Generalized methods for predicting biological response to mixed radiation types and calculating equieffective doses (EQDX)

BACKGROUND

Predicting biological responses to mixed radiation types is of considerable importance when combining radiation therapies that use multiple radiation types and delivery regimens. These may include the use of both low- and high-linear energy transfer (LET) radiations. A number of theoretical models have been developed to address this issue. However, model predictions do not consistently match published experimental data for mixed radiation exposures. Furthermore, the models are often computationally intensive. Accordingly, there is a need for efficient analytical models that can predict responses to mixtures of low- and high-LET radiations. Additionally, a general formalism to calculate equieffective dose (EQDX) for mixed radiations is needed.

PURPOSE

To develop a computationally efficient analytical model that can predict responses to complex mixtures of low- and high-LET radiations as a function of either absorbed dose or EQDX.

METHODS

The Zaider-Rossi model (ZRM) was modified by replacing the geometric mean of the quadratic coefficients in the interaction term with the arithmetic mean. This modified ZRM model (mZRM) was then further generalized to any number of radiation types and its validity was tested against published experimental observations. Comparisons between the predictions of the ZRM and mZRM, and other models, were made using two and three radiation types. In addition, a generalized formalism for calculating EQDX for mixed radiations was developed within the context of mZRM and validated with published experimental results.

RESULTS

The predictions of biological responses to mixed-LET radiations calculated with the mZRM are in better agreement with experimental observations than ZRM, especially when high- and low-LET radiations are mixed. In these situations, the ZRM overestimated the surviving fraction. Furthermore, the EQDX calculated with mZRM are in better agreement with experimental observations.

CONCLUSION

The mZRM is a computationally efficient model that can be used to predict biological response to mixed radiations that have low- and high-LET characteristics. Importantly, interaction terms are retained in the calculation of EQDX for mixed radiation exposures within the mZRM framework. The mZRM has application in a wide range of radiation therapies, including radiopharmaceutical therapy.

High-LET radiation induces large amounts of rapidly-repaired sublethal damage

There is agreement that high-LET radiation has a high Relative Biological Effectiveness (RBE) when delivered as a single treatment, but how it interacts with radiations of different qualities, such as X-rays, is less clear. We sought to clarify these effects by quantifying and modelling responses to X-ray and alpha particle combinations. Cells were exposed to X-rays, alpha particles, or combinations, with different doses and temporal separations. DNA damage was assessed by 53BP1 immunofluorescence, and radiosensitivity assessed using the clonogenic assay. Mechanistic models were then applied to understand trends in repair and survival. 53BP1 foci yields were significantly reduced in alpha particle exposures compared to X-rays, but these foci were slow to repair. Although alpha particles alone showed no inter-track interactions, substantial interactions were seen between X-rays and alpha particles. Mechanistic modelling suggested that sublethal damage (SLD) repair was independent of radiation quality, but that alpha particles generated substantially more sublethal damage than a similar dose of X-rays, [Formula: see text]. This high RBE may lead to unexpected synergies for combinations of different radiation qualities which must be taken into account in treatment design, and the rapid repair of this damage may impact on mechanistic modelling of radiation responses to high LETs.

Modelling Dose Effects from Space Irradiations: Combination of High-LET and Low-LET Radiations with a Modified Microdosimetric Kinetic Model

The Microdosimetric Kinetic Model (MKM) to predict the effects of ionizing radiation on cell colonies is studied and reformulated for the case of high-linear energy transfer (LET) radiations with a low dose. When the number of radiation events happening in a subnuclear domain follows a Poisson distribution, the MKM predicts a linear-quadratic (LQ) survival curve. We show that when few events occur, as for high-LET radiations at doses lower than the mean specific energy imparted to the nucleus, zF,n, a Poisson distribution can no longer be assumed and an initial pure linear relationship between dose and survival fraction should be observed. Predictions of survival curves for combinations of high-LET and low-LET radiations are produced under two assumptions for their comparison: independent and combined action. Survival curves from previously published articles of V79 cell colonies exposed to X-rays, α particles, Ar-ions, Fe-ions, Ne-ions and mixtures of X-rays and each one of the ions are predicted according to the modified MKM. We conclude that mixtures of high-LET and low-LET radiations may enhance the effect of individual actions due to the increase of events in domains provided by the low-LET radiation. This hypothesis is only partially validated by the analyzed experiments.

