Inactivation of individual mammalian cells by single alpha-particles

Biophysical modeling of low-energy ion irradiations with NanOx

BACKGROUND

Targeted radiotherapies with low-energy ions show interesting possibilities for the selective irradiation of tumor cells, a strategy particularly appropriate for the treatment of disseminated cancer. Two promising examples are boron neutron capture therapy (BNCT) and targeted radionuclide therapy with α $\alpha$ -particle emitters (TAT). The successful clinical translation of these radiotherapies requires the implementation of accurate radiation dosimetry approaches able to take into account the impact on treatments of the biological effectiveness of ions and the heterogeneity in the therapeutic agent distribution inside the tumor cells. To this end, biophysical models can be applied to translate the interactions of radiations with matter into biological endpoints, such as cell survival.

PURPOSE

The NanOx model was initially developed for predicting the cell survival fractions resulting from irradiations with the high-energy ion beams encountered in hadrontherapy. We present in this work a new implementation of the model that extends its application to irradiations with low-energy ions, as the ones found in TAT and BNCT.

METHODS

The NanOx model was adapted to consider the energy loss of primary ions within the sensitive volume (i.e., the cell nucleus). Additional assumptions were introduced to simplify the practical implementation of the model and reduce computation time. In particular, for low-energy ions the narrow-track approximation allowed to neglect the energy deposited by secondary electrons outside the sensitive volume, increasing significantly the performance of simulations. Calculations were performed to compare the original hadrontherapy implementation of the NanOx model with the present one in terms of the inactivation cross sections of human salivary gland cells as a function of the kinetic energy of incident α $\alpha$ -particles.

RESULTS

The predictions of the previous and current versions of NanOx agreed for incident energies higher than 1 MeV/n. For lower energies, the new NanOx implementation predicted a decrease in the inactivation cross sections that depended on the length of the sensitive volume.

CONCLUSIONS

We reported in this work an extension of the NanOx biophysical model to consider irradiations with low-energy ions, such as the ones found in TAT and BNCT. The excellent agreement observed at intermediate and high energies between the original hadrontherapy implementation and the present one showed that NanOx offers a consistent, self-integrated framework for describing the biological effects induced by ion irradiations. Future work will focus on the application of the latest version of NanOx to cases closer to the clinical setting.

Multi-scaled Monte Carlo calculation for radon-induced cellular damage in the bronchial airway epithelium

Radon is a leading cause of lung cancer in indoor public and mining workers. Inhaled radon progeny releases alpha particles, which can damage cells in the airway epithelium. The extent and complexity of cellular damage vary depending on the alpha particle's kinetic energy and cell characteristics. We developed a framework to quantitate the cellular damage on the nanometer and micrometer scales at different intensities of exposure to radon progenies Po-218 and Po-214. Energy depositions along the tracks of alpha particles that were slowing down were simulated on a nanometer scale using the Monte Carlo code Geant4-DNA. The nano-scaled track histories in a 5 μm radius and 1 μm-thick cylindrical volume were integrated into the tracking scheme of alpha trajectories in a micron-scale bronchial epithelium segment in the user-written SNU-CDS program. Damage distribution in cellular DNA was estimated for six cell types in the epithelium. Deep-sited cell nuclei in the epithelium would have less chance of being hit, but DNA damage from a single hit would be more serious, because low-energy alpha particles of high LET would hit the nuclei. The greater damage in deep-sited nuclei was due to the 7.69 MeV alpha particles emitted from Po-214. From daily work under 1 WL of radon concentration, basal cells would respond with the highest portion of complex DSBs among the suspected progenitor cells in the most exposed regions of the lung epithelium.

Determination of the Effective Local Lethal Function for the NanOx Model

NanOx is a biophysical model recently developed in the context of hadrontherapy to predict the cell survival probability from ionizing radiation. It postulates that this may be factorized into two independent terms describing the cell response to two classes of biological events that occur in the sequence of an irradiation: the local lethal events that occur at nanometric scale and can by themselves induce cell death, and the non-local lethal events that lead to cell death by an effect of accumulation and/or interaction at a larger scale. Here we address how local lethal events are modeled in terms of the inactivation of undifferentiated nanometric targets via an "effective local lethal function F", which characterizes the response of each cell line to the spectra of "restricted specific energy". F is initially determined as a linear combination of basis functions. Then, a parametric expression is used to reproduce the function's main features, a threshold and a saturation, while at the same time reducing the number of free parameters. This strategy was applied to three cell lines in response to ions of different type and energy, which allows for benchmarking of the α(LET) curves predicted with both effective local lethal functions against the experimental data.

