Repair processes and cell survival

A Unified Dose Response Relationship to Predict High Dose Fractionation Response in the Lung Cancer Stereotactic Body Radiation Therapy

AIM

This study is designed to investigate the superiority and applicability of the model among the linear-quadratic (LQ), linear-quadratic-linear (LQ-L) and universal-survival-curve (USC) models by fitting published radiation cell survival data of lung cancer cell lines.

MATERIALS AND METHOD

The radiation cell survival data for small cell (SC) and non-small cell (NSC) lung cancer cell lines were obtained from published reports, and were used to determine the LQ and cell survival curve parameters, which ultimately were used in the curve fitting of the LQ, LQ-L and USC models.

RESULTS

The results of this study demonstrate that the LQ-L(Dt-mt) model, compared with the LQ and USC models, provides best fit with smooth and gradual transition to the linear portion of the curve at transition dose Dt-mt, where the LQ model loses its validity, and the LQ-L(Dt-2α/β) and USC(Dt-mt) models do not transition smoothly to the linear portion of the survival curve.

CONCLUSION

The LQ-L(Dt-mt) model is able to fit wide variety of cell survival data over a very wide dose range, and retains the strength of the LQ model in the low-dose range.

Inter-comparison of population models for the calculation of radiation dose effects on wildlife

An inter-comparison of five models designed to predict the effect of ionizing radiation on populations of non-human wildlife, performed under the IAEA EMRAS II programme, is presented and discussed. A benchmark scenario 'Population response to chronic irradiation' was developed in which stable generic populations of mice, hare/rabbit, wolf/wild dog and deer were modelled as subjected to chronic low-LET radiation with dose rates of 0-5 × 10(-2) Gy day(-1) in increments of 10(-2) Gy day(-1). The duration of exposure simulations was 5 years. Results are given for the size of each surviving population for each of the applied dose rates at the end of the 1st to 5th years of exposure. Despite the theoretical differences in the modelling approaches, the inter-comparison allowed the identification of a series of common findings. At dose rates of about 10(-2) Gy day(-1) for 5 years, the survival of populations of short-lived species was better than that of long-lived species: significant reduction in large mammals was predicted whilst small mammals survive at 80-100 % of the control. Dose rates in excess of 2 × 10(-2) Gy day(-1) for 5 years produced considerable reduction in all populations. From this study, a potential relationship between higher reproduction rates and lower radiation effects at population level can be hypothesized. The work signals the direction for future investigations to validate and improve the predictive ability of different population dose effects models.

Dual-age-class population model to assess radiation dose effects on non-human biota populations

In the present paper, a two-age-class group, logistic growth model for generic populations of non-human biota is described in order to assess non-stochastic effects of low linear energy-transfer radiation using three endpoints: repairable radiation damage, impairment of reproductive ability and, at higher radiation dose rates, mortality. This model represents mathematically the exchange between two life stages considering fecundity, growth and mortality. Radiation effects are modeled with a built-in self-recovery pool whereupon individuals can repair themselves. In acute effects mode, the repairing pool becomes depleted due to radiation and the model tends to lethality mode. A base calibration of the model's two free parameters is possible assuming that in acute mode 50% of the individuals die on 30 days when a radiation dose equal to the LD(50/30) is applied during that period. The model, which requires 10 species-dependent life-history parameters, was applied to fish and mammals. Its use in the derivation of dose-rate screening values for the protection of non-human biota from the effects of ionizing radiation is demonstrated through several applications. First, results of model testing with radiation effects data for fish populations from the EPIC project show the predictive capability of the model in a practical case. Secondly, the model was further verified with FREDERICA radiation effects data for mice and voles. Then, consolidated predictions for mouse, rabbit, dog and deer were generated for use in a population model comparison made within the IAEA EMRAS II project. Taken together, model predictions suggest that radiation effects are more harmful for larger organisms that generate lower numbers of offspring. For small mammal and fish populations, dose rates that are below 0.02 Gy day(-1) are not fatal; in contrast, for large mammals, chronic exposure at this level is predicted to be harmful. At low exposure rates similar to the ERICA screening dose rate of 2.4 × 10(-4) Gy day(-1), long-term effects on the survivability of populations are negligible, supporting the appropriateness of this value for radiological assessments to wildlife.