Clustered DNA Double-Strand Breaks: Biological Effects and Relevance to Cancer Radiotherapy

Cells manage to survive, thrive, and divide with high accuracy despite the constant threat of DNA damage. Cells have evolved with several systems that efficiently repair spontaneous, isolated DNA lesions with a high degree of accuracy. Ionizing radiation and a few radiomimetic chemicals can produce clustered DNA damage comprising complex arrangements of single-strand damage and DNA double-strand breaks (DSBs). There is substantial evidence that clustered DNA damage is more mutagenic and cytotoxic than isolated damage. Radiation-induced clustered DNA damage has proven difficult to study because the spectrum of induced lesions is very complex, and lesions are randomly distributed throughout the genome. Nonetheless, it is fairly well-established that radiation-induced clustered DNA damage, including non-DSB and DSB clustered lesions, are poorly repaired or fail to repair, accounting for the greater mutagenic and cytotoxic effects of clustered lesions compared to isolated lesions. High linear energy transfer (LET) charged particle radiation is more cytotoxic per unit dose than low LET radiation because high LET radiation produces more clustered DNA damage. Studies with I-SceI nuclease demonstrate that nuclease-induced DSB clusters are also cytotoxic, indicating that this cytotoxicity is independent of radiogenic lesions, including single-strand lesions and chemically "dirty" DSB ends. The poor repair of clustered DSBs at least in part reflects inhibition of canonical NHEJ by short DNA fragments. This shifts repair toward HR and perhaps alternative NHEJ, and can result in chromothripsis-mediated genome instability or cell death. These principals are important for cancer treatment by low and high LET radiation.

Prediction of Cell Survival after Exposure to Mixed Radiation Fields with the Local Effect Model

Mixed radiation fields comprise the most common form of radiation exposure. Given their relevance in radiation protection, cancer radiotherapy and space research, accurate predictions of the corresponding radiation effects are essential. The local effect model (LEM) allows the prediction of cell survival after ion irradiation based on the knowledge of the cells' response to photon radiation. The assumption is made that the same spatial DNA double-strand break (DSB) distributions in the cell nucleus lead to the same effects, independent of the radiation quality that produced the DSBs. This makes the LEM an ideal tool for predictions of cell survival after exposure to any mixed radiation field. In this work, the LEM is applied to calculate cell survival for extreme mixed irradiation scenarios, i.e., high-linear energy transfer (LET) ion radiation combined with low-LET photon radiation, which can be understood as a consistency test for the high-LET model. Available experimental data covering several ion species and energies in combination with photon exposure are predicted with the LEM. Furthermore, the results are compared to the microdosimetric model by Zaider and Rossi and the lesion additivity model by Lam, which allow the prediction of cell survival after exposure to mixed radiation fields based on the knowledge of the survival curves of the two radiation components. Although the LEM uses only photon dose-response data as input, it is able to compete with the empirical radiobiological models that additionally require ion dose-response curves as input. Certain experimental scenarios are presented in which the specific consideration of spatial DSB distributions could be essential for an accurate prediction of the effect of mixed radiation fields.

A comparison of mechanism-inspired models for particle relative biological effectiveness (RBE)

BACKGROUND AND SIGNIFICANCE

The application of heavy ion beams in cancer therapy must account for the increasing relative biological effectiveness (RBE) with increasing penetration depth when determining dose prescriptions and organ at risk (OAR) constraints in treatment planning. Because RBE depends in a complex manner on factors such as the ion type, energy, cell and tissue radiosensitivity, physical dose, biological endpoint, and position within and outside treatment fields, biophysical models reflecting these dependencies are required for the personalization and optimization of treatment plans.

AIM

To review and compare three mechanism-inspired models which predict the complexities of particle RBE for various ion types, energies, linear energy transfer (LET) values and tissue radiation sensitivities.

METHODS

The review of models and mechanisms focuses on the Local Effect Model (LEM), the Microdosimetric-Kinetic (MK) model, and the Repair-Misrepair-Fixation (RMF) model in combination with the Monte Carlo Damage Simulation (MCDS). These models relate the induction of potentially lethal double strand breaks (DSBs) to the subsequent interactions and biological processing of DSB into more lethal forms of damage. A key element to explain the increased biological effectiveness of high LET ions compared to MV x rays is the characterization of the number and local complexity (clustering) of the initial DSB produced within a cell. For high LET ions, the spatial density of DSB induction along an ion's trajectory is much greater than along the path of a low LET electron, such as the secondary electrons produced by the megavoltage (MV) x rays used in conventional radiation therapy. The main aspects of the three models are introduced and the conceptual similarities and differences are critiqued and highlighted. Model predictions are compared in terms of the RBE for DSB induction and for reproductive cell survival.

