughters.

RESULTS

The human cell line showed increased radioresistance when exposed to low doses of alpha particles. In contrast the fish cell line, which demonstrated radioresistance to low dose gamma radiation, showed increased lethality when exposed to low doses of alpha particles. Significant and complex levels of non-targeted effects were induced in progeny of irradiated cells. The simulation showed that gamma and beta decay products did not contribute significant dose and the highest beta dose was below the threshold for inducing non-targeted effects.

CONCLUSIONS

The results confirm the need to consider the dose-response relationship when developing radiation weighting factors for low dose exposures, as well as the need to be aware of possible cell line and species differences.

Environmental and industrial developments in radiation cataractogenesis

Purpose: This review discusses recent developments in our understanding of biological and physiological mechanisms underlying radiation cataractogenesis. The areas discussed include effects of low-dose exposures to the lens including potential relevance of non-targeted effects, the development of new personal-protective equipment (PPE) and standards in clinical and nuclear settings motivated by the updated ICRP recommendations to mitigate exposures to the lens of the eye. The review also looks at evidence from the field linking cataracts in birds and mammals to low dose exposures.Conclusions: The review suggests that there is evidence that cataractogenesis is not a tissue reaction (deterministic effect) but rather is a low dose effect which shows a saturable dose response relationship similar to that seen for non-targeted effects in general. The review concludes that new research is needed to determine the dose response relationship in environmental studies where field data are contradictory and lab studies confined to rodent models for human exposure studies.

Relevance of Non-Targeted Effects for Radiotherapy and Diagnostic Radiology; A Historical and Conceptual Analysis of Key Players

Non-targeted effects (NTE) such as bystander effects or genomic instability have been known for many years but their significance for radiotherapy or medical diagnostic radiology are far from clear. Central to the issue are reported differences in the response of normal and tumour tissues to signals from directly irradiated cells. This review will discuss possible mechanisms and implications of these different responses and will then discuss possible new therapeutic avenues suggested by the analysis. Finally, the importance of NTE for diagnostic radiology and nuclear medicine which stems from the dominance of NTE in the low-dose region of the dose–response curve will be presented. Areas such as second cancer induction and microenvironment plasticity will be discussed.

The tubercular badger and the uncertain curve:- The need for a multiple stressor approach in environmental radiation protection

This article presents the results of a workshop held in Stirling, Scotland in June 2018, called to examine critically the effects of low-dose ionising radiation on the ecosphere. The meeting brought together participants from the fields of low- and high-dose radiobiology and those working in radioecology to discuss the effects that low doses of radiation have on non-human biota. In particular, the shape of the low-dose response relationship and the extent to which the effects of low-dose and chronic exposure may be predicted from high dose rate exposures were discussed. It was concluded that high dose effects were not predictive of low dose effects. It followed that the tools presently available were deemed insufficient to reliably predict risk of low dose exposures in ecosystems. The workshop participants agreed on three major recommendations for a path forward. First, as treating radiation as a single or unique stressor was considered insufficient, the development of a multidisciplinary approach is suggested to address key concerns about multiple stressors in the ecosphere. Second, agreed definitions are needed to deal with the multiplicity of factors determining outcome to low dose exposures as a term can have different meanings in different disciplines. Third, appropriate tools need to be developed to deal with the different time, space and organisation level scales. These recommendations permit a more accurate picture of prospective risks.

A Functional Immune System Is Required for the Systemic Genotoxic Effects of Localized Irradiation

PURPOSE

Nontargeted effects of ionizing radiation, by which unirradiated cells and tissues are also damaged, are a relatively new paradigm in radiobiology. We recently reported radiation-induced abscopal effects (RIAEs) in normal tissues; namely, DNA damage, apoptosis, and activation of the local and systemic immune responses in C57BL6/J mice after irradiation of a small region of the body. High-dose-rate, synchrotron-generated broad beam or multiplanar x-ray microbeam radiation therapy was used with various field sizes and doses. This study explores components of the immune system involved in the generation of these abscopal effects.

METHODS AND MATERIALS

The following mice with various immune deficiencies were irradiated with the microbeam radiation therapy beam: (1) SCID/IL2γR^-/- (NOD SCID gamma, NSG) mice, (2) wild-type C57BL6/J mice treated with an antibody-blocking macrophage colony-stimulating factor 1 receptor, which depletes and alters the function of macrophages, and (3) chemokine ligand 2/monocyte chemotactic protein 1 null mice. Complex DNA damage (ie, DNA double-strand breaks), oxidatively induced clustered DNA lesions, and apoptotic cells in tissues distant from the irradiation site were measured as RIAE endpoints and compared with those in wild-type C57BL6/J mice.

RESULTS

Wild-type mice accumulated double-strand breaks, oxidatively induced clustered DNA lesions, and apoptosis, enforcing our RIAE model. However, these effects were completely or partially abrogated in mice with immune disruption, highlighting the pivotal role of the immune system in propagation of systemic genotoxic effects after localized irradiation.

CONCLUSIONS

These results underline the importance of not only delineating the best strategies for tumor control but also mitigating systemic radiation toxicity.

Development, commissioning, and evaluation of a new intensity modulated minibeam proton therapy system

PURPOSE

To invent, design, construct, and commission an intensity modulated minibeam proton therapy system (IMMPT) without the need for physical collimation and to compare its resulting conformity to a conventional IMPT system.