Changing paradigms in radiobiology

The last 25 years have seen a major shift in emphasis in the field of radiobiology from a DNA-centric view of how radiation damage occurs to a much more biological view that appreciates the importance of macro-and micro-environments, hierarchical organization, underlying genetics, evolution, adaptation and signaling at all levels from atoms to ecosystems. The new view incorporates concepts of hormesis, nonlinear systems, bioenergy field theory, uncertainty and homeodynamics. While the mechanisms underlying these effects and responses are still far from clear, it is very apparent that their implications are much wider than the field of radiobiology. This reflection discusses the changing views and considers how they are influencing thought in environmental and medical science and systems biology.

An approach to the assessment of risk from chronic radiation to populations of European lobster, Homarus gammarus (L.)

The basic principles underlying a four-discrete age group, logistic, growth model for the European lobster Homarus gammarus are presented and discussed at proof-of-concept level. The model considers reproduction, removal by predation, natural death, fishing, radiation and migration. Non-stochastic effects of chronic low linear energy transfer (LET) radiation are modelled with emphasis on (99)Tc, using three endpoints: repairable radiation damage, impairment of reproductive ability and, at higher dose rates, mortality. An allometric approach for the calculation of LD(50/30) as a function of the mass of each life stage is used in model calibration. The model predicts that at a dose rate of 1 Gy day(-1), lobster population reproduction and survival become severely compromised, leading eventually to population extinction. At 0.01 Gy day(-1), the survival rate of an isolated population is reduced by 10%, mainly through loss of fecundity, comparable to natural migration losses. Fishing is the main ecological stress and only dose rates in the range 0.03-0.1 Gy day(-1) can achieve discernible effects above it. On the balance of radiation and other ecological stresses, a benchmark value of 0.01 Gy day(-1) is proposed for the protection of lobster populations. This value appears consistent with available information on radiation effects in wildlife.

Investigation of non-linear adaptive responses and split dose recovery induced by ionizing radiation in three human epithelial derived cell lines

Two almost completely exclusive fields in radiobiology deal with splitting doses of radiation and comparing the effect to a similar total dose given in one exposure. In radiotherapy, dose "fractionation" is used to "spare" normal tissue and in the low dose field, the adaptive response is well documented as a phenomenon where a small "priming" dose administered before the larger "challenge" dose reduces the effect of the large dose. There have been very few studies where these fields overlap, thus it is not possible to ascertain whether common or distinct mechanisms underlie both phenomena but this is certainly an interesting question and relevant to our understanding of high and low dose radiobiology. This paper presents data for three human cell lines with varying p53 status and radiation responses, treated at a range of times between first and second dose and for 3 different first doses (0.1, 0.5 and 2Gy). The data show that time between doses is critical. Protective (adaptive) effects were seen in each cell line but most prominently in the malignant HT 29 cell line. Surprisingly none of the cell lines showed pronounced split dose recovery. This suggests different mechanisms may underlie the two phenomena.

The linear-quadratic model is an appropriate methodology for determining isoeffective doses at large doses per fraction

The tool most commonly used for quantitative predictions of dose/fractionation dependencies in radiotherapy is the mechanistically based linear-quadratic (LQ) model. The LQ formalism is now almost universally used for calculating radiotherapeutic isoeffect doses for different fractionation/protraction schemes. In summary, the LQ model has the following useful properties for predicting isoeffect doses: (1) it is a mechanistic, biologically based model; (2) it has sufficiently few parameters to be practical; (3) most other mechanistic models of cell killing predict the same fractionation dependencies as does the LQ model; (4) it has well-documented predictive properties for fractionation/dose-rate effects in the laboratory; and (5) it is reasonably well validated, experimentally and theoretically, up to about 10 Gy/fraction and would be reasonable for use up to about 18 Gy per fraction. To date, there is no evidence of problems when the LQ model has been applied in the clinic.

Modelling of effects due to chronic exposure of a fish population to ionizing radiation

A dynamic model was developed for description of radiation effects in an isolated fish population chronically exposed at different dose rates. The induced effects were predicted based on damage created by the radiation, recovery by means of repair mechanisms, and natural growth of the population. Three types of radiation effects (umbrella endpoints) were simulated--decrease of population size, decrease of reproductive capacity, and effects on the morbidity of the population. The influence of ecological interactions on the irradiated fish population was simulated using the combined action of radiation and parasite infestation as an example (ecological interaction "host-parasite"). The model calculations demonstrate that influence of ecological interactions can considerably aggravate the effects of radiation to an exposed population. It was concluded that development of standards for wildlife protection against ionizing radiation requires consideration of possible ecological interactions and to take into account the ecological effects of radiation.