RESULTS AND CONCLUSIONS

Comparisons of the RBE for DSB induction and for cell survival are presented for proton (¹ H), helium (⁴ He), and carbon (¹² C) ions for the therapeutically most relevant range of ion beam energies. The reviewed models embody mechanisms of action acting over the spatial scales underlying the biological processing of potentially lethal DSB into more lethal forms of damage. Differences among the number and types of input parameters, relevant biological targets, and the computational approaches among the LEM, MK and RMF models are summarized and critiqued. Potential experiments to test some of the seemingly contradictory aspects of the models are discussed.

Simultaneous induction of dispersed and clustered DNA lesions compromises DNA damage response in human peripheral blood lymphocytes

Due to its ability to induce DNA damage in a space and time controlled manner, ionising radiation is a unique tool for studying the mechanisms of DNA repair. The biological effectiveness of ionising radiation is related to the ionisation density which is defined by the linear energy transfer (LET). Alpha particles are characterised by high LET, while X-rays by low LET values. An interesting question is how cells react when exposed to a mixed beam of high and low LET radiation. In an earlier study carried out with human peripheral blood lymphocytes (PBL) we could demonstrate that alpha radiation X-rays interact in producing more chromosomal aberrations than expected based on additivity. The aim of the present investigation was to look at the mechanism of the interaction, especially with respect to the question if it is due to an augmented level of initial damage or impaired DNA repair. PBL were exposed to various doses of alpha particles, X-rays and mixed beams. DNA damage and the kinetics of damage repair was quantified by the alkaline comet assay. The levels of phosphorylated, key DNA damage response (DDR) proteins ATM, p53 and DNA-PK were measured by Western blotting and mRNA levels of 6 damage-responsive genes were measured by qPCR. Alpha particles and X-rays interact in inducing DNA damage above the level predicted by assuming additivity and that the repair of damage occurs with a delay. The activation levels of DDR proteins and mRNA levels of the studied genes were highest in cells exposed to mixed beams. The results substantiate the idea that exposure to mixed beams presents a challenge for the cellular DDR system.

Interaction of low and high LET radiation in TK6 cells-mechanistic aspects and significance for radiation protection

Most environmental, occupational and medical exposures to ionising radiation are associated with a simultaneous action of different radiation types. An open question remains whether radiations of different qualities interact with each other to yield effects stronger than expected based on the assumption of additivity. It is possible that DNA damage induced by high linear energy transfer (LET) radiation will lead to an opening of the chromatin structure making the DNA more susceptible to attack by reactive oxygen species (ROS) generated by the low LET radiation. In such case, the effect of mixed beams should be strongly expressed in cells that are sensitive to ROS. The present investigation was carried out to test if cells with an impaired capacity to handle oxidative stress are particularly sensitive to the effect of mixed beams of alpha particles and x-rays. Clonogenic cell survival curves and mutant frequencies were analysed in TK6 wild type (wt) cells and in TK6 cells with a knocked down hMYH glycosylase. The results showed a synergistic effect of mixed beams on clonogenic cell survival of TK6^wt but not TK6^MYH- cells. The frequencies of mutants showed a high degree of interexperimental variability without any indications for synergistic effects of mixed beams. TK6^MYH- cells were generally more tolerant to radiation exposure with respect to clonogenic cell survival but showed a strong increase in mutant frequency. The results demonstrate that exposure of wt cells to a mixed beam of alpha particles and x-rays leads to a detrimental effect which is stronger than expected based on the assumption of additivity. The role of oxidative stress in the reaction of cells to mixed beams remains unclear.

Gamma-H2AX foci in cells exposed to a mixed beam of X-rays and alpha particles

Background

Little is known about the cellular effects of exposure to mixed beams of high and low linear energy transfer radiation. So far, the effects of combined exposures have mainly been assessed with clonogenic survival or cytogenetic methods, and the results are contradictory. The gamma-H2AX assay has up to now not been applied in this context, and it is a promising tool for investigating the early cellular response to mixed beam irradiation.

Purpose

To determine the dose response and repair kinetics of gamma-H2AX ionizing radiation-induced foci in VH10 human fibroblasts exposed to mixed beams of ²⁴¹Am alpha particles and X-rays.