METHODS

A proton therapy system (Hitachi, Ltd, Hitachi City, Japan; Model: Probeat-V) was specially modified to produce scanned minibeams without collimation. We performed integral depth dose acquisitions and calibrations using a large diameter parallel-plate ionization chamber in a scanning water phantom (PTW, Freiburg, Germany; Models: Bragg Peak ionization chamber, MP3-P). Spot size and shape was measured using radiochromic film (Ashland Advanced Materials, Bridgewater NJ; Type: EBT3), and a synthetic diamond diode type scanned point by point in air (PTW Models: MicroDiamond, MP3-P). The measured data were used as inputs to generate a Monte Carlo-based model for a commercial radiotherapy planning system (TPS) (Varian Medical Systems, Inc., Palo Alto, CA; Model: Eclipse v13.7.15). The regular ProBeat-V system (sigma ~2.5 mm) TPS model was available for comparison. A simulated base of skull case with small and medium targets proximal to brainstem was planned for both systems and compared.

RESULTS

The spot sigma is determined to be 1.4 mm at 221 MeV at the isocenter and below 1 mm at proximal distances. Integral depth doses were indistinguishable from the standard spot commissioning data. The TPS fit the spot profiles closely, giving a residual error maximum of 2.5% in the spot penumbra tails (below 5% of maximum) from the commissioned energies 69.4 to 221.3 MeV. The resulting IMMPT plans were more conformal than the IMPT plans due to a sharper dose gradient (90-10%) 1.5 mm smaller for the small target, and 1.3 mm for the large target, at a representative central axial water equivalent depth of 7 cm.

CONCLUSIONS

We developed, implemented, and tested a new IMMPT system. The initial results look promising in cases where treatments can benefit from additional dose sparing to neighboring sensitive structures.

Graphene-Based Raman Spectroscopy for pH Sensing of X-rays Exposed and Unexposed Culture Media and Cells

Graphene provides a unique way of sensing the local pH level of substances on the micrometric scale, with important implications for the monitoring of cellular metabolic activities where proton excretion could occur. Accordingly, an innovative biosensing approach for the quantification of the pH value of biological fluids, to be used also with small amounts of fluids, was realized and tested. It is based on the use of micro-Raman spectroscopy to detect the modifications of the graphene doping level induced by the contact of the graphene with the selected fluids. The approach was preliminarily tested on aqueous solutions of known pH values. It was then used to quantify the pH values of cell culture media directly exposed to different doses of X-ray radiation and to media exposed to X-ray-irradiated cells. The Raman response of cells placed on graphene layers was also examined.

Old Data-New Concepts: Integrating "Indirect Effects" Into Radiation Protection

PURPOSE

To address the following key question, what are the consequences of nontargeted and delayed effects for linear nonthreshold models of radiation risk? This paper considers low-dose "indirect" or nontargeted effects and how they might impact radiation protection, particularly at the level of the environment. Nontargeted effects refer to effects in cells, tissues, or organisms that were not targeted by irradiation and that did not receive direct energy deposition. They include genomic instability and lethal mutations in progeny of irradiated cells and bystander effects in neighboring cells, tissues, or organisms. Low-dose hypersensitivity and adaptive responses are sometimes included under the nontargeted effects umbrella, but these are not considered in this paper. Some concepts emerging in the nontargeted effects field that could be important include historic dose. This suggests that the initial exposure to radiation initiates the instability phenotype which is passed to progeny leading to a transgenerational radiation-response phenotype, which suggests that the system response rather than the individual response is critical in determining outcome.

CONCLUSION

Nontargeted effects need to be considered, and modeling, experimental, and epidemiological approaches could all be used to determine the impact of nontargeted effects on the currently used linear nonthreshold model in radiation protection.

Nanoscale Investigation into the Cellular Response of Glioblastoma Cells Exposed to Protons

Exposure to ionizing radiation can induce cellular defense mechanisms including cell activation and rapid proliferation prior to metastasis and in extreme cases can result in cell death. Herewith we apply infrared nano- and microspectroscopy combined with multidimensional data analysis to characterize the effect of ionizing radiation on single glioblastoma nuclei isolated from cells treated with 10 Gy of X-rays or 1 and 10 Gy of protons. We observed chromatin fragmentation related to the formation of apoptotic bodies following X-ray exposure. Following proton irradiation we detected evidence of a DNA conformational change (B-DNA to A-DNA transition) related to DNA repair and accompanied by an increase in protein content related to the synthesis of peptide enzymes involved in DNA repair. We also show that proton exposure can increase cholesterol and sterol ester synthesis, which are important lipids involved in the metastatic process changing the fluidity of the cellular membrane in preparation for rapid proliferation.