Mathematical simulation of dose-effect relationships for fish eggs exposed chronically to ionizing radiation

A mathematical model which simulates the observed dose-effect relationships for fish eggs exposed to chronic irradiation is presented. The model assumes that the exposed fish eggs may exist in one of the following states: normally developing, reversibly damaged, and lethally damaged. Reversible damages may be recovered by repairing mechanisms which are spent for the repairing processes. The model was applied to describe the observed differences in effects of chronic exposure for quickly (2 weeks) and slowly (up to 20 weeks) developing fish eggs. Calculations were performed for dose rates of chronic irradiation ranging from 10 to 300 mGy/day. Two types of radiation effects were considered-the effect on eggs survival (percentage of survived eggs at time t), and the depletion of the repairing pool (in percentage of its maximal value). The model predictions have been compared with the experimental data from the EPIC database. This comparison showed that the model adequately describes the radiation effects in fish eggs of different species, within a wide range of chronic radiation exposures.

Radiation-induced bystander effects and the DNA paradigm: an "out of field" perspective

Over the past 20 years there has been increasing evidence that cells and the progeny of cells surviving a very low dose of ionizing radiation [micro-mGy] can exhibit a wide range of non-monotonic effects such as adaptive responses, low dose hypersensitivity and other delayed effects. These effects are inconsistent with the expected dose-response, when based on extrapolation of high dose data and cast doubt on the reliability of extrapolating from high dose data to predict low dose effects. Recently the cause of many of these effects has been tentatively ascribed to so-called "bystander effects". These are effects that occur in cells not directly hit by an ionizing track but which are influenced by signals from irradiated cells and are thus highly relevant in situations where the dose is very low. Not all bystander effects may be deleterious although most endpoints measured involve cell damage or death. In this commentary, we consider how these effects impact the historical central dogma of radiobiology and radiation protection, which is that DNA double strand breaks are the primary radiation-induced lesion which can be quantifiably related to received dose and which determine the probability that a cancer will result from a radiation exposure. We explore the low dose issues and the evidence and conclude that in the very low dose region, the primary determinant of radiation exposure outcome is the genetic and epigenetic background of the individual and not solely the dose. What this does is to dissociate dose from effect as a quantitative relationship, but it does not necessarily mean that the effect is ultimately unrelated to DNA damage. The fundamental thesis we present is that at low doses fundamentally different mechanisms underlie radiation action and that at these doses, effect is not quantitatively related to dose.

Actions of radiation on living cells in the "post-bystander" era

Over the past 20 years there has been increasing evidence that cells and the progeny of cells surviving a dose of ionizing radiation can exhibit a wide range of effects inconsistent with the level of dose received. Recently, the cause of these delayed effects has been ascribed to so-called bystander effects, occurring in cells not directly hit by an ionizing track, but which are influenced by signals from irradiated cells. These effects are not necessarily deleterious, although most of the literature deals with adverse delayed effects. What is important to consider is what, if anything, these effects mean for what is still the central dogma of radiobiology and radiation protection, i.e., that DNA double-strand breaks are the primary radiation-induced lesion that can be quantifiably related to received dose, and which determine the probability that a cancer will result from a radiation exposure. In this chapter we review the history of radiation biology which led to the DNA paradigm. We explore the issues and the evidence which are now challenging the view that dose deposition in DNA is all important. We conclude that in the low-dose region, the primary determinant of radiation exposure outcome is the genetic and epigenetic background of the individual and not the dose. This effectively dissociates dose from effect as a quantitative relationship, but it does not necessarily mean that the effect is unrelated to DNA damage somewhere in the system.

Low-dose radiation effects: experimental hematology and the changing paradigm

This review looks at the emerging field of nontargeted radiation effects and their impact on low-dose radiation risk assessment and radiotherapy. It identifies the major role of experimental hematologists and cytogeneticists in changing the old view of radiation action on living things. It also considers the history of radiobiology, seeking to explain why it is only now that we are considering indirect or nontargeted effects of low doses even though the evidence was there, though buried, in the old literature. Effects receiving major attention worldwide now include genomic instability and bystander effects. The impact of these effects, both on radiotherapy used to treat cancer and on radiation induction of cancer, still need to be clarified. Techniques developed by experimental hematologists are central to these efforts and have been instrumental in causing radiobiologists to consider that a paradigm shift is necessary. Throughout, we make a plea to think "outside the box" since the very construction of a framework necessarily limits our thinking and our experimental design.