Results

VH10 human fibroblasts were irradiated with each radiation type individually or both in combination at 37°C. Foci were scored for repair kinetics 0.5, 1, 3 and 24 h after irradiation (one dose per irradiation type), and for dose response at the 1 h time point. The dose response effect of mixed beam was additive, and the relative biological effectiveness for alpha particles (as compared to X-rays) was of 0.76 ± 0.52 for the total number of foci, and 2.54 ± 1.11 for large foci. The repair kinetics for total number of foci in cells exposed to mixed beam irradiation was intermediate to that of cells exposed to alpha particles and X-rays. However, for mixed beam-irradiated cells the frequency and area of large foci were initially lower than predicted and increased during the first 3 hours of repair (while the predicted number and area did not).

Conclusions

The repair kinetics of large foci after mixed beam exposure was significantly different from predicted based on the effect of the single dose components. The formation of large foci was delayed and they did not reach their maximum area until 1 h after irradiation. We hypothesize that the presence of low X-ray-induced damage engages the DNA repair machinery leading to a delayed DNA damage response to the more complex DNA damage induced by alpha particles.

Chromosomal aberrations in peripheral blood lymphocytes exposed to a mixed beam of low energy neutrons and gamma radiation

Cells exposed to thermal neutrons are simultaneously damaged by radiations with high and low linear energy transfer (LET). A question relevant for the assessment of risk of exposure to a mixed beam is whether the biological effect of both radiation types is additive or synergistic. The aim of the present investigation was to calculate whether the high and low LET components of a thermal neutron field interact when damaging cells. Human peripheral blood lymphocytes were exposed to neutrons from the HB11 beam at the Institute for Energy and Transport, Petten, Netherlands, in a 37 °C water phantom at varying depths, where the mix of high and low LET beam components differs. Chromosomal aberrations were analysed and the relative biological effectiveness (RBE) values as well as the expected contributions of protons and photons to the aberration yield were calculated based on a dose response of aberrations in lymphocytes exposed to (60)Co gamma radiation. The RBE for 10 dicentrics per 100 cells was 3 for mixed beam and 7.2 for protons. For 20 dicentrics per 100 cells the respective values were 2.4 and 5.8. Within the limitations of the experimental setup the results indicate that for this endpoint there is no synergism between the high and low LET radiations.

Micronuclei in human peripheral blood lymphocytes exposed to mixed beams of X-rays and alpha particles

The purpose of this study was to analyse the cytogenetic effect of exposing human peripheral blood lymphocytes (PBL) to a mixed beam of alpha particles and X-rays. Whole blood collected from one donor was exposed to different doses of alpha particles ((241)Am), X-rays and a combination of both. All exposures were carried out at 37 °C. Three independent experiments were performed. Micronuclei (MN) in binucleated PBL were scored as the endpoint. Moreover, the size of MN was measured. The results show that exposure of PBL to a mixed beam of high and low linear energy transfer radiation led to significantly higher than expected frequencies of MN. The measurement of MN size did not reveal any differences between the effect of alpha particles and mixed beam. In conclusion, a combined exposure of PBL to alpha particles and X-rays leads to a synergistic effect as measured by the frequency of MN. From the analysis of MN distributions, we conclude that the increase was due to an impaired repair of X-ray-induced DNA damage.

Do the various radiations present in BNCT act synergistically? Cell survival experiments in mixed alpha-particle and gamma-ray fields

In many radiotherapy situations patients are exposed to mixed field radiation. In particular in BNCT, as with all neutron beam exposures, a significant fraction of the dose is contributed by low LET gamma ray photons. The components of such a mixed field may show a synergistic interaction and produce a greater cell kill effect than would be anticipated from the independent action of the different radiation types. Such a synergy would have important implications for treatment planning and in the interpretation of clinical results. An irradiation setup has been created at the Medical Research Council in Harwell to allow simultaneous irradiation of cells by cobalt-60 gamma rays and plutonium-238 alpha-particles. The setup allows for variation of dose and dose rates for both sources along with variation of the alpha particle energy. A series of cell survival assays for this mixed field have been carried out using V79-4 cells and compared to exposures to the individual components of the field under identical conditions. In the experimental setup described no significant synergistic effect was observed.