Homogenous and Microbeam X-Ray Radiation Induces Proteomic Changes in the Brains of Irradiated Rats and in the Brains of Nonirradiated Cage Mate Rats

To evaluate microbeam radiation therapy (MRT), for brain tumor treatment, the bystander effect in nonirradiated companion animals was investigated. Adult rats were irradiated with 35 or 350 Gy at the European Synchrotron Research Facility using homogenous irradiation (HR) or MRT to the right brain hemisphere. The irradiated rats were housed with nonirradiated rats. After 48 hours, all rats were euthanized and the frontal lobe proteome was analyzed using 2-dimensional electrophoresis and mass spectrometry. Proteome changes were determined by analysis of variance (P < .05). Homogenous irradiation increased serum albumin, heat shock protein 71 (HSP-71), triosephosphate isomerase (TPI), fructose bisphosphate aldolase (FBA), and prohibitin and decreased dihydrolipoyl dehydrogenase (DLD) and pyruvate kinase. Microbeam radiation therapy increased HSP-71, FBA, and prohibitin, and decreased aconitase, dihydropyrimidinase, TPI, tubulin DLD, and pyruvate kinase. Cage mates with HR irradiated rats showed increased HSP-71 and FBA and decreased pyruvate kinase, DLD, and aconitase. Cage mates with MRT irradiated rats showed increased HSP-71, prohibitin, and FBA and decreased aconitase and DLD. Homogenous irradiation proteome changes indicated tumorigenesis, while MRT proteome changes indicated an oxidative stress response. The bystander effect of proteome changes appeared antitumorigenic and inducing radioresistance. This investigation also supports the need for research into prohibitin interaction with HSP-70/71 chaperones and cancer therapy.

Low doses and non-targeted effects in environmental radiation protection; where are we now and where should we go?

The field of low dose radiobiology has advanced considerably in the last 30 years from small indications in the 1980's that all was not simple, to a paradigm shift which occurred during the 1990's, which severely dented the dose-driven models and DNA centric theories which had dominated until then. However while the science has evolved, the application of that science in environmental health protection has not. A reason for this appears to be the uncertainties regarding the shape of the low dose response curve, which lead regulators to adopt a precautionary approach to radiation protection. Radiation protection models assume a linear relationship between dose (i.e. energy deposition) and effect (in this case probability of an adverse DNA interaction leading to a mutation). This model does not consider non-targeted effects (NTE) such as bystander effects or delayed effects, which occur in progeny cells or offspring not directly receiving energy deposition from the dose. There is huge controversy concerning the role of NTE with some saying they reflect "biology" and that repair and homeostatic mechanisms sort out the apparent damage while others consider them to be a class of damage which increases the size of the target. One thing which has recently become apparent is that NTE may be very critical for modelling long-term effects at the level of the population rather than the individual. The issue is that NTE resulting from an acute high dose such as occurred after the A-bomb or Chernobyl occur in parallel with chronic effects induced by the continuing residual effects due to radiation dose decay. This means that if ambient radiation doses are measured for example 25 years after the Chernobyl accident, they only represent a portion of the dose effect because the contribution of NTE is not included.

Systemic mechanisms and effects of ionizing radiation: A new 'old' paradigm of how the bystanders and distant can become the players

Exposure of cells to any form of ionizing radiation (IR) is expected to induce a variety of DNA lesions, including double strand breaks (DSBs), single strand breaks (SSBs) and oxidized bases, as well as loss of bases, i.e., abasic sites. The damaging potential of IR is primarily related to the generation of electrons, which through their interaction with water produce free radicals. In their turn, free radicals attack DNA, proteins and lipids. Damage is induced also through direct deposition of energy. These types of IR interactions with biological materials are collectively called 'targeted effects', since they refer only to the irradiated cells. Earlier and sometimes 'anecdotal' findings were pointing to the possibility of IR actions unrelated to the irradiated cells or area, i.e., a type of systemic response with unknown mechanistic basis. Over the last years, significant experimental evidence has accumulated, showing a variety of radiation effects for 'out-of-field' areas (non-targeted effects-NTE). The NTE involve the release of chemical and biological mediators from the 'in-field' area and thus the communication of the radiation insult via the so called 'danger' signals. The NTE can be separated in two major groups: bystander and distant (systemic). In this review, we have collected a detailed list of proteins implicated in either bystander or systemic effects, including the clinically relevant abscopal phenomenon, using improved text-mining and bioinformatics tools from the literature. We have identified which of these genes belong to the DNA damage response and repair pathway (DDR/R) and made protein-protein interaction (PPi) networks. Our analysis supports that the apoptosis, TLR-like and NOD-like receptor signaling pathways are the main pathways participating in NTE. Based on this analysis, we formulate a biophysical hypothesis for the regulation of NTE, based on DNA damage and apoptosis gradients between the irradiation point and various distances corresponding to bystander (5mm) or distant effects (5cm). Last but not least, in order to provide a more realistic support for our model, we calculate the expected DSB and non-DSB clusters along the central axis of a representative 200.6MeV pencil beam calculated using Monte Carlo DNA damage simulation software (MCDS) based on the actual beam energy-to-depth curves used in therapy.

Is the Linear No-Threshold Dose-Response Paradigm Still Necessary for the Assessment of Health Effects of Low Dose Radiation?

Inevitable human exposure to ionizing radiation from man-made sources has been increased with the proceeding of human civilization and consequently public concerns focus on the possible risk to human health. Moreover, Fukushima nuclear power plant accidents after the 2011 East-Japan earthquake and tsunami has brought the great fear and anxiety for the exposure of radiation at low levels, even much lower levels similar to natural background. Health effects of low dose radiation less than 100 mSv have been debated whether they are beneficial or detrimental because sample sizes were not large enough to allow epidemiological detection of excess effects and there was lack of consistency among the available experimental data. We have reviewed an extensive literature on the low dose radiation effects in both radiation biology and epidemiology, and highlighted some of the controversies therein. This article could provide a reasonable view of utilizing radiation for human life and responding to the public questions about radiation risk. In addition, it suggests the necessity of integrated studies of radiobiology and epidemiology at the national level in order to collect more systematic and profound information about health effects of low dose radiation.