Queueing models of potentially lethal damage repair in irradiated cells

Some of the ideas arising in queueing theory are applied to describe the repair mechanisms responsible for recovery of cells from potentially lethal radiation damage. Two alternative versions are presented of a queueing model of damage repair after a single dose of irradiation. The first version represents a linear misrepair model, and the second invokes the idea of spontaneous lesion fixation. They are pieced together in the third model, allowing for both mechanisms. The consistency of the proposed models with published experimental data is tested.

Lethal mutations, the survival curve shoulder and split-dose recovery

Our group has shown that many of the progeny of cells which survive irradiation, as determined using a clonogenic assay, die out after 8-30 further cell divisions. Correction of conventional survival curves for this extra component of defective cells (termed lethally mutated cells) reduces or eliminates the 'shoulder' which is characteristic of the radiation response of many mammalian cell lines. Since the size of the shoulder is theoretically and experimentally linked with the extent of split-dose recovery, this paper examines the occurrence of lethal mutations following fractionated irradiation using a variety of experimental conditions. The results show that, when curves are corrected for lethal mutations to give residual survival, the size of the single-dose survival-curve shoulder is indeed reduced but the extent of recovery after split-dose irradiation remains the same or, under some conditions, is actually increased. However, the rate of increase in survival with time between doses is reduced over the first 2 h if the data are corrected for lethal mutations, suggesting that early postirradiation repair may be error prone. When a metabolic inhibitor which depletes cellular ATP was used, the single-dose and split-dose curves corrected for lethal mutations were coincident with each other and with the corrected single-dose control curve, all being exponential and with an extrapolation number of one. It is concluded that the mechanisms leading to the production of the primary survival-curve shoulder are different from those leading to split-dose recovery. The results strongly suggest that a mechanism involving induction of repair/resistance by an initial dose of radiation is involved in split-dose recovery.

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.

Dose-rate effects and the repair of radiation damage

The extent of dose-sparing that occurs in a variety of cell lines and in vivo cell systems as a result of a reduction in dose-rate is reviewed. The emphasis is on the range from around 200 cGy/min down to 5 cGy/min, in which the predominant reason for dose-sparing is the repair of radiation damage. Dose-rate dependence is considered in relation to the Lethal-Potentially Lethal model of cell inactivation, which satisfactorily fits 4 sets of data that we have tested; estimates of half-time for repair varied from 0.07 to 1.4 h. The model shows that in spite of these short half-times, repair will often continue to influence response down to dose-rates below 5 cGy/min. The steepness of the dose-rate dependence varies widely among in vitro cell lines and among mouse normal tissues, indeed the ranges in vitro and in vivo are similar. Haemopoietic tissues are much less spared by a lowering of dose-rate than are other normal tissues. Uncertainties about the rate of reoxygenation preclude similar considerations in experimental tumours in vivo. There is a need for detailed studies of dose-rate dependence in human tumour cell lines, and the present review outlines the basis (including the optimum dose-rate range) for such studies.

Inactivation and mutation of cultured mammalian cells by aluminium characteristic ultrasoft X-rays. II. Dose-responses of Chinese hamster and human diploid cells to aluminium X-rays and radiations of different LET

The induction of inactivation and mutation to thioguanine-resistance of two types of cultured mammalian cells, V79 Chinese hamster and HF19 human diploid, was studied after irradiation with aluminium K characteristic ultrasoft X-rays, helium ion track intersections of different LET, 42 MeV d-Be neutrons, and hard X- or gamma-rays. The form of the dose-response curves was different for the two cell-types, and there was an overall difference in radiosensitivity, the human cells being the more sensitive to all radiations. However, for both inactivation and mutation-induction, the relative responses of both cell-types to these radiations was similar. Aluminium X-rays were considerably more effective than hard X- or gamma-rays and were at least as effective as helium ions of 20-28 keV micron-1, although aluminium X-rays produce tracks of very limited range (less than about 0.07 micron). Single track effects by aluminium X-rays cannot, therefore, extend beyond about 0.07 micron, and the subcellular sites involved in inactivation and mutation cannot be greater than this dimension or else the effectiveness of aluminium X-rays would be similar to that of low-LET radiations. This observation is in contradiction to models of radiation action which require relatively large sensitive sites; for example the 'theory of dual radiation action' requires a site diameter of about 0.4 micron to explain the shape of the dose-response curves for V79 hamster cells.