Experimental validation of the new nanodosimetry-based cell survival model for mixed neutron and gamma-ray irradiation

The new nanodosimetry-based linear-quadratic (LQ) formula has been reviewed for mixed-LET irradiation. V-79 Chinese hamster cells have been irradiated with a mixed-LET field of fission neutrons and gamma rays at the University of Maryland Training Reactor (MUTR). The results show that the experimental survival curve agrees well with that predicted by the new nanodosimetry-based LQ model. The experimental study described in this note, therefore, serves as a validation for the new model to be used for mixed-LET radiotherapies, e.g. 252Cf brachytherapy.

Cell killing, nuclear damage and apoptosis in Chinese hamster V79 cells after irradiation with heavy-ion beams of (16)O, (12)C and (7)Li

Chinese hamster V79 cells were exposed to high LET (linear energy transfer) (16)O-beam (625keV/mum) radiation in the dose range of 0-9.83Gy. Cell survival, micronuclei (MN), chromosomal aberrations (CA) and induction of apoptosis were studied as a follow up of our earlier study on high LET radiations ((7)Li-beam of 60keV/mum and (12)C-beam of 295keV/mum) as well as (60)Co gamma-rays. Dose dependent decline in surviving fraction was noticed along with the increase of MN frequency, CA frequency as well as percentage of apoptosis as detected by nuclear fragmentation assay. The relative intensity of DNA ladder, which is a useful marker for the determination of the extent of apoptosis induction, was also increased in a dose dependent manner. Additionally, expression of tyrosine kinase lck-1 gene, which plays an important role in response to ionizing radiation induced apoptosis, was increased with the increase of radiation doses and also with incubation time. The present study showed that all the high LET radiations were generally more effective in cell killing and inflicting other cytogenetic damages than that of low LET gamma-rays. The dose response curves revealed that (7)Li-beam was most effective in cell killing as well as inducing other nuclear damages followed by (12)C, (16)O and (60)Co gamma-rays, in that order. The result of this study may have some application in biological dosimetry for assessment of genotoxicity in heavy ion exposed subjects and in determining suitable doses for radiotherapy in cancer patients where various species of heavy ions are now being generally used.

Response to high LET radiation 12C (LET, 295 keV/microm) in M5 cells, a radio resistant cell strain derived from Chinese hamster V79 cells

PURPOSE

To study the effects of 12C-beam of 295 keV/microm (57.24 MeV) on M5 and Chinese hamster V79 cells by using cytogenetic assays like micronuclei (MN) induction, chromosomal aberrations (CA) and apoptosis. Additionally, the relative survival of these two cell lines was tested by the colony forming ability of the cells, with a view to understanding the mechanism of cellular damages that lead to difference in cell survival.

MATERIALS AND METHODS

Confluent cells were irradiated with 12C-beam at various doses using 15UD Pelletron accelerator. Cell survival was studied by the colony forming ability of cells. MN assay was done by fluorescent staining. Different types of chromosomal aberrations in metaphase cells were scored at 12 h after irradiation. Apoptosis was measured at different post irradiation times as detected by nuclear fragmentation and DNA ladder was prepared after 48 h of incubation.

RESULTS

Dose-dependent decrease in surviving fractions was found in both the cell lines. However, the surviving fractions were higher in M5 cells in comparison to V79 cells when exposed to the same radiation doses. On the other hand, induced MN frequencies, CA frequencies and apoptosis percentages were less in M5 cells than V79 cells. Very good correlations between surviving fractions and induced MN frequencies or induced total CA or induced apoptosis percentages were obtained in this study.

CONCLUSIONS

The cell strain M5 showed relatively more radio-resistance to 12C-beam compared to Chinese hamster V79 cells in this study. As the MN formation, CA and apoptosis induction were less in M5 cells as compared to parental V79 cells, the higher cell survival in the former could possibly be attributed to their better repairing ability leading to higher cell survival.

A nanodosimetry-based linear-quadratic model of cell survival for mixed-LET radiations

A new nanodosimetry-based linear-quadratic (LQ) model of cell survival for mixed-LET radiations has been developed. The new model employs three physical quantities and three biological quantities. The three physical quantities are related to energy depositions at two nanometre scales, 5 nm and 25 nm. The three biological quantities are related to the lesion production and interaction probabilities and the lesion repair rate. The coefficients alpha and beta of the LQ formula (alpha D + beta D(2)) are explicitly expressed in terms of the three physical quantities and the three biological quantities. The new model is shown to be consistent with the previously published cell survival curves of V-79 cells. The advantage of this new model is that it can be conveniently adopted to estimate the iso-effect for radiotherapies that involve ionizing radiation of mixed LET. An example is given to estimate the cell survival fractions for a high-dose-rate mixed neutron and gamma-ray field from a (252)Cf source.