Vibrational spectroscopy in sensing radiobiological effects: analyses of targeted and non-targeted effects in human keratinocytes

Modern models of radiobiological effects include mechanisms of damage initiation, sensing and repair, for those cells that directly absorb ionizing radiation as well as those that experience molecular signals from directly irradiated cells. In the former case, the effects are termed targeted effects while, in the latter, non-targeted effects. It has emerged that phenomena occur at low doses below 1 Gy in directly irradiated cells that are associated with cell-cycle-dependent mechanisms of DNA damage sensing and repair. Likewise in non-targeted bystander-irradiated cells the effect saturates at 0.5 Gy. Both effects at these doses challenge the limits of detection of vibrational spectroscopy. In this paper, a study of the sensing of both targeted and non-targeted effects in HaCaT human keratinocytes irradiated with gamma ray photons is conducted with vibrational spectroscopy. In the case of directly irradiated cells, it is shown that the HaCaT cell line does exhibit both hyperradiosensitivity and increased radioresistance at low doses, a transition between the two effects occurring at a dose of 200 mGy, and that cell survival and other physiological effects as a function of dose follow the induced repair model. Both Raman and FTIR signatures are shown to follow a similar model, suggesting that the spectra include signatures of DNA damage sensing and repair. In bystander-irradiated cells, pro- and anti-apoptotic signalling and mechanisms of ROS damage were inhibited in the mitogen-activated protein kinase (MAPK) transduction pathway. It is shown that Raman spectral profiles of bystander-irradiated cells are correlated with markers of bystander signalling and molecular transduction. This work demonstrates for the first time that both targeted and non-targeted effects of ionizing radiation damage are detected by vibrational spectroscopy in vitro.

Potential strategies to ameliorate risk of radiotherapy-induced second malignant neoplasms

This review is aimed at the issue of radiation-induced second malignant neoplasms (SMN), which has become an important problem with the increasing success of modern cancer radiotherapy (RT). It is imperative to avoid compromising the therapeutic ratio while addressing the challenge of SMN. The dilemma is illustrated by the role of reactive oxygen species in both the mechanisms of tumor cell kill and of radiation-induced carcinogenesis. We explore the literature focusing on three potential routes of amelioration to address this challenge. An obvious approach to avoiding compromise of the tumor response is the use of radioprotectors or mitigators that are selective for normal tissues. We also explore the opportunities to avoid protection of the tumor by topical/regional radioprotection of normal tissues, although this strategy limits the scope of protection. Finally, we explore the role of the bystander/abscopal phenomenon in radiation carcinogenesis, in association with the inflammatory response. Targeted and non-targeted effects of radiation are both linked to SMN through induction of DNA damage, genome instability and mutagenesis, but differences in the mechanisms and kinetics between targeted and non-targeted effects may provide opportunities to lessen SMN. The agents that could be employed to pursue each of these strategies are briefly reviewed. In many cases, the same agent has potential utility for more than one strategy. Although the parallel problem of chemotherapy-induced SMN shares common features, this review focuses on RT associated SMN. Also, we avoid the burgeoning literature on the endeavor to suppress cancer incidence by use of antioxidants and vitamins either as dietary strategies or supplementation.

Impact of fractionation on out-of-field survival and DNA damage responses following exposure to intensity modulated radiation fields

To limit toxicity to normal tissues adjacent to the target tumour volume, radiotherapy is delivered using fractionated regimes whereby the total prescribed dose is given as a series of sequential smaller doses separated by specific time intervals. The impact of fractionation on out-of-field survival and DNA damage responses was determined in AGO-1522 primary human fibroblasts and MCF-7 breast tumour cells using uniform and modulated exposures delivered using a 225 kVp x-ray source. Responses to fractionated schedules (two equal fractions delivered with time intervals from 4 h to 48 h) were compared to those following acute exposures. Cell survival and DNA damage repair measurements indicate that cellular responses to fractionated non-uniform exposures differ from those seen in uniform exposures for the investigated cell lines. Specifically, there is a consistent lack of repair observed in the out-of-field populations during intervals between fractions, confirming the importance of cell signalling to out-of-field responses in a fractionated radiation schedule, and this needs to be confirmed for a wider range of cell lines and conditions.

Non-targeted effects of radiation exposure: recent advances and implications

The target theory of radiation-induced effects has been challenged by numerous studies, which indicate that in addition to biological effects resulting from direct DNA damage within the cell, a variety of non-DNA targeted effects (NTE) may make important contributions to the overall outcome. Ionising radiation induces complex, global cellular responses, such as genomic instability (GI) in both irradiated and never-irradiated 'bystander' cells that receive molecular signals produced by irradiated cells. GI is a well-known feature of many cancers, increasing the probability of cells to acquire the 'hallmarks of cancer' during the development of tumours. Although epidemiological data include contributions of both direct and NTE, they lack (i) statistical power at low dose where differences in dose response for NTE and direct effects are likely to be more important and (ii) heterogeneity of non-targeted responses due to genetic variability between individuals. In this article, NTE focussing on GI and bystander effects were critically examined, the specific principles of NTE were discussed and the potential influence on human health risk assessment from low-dose radiation was considered.