Single α-particle irradiation permits real-time visualization of RNF8 accumulation at DNA damaged sites

As well as being a significant source of environmental radiation exposure, α-particles are increasingly considered for use in targeted radiation therapy. A better understanding of α-particle induced damage at the DNA scale can be achieved by following their tracks in real-time in targeted living cells. Focused α-particle microbeams can facilitate this but, due to their low energy (up to a few MeV) and limited range, α-particles detection, delivery, and follow-up observations of radiation-induced damage remain difficult. In this study, we developed a thin Boron-doped Nano-Crystalline Diamond membrane that allows reliable single α-particles detection and single cell irradiation with negligible beam scattering. The radiation-induced responses of single 3 MeV α-particles delivered with focused microbeam are visualized in situ over thirty minutes after irradiation by the accumulation of the GFP-tagged RNF8 protein at DNA damaged sites.

Evaluation of cetuximab as a candidate for targeted α-particle radiation therapy of HER1-positive disseminated intraperitoneal disease

Although the epidermal growth factor receptor (EGFR), also known as HER1, has been studied for over a decade, it continues to be a molecule of great interest and focus of investigators for development of targeted therapies. The marketed monoclonal antibody cetuximab binds to HER1, and thus might serve as the basis for creation of imaging or therapies that target this receptor. The potential of cetuximab as a vehicle for the delivery of α-particle radiation was investigated in an intraperitoneal tumor mouse model. The effective working dose of 10 μCi of (212)Pb-cetuximab was determined from a dose (10-50 μCi) escalation study. Toxicity, as indicated by the lack of animal weight loss, was not evident at the 10 μCi dose of (212)Pb-cetuximab. A subsequent study demonstrated (212)Pb-cetuximab had a therapeutic efficacy similar to that of (212)Pb-trastuzumab (p = 0.588). Gemcitabine given 24 h prior to (212)Pb-cetuximab increased the median survival from 174 d to 283 d, but carboplatin suppressed the effectiveness of (212)Pb-cetuximab. Notably, concurrent treatment of tumor-bearing mice with (212)Pb-labeled cetuximab and trastuzumab provided therapeutic benefit that was greater than either antibody alone. In conclusion, cetuximab proved to be an effective vehicle for targeting HER1-expressing tumors with α-radiation for the treatment of disseminated intraperitoneal disease. These studies provide further evidence that the multimodality therapy regimens may have greater efficacy and benefit in the treatment of cancer patients.

Effects of radiation type and delivery mode on a radioresistant eukaryote Cryptococcus neoformans

INTRODUCTION

Most research on radioresistant fungi, particularly on human pathogens such as Cryptococcus neoformans, involves sparsely-ionizing radiation. Consequently, fungal responses to densely-ionizing radiation, which can be harnessed to treat life-threatening fungal infections, remain incompletely understood.

METHODS

We addressed this issue by quantifying and comparing the effects of densely-ionizing α-particles (delivered either by external beam or by (213)Bi-labeled monoclonal antibodies), and sparsely-ionizing (137)Cs γ-rays, on Cryptococcus neoformans.

RESULTS

The best-fit linear-quadratic parameters for clonogenic survival were the following: α = 0.24 × 10(-2) Gy(-1) for γ-rays and 1.07 × 10(-2) Gy(-1) for external-beam α-particles, and β = 1.44 × 10(-5) Gy(-2) for both radiation types. Fungal cell killing by radiolabeled antibodies was consistent with predictions based on the α-particle dose to the cell nucleus and the linear-quadratic parameters for external-beam α-particles. The estimated RBE (for α-particles vs. γ-rays) at low doses was 4.47 for the initial portion of the α-particle track, and 7.66 for the Bragg peak. Non-radiological antibody effects accounted for up to 23% of cell death.

CONCLUSIONS

These results quantify the degree of C. neoformans resistance to densely-ionizing radiations, and show how this resistance can be overcome with fungus-specific radiolabeled antibodies.

Influence of the nucleus area distribution on the survival fraction after charged particles broad beam irradiation

It is well known that broad beam irradiation with heavy ions leads to variation in the number of hit(s) received by each cell as the distribution of particles follows the Poisson statistics. Although the nucleus area will determine the number of hit(s) received for a given dose, variation amongst its irradiated cell population is generally not considered. In this work, we investigate the effect of the nucleus area's distribution on the survival fraction. More specifically, this work aims to explain the deviation, or tail, which might be observed in the survival fraction at high irradiation doses. For this purpose, the nucleus area distribution was added to the beam Poisson statistics and the Linear-Quadratic model in order to fit the experimental data. As shown in this study, nucleus size variation, and the associated Poisson statistics, can lead to an upward survival trend after broad beam irradiation. The influence of the distribution parameters (mean area and standard deviation) was studied using a normal distribution, along with the Linear-Quadratic model parameters (α and β). Finally, the model proposed here was successfully tested to the survival fraction of LN18 cells irradiated with a 85 keV µm(- 1) carbon ion broad beam for which the distribution in the area of the nucleus had been determined.