Cell biological basis for combination radiotherapy using heavy-ion beams and high-energy X-rays

We investigated the biological effect of combining carbon-beam and X-ray in vitro. The results showed that when we employed Gray equivalent as the indication of therapeutic dose, the effects could be explained with simple additive way in the treatment plan. This fact provides important information about the combined therapy of carbon-beam and X-ray.

Simultaneous exposure of mammalian cells to heavy ions and X-rays

Crews of space missions are exposed to a mixed radiation field, including sparsely and densely ionizing radiation. To determine the biological effectiveness of mixed high-/low-LET radiation fields, mammalian cells were exposed in vitro simultaneously to X-rays and heavy ions, accelerated at the HIMAC accelerator. X-ray doses ranged from 1 to 11 Gy. At the same time, cells were exposed to either 40Ar (550 MeV/n, 86 keV/micrometers), 28Si (100 MeV/n, 150 keV/micrometers), or 56Fe (115 MeV/n, 442 keV/micrometers) ions. Survival was measured in hamster V79 fibroblasts. Structural aberrations in chromosome 2 were measured by chemical-induced premature chromosome condensation combined with fluorescence in situ hybridization in isolated human lymphocytes. For argon and silicon experiments, measured damage in the mixed radiation field was consistent with the value expected using an additive function for low- and high-LET separated data. A small deviation from a simple additive function is observed with very high-LET iron ions combined to X-rays.

Deletion-pattern analysis of alpha-particle and X-ray induced mutations at the HPRT locus of V79 Chinese hamster cells

To investigate the mutagenic mechanisms of low-energy alpha particles V79 Chinese hamster cells were irradiated with 241Am-alpha particles (mean LET of 112 keV/micron). Parallel experiments were performed using 300 kV X-rays. Cell inactivation and mutation induction cross sections were measured. At approximately 20%--survival level, DNA deletions were analysed at the HPRT locus by multiplex-PCR-analysis of all nine exons of 47 alpha-irradiated and 36 background mutants. 92 HPRT- mutants isolated after 300 kV-X-irradiation were analysed similarly for comparison, along with 15 corresponding background mutants. The resulting mutant deletion-pattern distributions were corrected for background mutations. alpha Particles induced a larger fraction of deletions than X-rays. Furthermore, non-contiguous partial deletions were present among the alpha-induced mutants, a type not found after X-irradiation.

The contribution of secondary heavy particles to the absorbed dose from high-energy photon beams

High-energy photon radiotherapy sources produce non-negligible numbers of secondary heavy particles (neutrons, protons and alphas) which contribute to the dose in a patient. They also present a radiation protection problem. The contribution of such particles to the absorbed dose in a tissue-like medium and to the equivalent dose is considered in this study experimentally. Track etch detectors were chosen as a basic type of measuring instrument: cellulose nitrate Kodak LR115 and polyallyldiglycol carbonate CR39 were used as detectors. Several methods of directly read datum (track density, track parameter, LET spectrum etc) interpretation have been analysed. It has been ascertained that the contribution of secondary heavy particles represents approximately 0.002 of the photon dose and 0.02 of the equivalent dose, both at 50 MeV as maximum photon energy. The contribution of a 20 MV bremsstrahlung beam is roughly one-fifth of that at 50 MV.

RBE-LET relationships for different types of lethal radiation damage in mammalian cells: comparison with DNA dsb and an interpretation of differences in radiosensitivity

Relative biological effectiveness (RBE), as a function of linear energy transfer (LET), is evaluated for different types of damage contributing to mammalian cell reproductive death. Survival curves are analysed assuming a linear-quadratic dose dependence of lethal lesions. The linear term represents lethal damage due to single particle tracks, the quadratic term represents lethality due to interaction of lesions from independent tracks. RBE-LET relationships of single-track lethal damage, sublethal damage, potentially lethal damage and DNA double-strand breaks (dsb) are compared. Single-track lethal damage is shown to be composed of two components: damage that remains unrepaired in an interval between irradiation and assay, characterized by a very strong dependence on LET, with RBEs up to 20, and potentially lethal damage, which is weakly dependent on LET with RBEs < 3. Potentially lethal damage and sublethal damage depend similarly on LET as DNA dsb. The identification of these different components of damage leads to an interpretation of differences in radiosensitivity and in RBEs among various types of cells.