Low-radiation environment affects the development of protection mechanisms in V79 cells

Very little is known about the influence of environmental radiation on living matter. In principle, important information can be acquired by analysing possible differences between parallel biological systems, one in a reference-radiation environment (RRE) and the other in a low-radiation environment (LRE). We took advantage of the unique opportunity represented by the cell culture facilities at the Gran Sasso National Laboratories of the Istituto Nazionale di Fisica Nucleare, where environment dose rate reduction factors in the underground (LRE), with respect to the external laboratory (RRE), are as follows: 10(3) for neutrons, 10(7) for directly ionizing cosmic rays and 10 for total γ-rays. Chinese hamster V79 cells were cultured for 10 months in both RRE and LRE. At the end of this period, all the cultures were kept in RRE for another 6 months. Changes in the activities of antioxidant enzymes (superoxide dismutase, SOD; catalase, CAT; glutathione peroxidase, GPX) and spontaneous mutation frequency at the hypoxanthine-guanine phosphoribosyl transferase (hprt) locus were investigated. The results obtained suggest that environmental radiation might act as a trigger of defence mechanisms in V79 cells, specifically those in reference conditions, showing a higher degree of defence against endogenous damage as compared to cells grown in a very low-radiation environment. Our findings corroborate the hypothesis that environmental radiation contributes to the development of defence mechanisms in today living organisms/systems.

Use of synchrotron medical microbeam irradiation to investigate radiation-induced bystander and abscopal effects in vivo

The question of whether bystander and abscopal effects are the same is unclear. Our experimental system enables us to address this question by allowing irradiated organisms to partner with unexposed individuals. Organs from both animals and appropriate sham and scatter dose controls are tested for expression of several endpoints such as calcium flux, role of 5HT, reporter assay cell death and proteomic profile. The results show that membrane related functions of calcium and 5HT are critical for true bystander effect expression. Our original inter-animal experiments used fish species whole body irradiated with low doses of X-rays, which prevented us from addressing the abscopal effect question. Data which are much more relevant in radiotherapy are now available for rats which received high dose local irradiation to the implanted right brain glioma. The data were generated using quasi-parallel microbeams at the biomedical beamline at the European Synchrotron Radiation Facility in Grenoble France. This means we can directly compare abscopal and "true" bystander effects in a rodent tumour model. Analysis of right brain hemisphere, left brain and urinary bladder in the directly irradiated animals and their unirradiated partners strongly suggests that bystander effects (in partner animals) are not the same as abscopal effects (in the irradiated animal). Furthermore, the presence of a tumour in the right brain alters the magnitude of both abscopal and bystander effects in the tissues from the directly irradiated animal and in the unirradiated partners which did not contain tumours, meaning the type of signal was different.

Mechanisms and biological importance of photon-induced bystander responses: do they have an impact on low-dose radiation responses

Elucidating the biological effect of low linear energy transfer (LET), low-dose and/or low-dose-rate ionizing radiation is essential in ensuring radiation safety. Over the past two decades, non-targeted effects, which are not only a direct consequence of radiation-induced initial lesions produced in cellular DNA but also of intra- and inter-cellular communications involving both targeted and non-targeted cells, have been reported and are currently defining a new paradigm in radiation biology. These effects include radiation-induced adaptive response, low-dose hypersensitivity, genomic instability, and radiation-induced bystander response (RIBR). RIBR is generally defined as a cellular response that is induced in non-irradiated cells that receive bystander signals from directly irradiated cells. RIBR could thus play an important biological role in low-dose irradiation conditions. However, this suggestion was mainly based on findings obtained using high-LET charged-particle radiations. The human population (especially the Japanese, who are exposed to lower doses of radon than the world average) is more frequently exposed to low-LET photons (X-rays or γ-rays) than to high-LET charged-particle radiation on a daily basis. There are currently a growing number of reports describing a distinguishing feature between photon-induced bystander response and high-LET RIBR. In particular, photon-induced bystander response is strongly influenced by irradiation dose, the irradiated region of the targeted cells, and p53 status. The present review focuses on the photon-induced bystander response, and discusses its impact on the low-dose radiation effect.

Implications for human and environmental health of low doses of ionising radiation

The last 20 years have seen a major paradigm shift in radiation biology. Several discoveries challenge the DNA centric view which holds that DNA damage is the critical effect of radiation irrespective of dose. This theory leads to the assumption that dose and effect are simply linked - the more energy deposition, the more DNA damage and the greater the biological effect. This is embodied in radiation protection (RP) regulations as the linear-non-threshold (LNT) model. However the science underlying the LNT model is being challenged particularly in relation to the environment because it is now clear that at low doses of concern in RP, cells, tissues and organisms respond to radiation by inducing responses which are not readily predictable by dose. These include adaptive responses, bystander effects, genomic instability and low dose hypersensitivity, and are commonly described as stress responses, while recognizing that "stress" can be good as well as bad. The phenomena contribute to observed radiation responses and appear to be influenced by genetic, epigenetic and environmental factors, meaning that dose and response are not simply related. The question is whether our discovery of these phenomena means that we need to re-evaluate RP approaches. The so-called "non-targeted" mechanisms mean that low dose radiobiology is very complex and supra linear or sub-linear (even hormetic) responses are possible but their occurrence is unpredictable for any given system level. Issues which may need consideration are synergistic or antagonistic effects of other pollutants. RP, at present, only looks at radiation dose but the new (NTE) radiobiology means that chemical or physical agents, which interfere with tissue responses to low doses of radiation, could critically modulate the predicted risk. Similarly, the "health" of the organism could determine the effect of a given low dose by enabling or disabling a critical response. These issues will be discussed.

Neutron exposures in human cells: bystander effect and relative biological effectiveness

Bystander effects have been observed repeatedly in mammalian cells following photon and alpha particle irradiation. However, few studies have been performed to investigate bystander effects arising from neutron irradiation. Here we asked whether neutrons also induce a bystander effect in two normal human lymphoblastoid cell lines. These cells were exposed to fast neutrons produced by targeting a near-monoenergetic 50.5 MeV proton beam at a Be target (17 MeV average neutron energy), and irradiated-cell conditioned media (ICCM) was transferred to unirradiated cells. The cytokinesis-block micronucleus assay was used to quantify genetic damage in radiation-naïve cells exposed to ICCM from cultures that received 0 (control), 0.5, 1, 1.5, 2, 3 or 4 Gy neutrons. Cells grown in ICCM from irradiated cells showed no significant increase in the frequencies of micronuclei or nucleoplasmic bridges compared to cells grown in ICCM from sham irradiated cells for either cell line. However, the neutron beam has a photon dose-contamination of 5%, which may modulate a neutron-induced bystander effect. To determine whether these low doses of contaminating photons can induce a bystander effect, cells were irradiated with cobalt-60 at doses equivalent to the percent contamination for each neutron dose. No significant increase in the frequencies of micronuclei or bridges was observed at these doses of photons for either cell line when cultured in ICCM. As expected, high doses of photons induced a clear bystander effect in both cell lines for micronuclei and bridges (p<0.0001). These data indicate that neutrons do not induce a bystander effect in these cells. Finally, neutrons had a relative biological effectiveness of 2.0 ± 0.13 for micronuclei and 5.8 ± 2.9 for bridges compared to cobalt-60. These results may be relevant to radiation therapy with fast neutrons and for regulatory agencies setting standards for neutron radiation protection and safety.

Radiation-induced bystander effects in the Atlantic salmon (salmo salar L.) following mixed exposure to copper and aluminum combined with low-dose gamma radiation

Very little is known about the combined effects of low doses of heavy metals and radiation. However, such "multiple stressor" exposure is the reality in the environment. In the work reported in this paper, fish were exposed to cobalt 60 gamma irradiation with or without copper or aluminum in the water. Doses of radiation ranged from 4 to 75 mGy delivered over 48 or 6 h. Copper doses ranged from 10 to 80 μg/L for the same time period. The aluminum dose was 250 μg/L. Gills and skin were removed from the fish after exposure and explanted in tissue culture flasks for investigation of bystander effects of the exposures using a stress signal reporter assay, which has been demonstrated to be a sensitive indicator of homeostatic perturbations in cells. The results show complex synergistic interactions of radiation and copper. Gills on the whole produce more toxic bystander signals than skin, but the additivity scores show highly variable results which depend on dose and time of exposure. The impacts of low doses of copper and low doses of radiation are greater than additive, medium levels of copper alone have a similar level of effect of bystander signal toxicity to the low dose. The addition of radiation stress, however, produces clear protective effects in the reporters treated with skin-derived medium. Gill-derived medium from the same fish did not show protective effects. Radiation exposure in the presence of 80 μg/L led to highly variable results, which due to animal variation were not significantly different from the effect of copper alone. The results are stressor type, stressor concentration and time dependent. Clearly co-exposure to radiation and heavy metals does not always lead to simple additive effects.

Radiotherapy and the tumor microenvironment: mutual influence and clinical implications

Ionizing radiation has been employed in targeted cancer treatments for more than a century because of its cytotoxic effects on cancer cells. However, the responsiveness to radiation and the behavior of tumors in vivo may differ dramatically from observed behaviors of isolated cancer cells in vitro. While not fully understood, these discrepancies are due to a complex constellation of extracellular and intercellular factors that are together termed the tumor microenvironment. Radiation may alter or affect the components of the adjacent tumor microenvironment in significant ways, often with consequences for cancer cells beyond the direct effects of the radiation itself. Moreover, different microenvironmental states, whether induced or at baseline, can modulate or even attenuate the effects of radiation, with consequences for therapeutic efficacy. This chapter describes this bidirectional relationship in detail, exploring the role and clinical implications of the tumor microenvironment with respect to therapeutic irradiation.