Mechanisms of cell reproductive death and shapes of radiation dose--survival curves of mammalian cells

A 'paired dsb' mechanism of action for cell reproductive death by ionizing radiations is proposed, which allows interpretation of differences in shapes of survival curves caused by variation of linear energy transfer of the radiation, by the stage in the cell cycle, but cell culture conditions and by sensitizing and protecting compounds. It is based on the analysis of shapes of survival curves in terms of S(D)/S(O) = exp - (a1D + a2D2) and the suggestion that paired dsb in DNA, produced within distances of the order of 10 nm, are efficient in initiating the sequence of events causing cell reproductive death by individual particle tracks. Part of the lethality may result from two dsb's produced by single tracks at larger distances, and this might constitute potentially lethal damage which in favourable conditions can be repaired. Thus, the initial slope of a survival curve is not independent of the repair capacity of a cell, but indeed can be modified by cell conditions. The damage causing the quadratic term in the survival equation may be interpreted as a consequence of two dsb produced by two different ionizing particles, although other interactions cannot be excluded. The suggested mechanism of 'paired dsb' damage is consistent with information concerning the LET dependence of different effects in cells and their constituents.

Analysis of interaction for mixtures of agents using the linear isobole

The linear isobole that is commonly used as a reference for the study of interaction is derived from the interaction of an agent with itself. It is shown that the general use of the linear isobole in the study of the combined effects of mixtures of agents implies interaction between the agents whether the dose-effect curves of the agents are the same or not. It is difficult to generalize the interaction between two doses of the same agent to the interaction between two doses of different agents with different action mechanisms without the use of a mechanistic model. Predictions using non-interaction defined as independent action are generally different from those using linear isobole. A simple mechanistic framework based on the concept of common intermediate lesions is introduced in this paper to relate these two methods used for the analysis of synergism and antagonism. In this framework of lesion development, two agents that have no common intermediate lesion in their action will be non-interactive (referred to as independent action). When the two agents share a common intermediate, it is shown that the combined effect will follow the linear isobole (referred to as common action). This simple framework of analysis is applicable to the general study of interaction between two agents with different types of dose-effect curves.

Cell-cycle radiation response: role of intracellular factors

We have been studying variations of radiosensitivity and endogenous cellular factors during the course of progression through the human and hamster cell cycle. After exposure to low-LET radiations, the most radiosensitive cell stages are mitosis and the G1/S interface. The increased activity of a specific antioxidant enzyme such as superoxide dismutase in G1-phase, and the variations of endogenous thiols during cell division are thought to be intracellular factors of importance to the radiation survival response. These factors may contribute to modifying the age-dependent yield of lesions or more likely, to the efficiency of the repair processes. These molecular factors have been implicated in our cellular measurements of the larger values for the radiobiological oxygen effect late in the cycle compared to earlier cell ages. Low-LET radiation also delays progression through S phase which may allow more time for repair and hence contribute to radioresistance in late-S-phase. The cytoplasmic and intranuclear milieu of the cell appears to have less significant effects on lesions produced by high-LET radiation compared to those made by low-LET radiation. High-LET radiation fails to slow progression through S phase, and there is much less repair of lesions evident at all cell ages; however, high-LET particles cause a more profound block in G2 phase than that observed after low-LET radiation. Hazards posed by the interaction of damage from sequential doses of radiations of different qualities have been evaluated and are shown to lead to a cell-cycle-dependent enhancement of radiobiological effects. A summary comparison of various cell-cycle-dependent endpoints measured with low- or high-LET radiations is given and includes a discussion of the possible additional effects introduced by microgravity.

The combined effects of mixtures of ionizing radiations

Ionizing radiation is a special group of toxic agents whose general interaction can be calculated. This was demonstrated using a radiation interaction model previously published. In this paper, this model is refined and mathematically reformulated using a unified set of assumptions. It postulates the existence of a common intermediate lesion and the relative action of lesions before, at and after this common stage. General quantitative dose-effect relationships of mixed radiations can be derived from the dose-effect relationships of the components in the mixture.