X-ray-induced bystander responses reduce spontaneous mutations in V79 cells

The potential for carcinogenic risks is increased by radiation-induced bystander responses; these responses are the biological effects in unirradiated cells that receive signals from the neighboring irradiated cells. Bystander responses have attracted attention in modern radiobiology because they are characterized by non-linear responses to low-dose radiation. We used a synchrotron X-ray microbeam irradiation system developed at the Photon Factory, High Energy Accelerator Research Organization, KEK, and showed that nitric oxide (NO)-mediated bystander cell death increased biphasically in a dose-dependent manner. Here, we irradiated five cell nuclei using 10 × 10 µm(2) 5.35 keV X-ray beams and then measured the mutation frequency at the hypoxanthine-guanosine phosphoribosyl transferase (HPRT) locus in bystander cells. The mutation frequency with the null radiation dose was 2.6 × 10(-)(5) (background level), and the frequency decreased to 5.3 × 10(-)(6) with a dose of approximately 1 Gy (absorbed dose in the nucleus of irradiated cells). At high doses, the mutation frequency returned to the background level. A similar biphasic dose-response effect was observed for bystander cell death. Furthermore, we found that incubation with 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (carboxy-PTIO), a specific scavenger of NO, suppressed not only the biphasic increase in bystander cell death but also the biphasic reduction in mutation frequency of bystander cells. These results indicate that the increase in bystander cell death involves mechanisms that suppress mutagenesis. This study has thus shown that radiation-induced bystander responses could affect processes that protect the cell against naturally occurring alterations such as mutations.

Embryos of the zebrafish Danio rerio in studies of non-targeted effects of ionizing radiation

The use of embryos of the zebrafish Danio rerio as an in vivo tumor model for studying non-targeted effects of ionizing radiation was reviewed. The zebrafish embryo is an animal model, which enables convenient studies on non-targeted effects of both high-linear-energy-transfer (LET) and low-LET radiation by making use of both broad-beam and microbeam radiation. Zebrafish is also a convenient embryo model for studying radiobiological effects of ionizing radiation on tumors. The embryonic origin of tumors has been gaining ground in the past decades, and efforts to fight cancer from the perspective of developmental biology are underway. Evidence for the involvement of radiation-induced genomic instability (RIGI) and the radiation-induced bystander effect (RIBE) in zebrafish embryos were subsequently given. The results of RIGI were obtained for the irradiation of all two-cell stage cells, as well as 1.5 hpf zebrafish embryos by microbeam protons and broad-beam alpha particles, respectively. In contrast, the RIBE was observed through the radioadaptive response (RAR), which was developed against a subsequent challenging dose that was applied at 10 hpf when <0.2% and <0.3% of the cells of 5 hpf zebrafish embryos were exposed to a priming dose, which was provided by microbeam protons and broad-beam alpha particles, respectively. Finally, a perspective on the field, the need for future studies and the significance of such studies were discussed.

The complex interactions between radiation induced non-targeted effects and cancer

Radiation induced non-targeted effects have been widely investigated in the last two decades for their potential impact on low dose radiation risk. In this paper we will give an overview of the most relevant aspects related to these effects, starting from the definition of the low dose scenarios. We will underline the role of radiation quality, both in terms of mechanisms of interaction with the biological matter and for the importance of charged particles as powerful tools for low dose effects investigation. We will focus on cell communication, representing a common feature of non-targeted effects, giving also an overview of cancer models that have explicitly considered such effects.

Toward computer simulation of high-LET in vitro survival curves

We developed a Monte Carlo based computer program called MCSC (Monte Carlo Survival Curve) able to predict the survival fraction of cells irradiated in vitro with a broad beam of high linear energy transfer particles. Three types of cell responses are studied: the usual high dose response, the bystander effect and the low-dose hypersensitivity (HRS). The program models the broad beam irradiation and double strand break distribution following Poisson statistics. The progression of cells through the cell cycle is taken into account while the repair takes place. Input parameters are experimentally determined for A549 lung carcinoma cells irradiated with 10 and 20 keV µm(-1) protons, 115 keV µm(-1) alpha particles and for EAhy926 endothelial cells exposed to 115 keV µm(-1) alpha particles. Results of simulations are presented and compared with experimental survival curves obtained for A549 and EAhy296 cells. Results are in good agreement with experimental data for both cell lines and all irradiation protocols. The benefits of MCSC are several: the gain of time that would have been spent performing time-consuming clonogenic assays, the capacity to estimate survival fraction of cell lines not forming colonies and possibly the evaluation of radiosensitivity parameters of given individuals.

Uncomfortable issues in radiation protection posed by low-dose radiobiology

This paper aims to stimulate discussion about the relevance for radiation protection of recent findings in low-dose radiobiology. Issues are raised which suggest that low-dose effects are much more complex than has been previously assumed. These include genomic instability, bystander effects, multiple stressor exposures and chronic exposures. To date, these have been accepted as being relevant issues, but there is no clear way to integrate knowledge about these effects into the existing radiation protection framework. A further issue which might actually lead to some fruitful approaches for human radiation protection is the need to develop a new framework for protecting non-human biota. The brainstorming that is being applied to develop effective and practical ways to protect ecosystems widens the debate from the narrow focus of human protection which is currently about protecting humans from radiation-induced cancers.

Alternative medicine techniques have non-linear effects on radiation response and can alter the expression of radiation induced bystander effects

Many so-called "alternative medicine" techniques such as Reiki and acupuncture produce very good outcomes for intractable pain and other chronic illnesses but the efficacy is often dismissed as being psychosomatic. However a plausible mechanism does exist i.e. that the treatments alter the electromagnetic fields in living organisms and thereby prevent or reduce activity of neurons which lead to the pain. Low doses of ionising radiation have similar effects on electromagnetic fields and are known to induce signaling cascades in tissues due to ion gradients. To test this hypothesis cell cultures were exposed to Reiki - like and to acupuncture - like treatments, both performed by qualified practitioners. The cells were exposed either before or after the treatment to x-rays and were monitored for production of direct damage or bystander signals. The data suggest that the alternative techniques altered the response of cells to direct irradiation and altered bystander signal mechanisms. We conclude that alternative medicine techniques involving electromagnetic perturbations may modify the response of cells to ionizing radiation. In addition to the obvious implications for mechanistic studies of low dose effects, this could provide a novel target to exploit in radiation protection and in optimizing therapeutic gain during radiotherapy.

Non-targeted effects of ionising radiation--implications for low dose risk

Non-DNA targeted effects of ionising radiation, which include genomic instability, and a variety of bystander effects including abscopal effects and bystander mediated adaptive response, have raised concerns about the magnitude of low-dose radiation risk. Genomic instability, bystander effects and adaptive responses are powered by fundamental, but not clearly understood systems that maintain tissue homeostasis. Despite excellent research in this field by various groups, there are still gaps in our understanding of the likely mechanisms associated with non-DNA targeted effects, particularly with respect to systemic (human health) consequences at low and intermediate doses of ionising radiation. Other outstanding questions include links between the different non-targeted responses and the variations in response observed between individuals and cell lines, possibly a function of genetic background. Furthermore, it is still not known what the initial target and early interactions in cells are that give rise to non-targeted responses in neighbouring or descendant cells. This paper provides a commentary on the current state of the field as a result of the non-targeted effects of ionising radiation (NOTE) Integrated Project funded by the European Union. Here we critically examine the evidence for non-targeted effects, discuss apparently contradictory results and consider implications for low-dose radiation health effects.

The threshold number of protons to induce an adaptive response in zebrafish embryos

In this study, microbeam protons were used to provide the priming dose to induce an in vivo radioadaptive response (RAR) in the embryos of zebrafish, Danio rerio, against subsequent challenging doses provided by x-ray photons. The microbeam irradiation system (Single-Particle Irradiation System to Cell, acronym SPICE) at the National Institute of Radiological Sciences (NIRS), Japan, was employed. The embryos were dechorionated at 4 h post fertilisation (hpf) and irradiated at 5 hpf by microbeam protons. For each embryo, one irradiation point was chosen, to which 5, 10, 20, 30, 40, 50, 100, 200, 300 and 500 protons each with an energy of 3.4 MeV were delivered. The embryos were returned to the incubator until 10 hpf to further receive the challenging exposure, which was achieved using 2 Gy of x-ray irradiation, and then again returned to the incubator until 24 hpf for analyses. The levels of apoptosis in zebrafish embryos at 25 hpf were quantified through terminal dUTP transferase-mediated nick end-labelling (TUNEL) assay. The results revealed that at least 200 protons (with average radiation doses of about 300 and 650 mGy absorbed by an irradiated epithelial and deep cell, respectively) would be required to induce RAR in the zebrafish embryos in vivo. Our previous investigation showed that 5 protons delivered at 10 points on an embryo would already be sufficient to induce RAR in the zebrafish embryos. The difference was explained in terms of the radiation-induced bystander effect as well as the rescue effect.

A kinetic-based model of radiation-induced intercellular signalling

It is now widely accepted that intercellular communication can cause significant variations in cellular responses to genotoxic stress. The radiation-induced bystander effect is a prime example of this effect, where cells shielded from radiation exposure see a significant reduction in survival when cultured with irradiated cells. However, there is a lack of robust, quantitative models of this effect which are widely applicable. In this work, we present a novel mathematical model of radiation-induced intercellular signalling which incorporates signal production and response kinetics together with the effects of direct irradiation, and test it against published data sets, including modulated field exposures. This model suggests that these so-called “bystander” effects play a significant role in determining cellular survival, even in directly irradiated populations, meaning that the inclusion of intercellular communication may be essential to produce robust models of radio-biological outcomes in clinically relevant in vivo situations.

Dose response of soft X-ray-induced bystander cell killing affected by p53 status

A radiation-induced bystander response, which is generally defined as a cellular response that is induced in nonirradiated cells that received bystander signals from directly irradiated cells within an irradiated cell population. In our earlier X-ray microbeam studies, bystander cell killing in normal human fibroblasts had a parabolic relationship to the irradiation dose. To elucidate the role of p53 in the bystander cell killing, the effects were assessed using human non-small cell lung cancer cells expressing wild-type or temperature-sensitive mutated p53. The surviving fraction of bystander wild-type p53 cells showed a parabolic relationship to the irradiation dose; survival was steeply reduced up to 0.45 Gy, recovered toward to 2 Gy, and remained at control levels up to 5 Gy. In contrast, in the mutated p53 cells at a nonpermissive temperature, the surviving fraction was steeply reduced up to 1 Gy and remained at the reduced level up to 5 Gy. When the mutated p53 cells were incubated at a permissive temperature, the decrease in the surviving fraction at 2 Gy was suppressed. The wild-type p53 cells were not only restrained in releasing bystander signals at 2 Gy, but were also resistant to the signals released by the mutated p53 cells. These results suggest that the X-ray-induced bystander cell killing depends on both the irradiation dose and the p53 status of the targeted cells and the bystander cells.