The interaction of radiations of different LET

Experimental data for mixed radiation of different LET are analysed using an isobologram approach and are shown to be consistent with the special case of 'zero interaction' in a general formulation of the effect of a combination of agents proposed by Berenbaum. This zero interaction is defined in the same sense as interaction between the lesions produced by the same radiation. This makes analysis and predictions of mixed radiation results particularly straightforward. The zero-interaction formula can be mathematically derived from a simple model of mixed radiation called the lesion additivity model using the assumption that different early lesions produced in the radiation mixture can gradually develop into an indistinguishable intermediate state and hence become additive to produce the same end point. The model is shown to be applicable to irradiation under both hypoxic and oxygenated conditions and hence is extended to the use of hypoxic cell sensitisers.

Repair of potentially lethal damage in yeast induced by fast neutrons

Survival curves of 3 diploid (D7) yeast strains: one wild-type, one deficient in excision of pyrimidine dimers (UV-sensitive) and one blocked in DNA double-strand-break repair (X-ray-sensitive), were compared after irradiation with cyclotron-produced fast neutrons. It was observed that both the UV-sensitive (rad3/rad3) and the X-ray-sensitive (rad52/rad52) mutants were more sensitive to neutrons than the wild-type. The role of DNA double-strand-breaks in neutron-induced cell death was further studied by comparing the relative sensitivity of the rad52/rad52 mutant to gamma-rays and fast neutrons. A comparison of the dose modification factors revealed that the deficiency in DNA double-strand-break repair did not make the yeast cells more sensitive to neutrons than to photons, which suggests that lesions of a different type may also be produced by neutrons. Survival curves obtained upon immediate plating and after delayed plating of neutron-irradiated cells showed that all 3 yeast strains were efficient in liquid holding recovery. The role of different repair pathways in cellular recovery from neutron-induced lethal damage is discussed.

The interaction between X-rays and 3 MeV neutrons in the skin of the mouse foot

Mouse feet were irradiated with mixtures of 3 MeV neutrons and 140 kVp X-rays given simultaneously or within 24 hours of each other. The effects of different treatments were contrasted by comparing the doses required to give equal skin reactions. Irradiation was given as 1, 2, 4 or 8 equal fractions, in order to assess r.b.e. and the shapes of the underlying dose-response curves for mixed beams over a wide range of dose per fraction. All dose-effect curves were well fitted by a linear-quadratic (alpha, beta) model. For X-rays and neutrons given simultaneously, the linear coefficient (alpha) decreased by a factor of 4.80 while the quadratic coefficient (beta) increased by a factor of only 1.44 when the proton contamination in the beam increased from 11 to 100 per cent, with alpha/beta changing from 95.0 to 13.8. The data from simultaneous X-ray and neutron irradiation were consistent with full interaction of those effects from the two radiations which give rise to the total quadratic component of effect. When the two radiations are separated by up to 24 h, this interaction decreases but does not entirely disappear.

The effect of sequential irradiation with X-rays and fast neutrons on the survival of V79 Chinese hamster cells

V79 Chinese hamster cells have been exposed to X-rays or fast neutrons or to the two radiations given sequentially. Cells exposed to a priming dose of X-rays and then exposed immediately to a series of neutron doses regard the X-ray dose as equivalent to a neutron dose giving the same surviving fraction (iso-effective). If the cells are exposed to a neutron dose followed by X-rays the resulting survival is higher than would be obtained if the primary dose had been an iso-effective X-ray dose. However, it is lower than would be expected if the two radiations acted independently. The results imply that there is interaction between the damage caused by X-rays and fast neutrons. If the two radiations are given 3 hours apart they act independently.

Unique radiobiological aspects of high-LET radiation

Since the beg inning of manned space flight the potentially unique radiobiological properties of the heavy ions of the cosmic radiation had been, apart from possible interactions of radiation effects with biological effects of weightlessness, of major concern with respect to the assessment of radiation hazards in manned space flight. Radiobiological findings obtained from space flight experiments and ground based experiments with densely ionizing radiation are discussed, which suggest qualitative differences between the radiobiological mechanisms of sparsely ionizing and densely ionizing radiation. These findings comprise the observation of a long lateral range of radiobiological effectiveness around tracks of single heavy ions, the observation of micro lesions induced in biological targets by the penetration of heavy ions, the nonadditivity of radiobiological effects from sparsely and densely ionizing radiation, the different kinetics for the expression of late effects induced by sparsely or densely ionizing radiation, and the observation of a reversed dose rate effect for early and late effects induced by densely ionizing radiation. These findings bear on the radiation protection standards to be installed for a general public in manned space flight and on the design of experiments, which intend to contribute to their specification.