The fate of a normal human cell traversed by a single charged particle

The long-term “fate” of normal human cells after single hits of charged particles is one of the oldest unsolved issues in radiation protection and cellular radiobiology. Using a high-precision heavy-ion microbeam we could target normal human fibroblasts with exactly one or five carbon ions and measured the early cytogenetic damage and the late behaviour using single-cell cloning. Around 70% of the first cycle cells presented visible aberrations in mFISH after a single ion traversal, and about 5% of the cells were still able to form colonies. In one third of selected high-proliferative colonies we observed clonal (radiation-induced) aberrations. Terminal differentiation and markers of senescence (PCNA, p16) in the descendants of cells traversed by one carbon ion occurred earlier than in controls, but no evidence of radiation-induced chromosomal instability was found. We conclude that cells surviving single-ion traversal, often carrying clonal chromosome aberrations, undergo accelerated senescence but maintain chromosomal stability.

Investigation of the mechanisms underpinning IL-6 cytokine release in bystander responses: the roles of radiation dose, radiation quality and specific ROS/RNS scavengers

PURPOSE

To investigate the mechanisms regulating the pathways of the bystander transmission in vitro, focusing on the radiation-perturbed signalling (via Interleukine 6, IL-6) of the irradiated cells after exposure to low doses of different radiation types.

MATERIALS AND METHODS

An integrated 'systems radiation biology' approach was adopted. Experimentally the level of the secreted cytokine from human fibroblasts was detected with ELISA (Enzyme-Linked ImmunoSorbent Assay) method and subsequently the data were analyzed and coupled with a phenomenological model based on differential equations to evaluate the single-cell release mechanisms.

RESULTS

The data confirmed the important effect of radiation on the IL-6 pathway, clearly showing a crucial role of the ROS (Reactive Oxygen Species) in transducing the effect of initial radiation exposure and the subsequent long-term release of IL-6. Furthermore, a systematic investigation of radiation dose/radiation quality dependence seems to indicate an increasing efficiency of high LET (Linear Energy Transfer) irradiation in the release of the cytokine. Basic hypotheses were tested, on the correlation between direct radiobiological damage and signal release and on the radiation target for this endpoint (secretion of IL-6).

CONCLUSIONS

The results demonstrate the role of reactive oxygen and nitrogen species in the signaling pathways of IL-6. Furthermore the systems radiation biology approach here adopted, allowed us to test and verify hypotheses on the behavior of the single cell in the release of cytokine, after the exposure to different doses and different qualities of ionizing radiation.

Towards translation of 212Pb as a clinical therapeutic; getting the lead in!

Targeted α-particle therapy offers the potential for more specific tumor cell killing with less damage to surrounding normal tissue than β-emitters because of the combination of short path length (50-80 μm) with the high linear energy transfer (100 keV μm(-1)) of this emission. These physical properties offer the real possibility of targeted (pre-targeted) α-therapy suitable for the elimination of minimal residual or micrometastatic disease. Targeted and pre-targeted radioimmunotherapy (RIT) using α-emitters such as (212)Bi (T(1/2) = 1.01 h) and (212)Pb (T(1/2) = 10.6 h) has demonstrated significant utility in both in vitro and in vivo model systems. (212)Pb, a promising α-particle emitting source, is the longer-lived parent nuclide of (212)Bi, and serves as an in vivo generator of (212)Bi. The radionuclide has been successfully used in RIT and pre-targeted RIT and demonstrated an enhanced therapeutic efficacy in combination with chemotherapeutics, such as gemcitabine and paclitaxel. The following perspective addresses the modes of radionuclide production, radiolabelling and chelation chemistry, as well as the application of (212)Pb to targeted and pre-targeted radiation therapy.

Radiation response of primary human skin fibroblasts and their bystander cells after exposure to counted particles at low and high LET

PURPOSE

To investigate the dependence of bystander effects on linear energy transfer (LET) in the low dose region.

MATERIALS AND METHODS

The single-ion microbeam of the Physikalisch-Technische Bundesanstalt (PTB) was used to irradiate confluent primary human skin fibroblasts. Cells plated on a special irradiation dish were targeted with 10 MeV protons (LET 4.7 keV/microm) and 4.5 MeV a-particles (LET 100 keV/microm). During exposure, the cells were confluent allowing signal transfers through both gap junctions and diffusion.

RESULTS

For 10 MeV protons the clonogenic capability was significantly higher after exposure to 70 protons (0.31 Gy) compared with unirradiated cells. For higher doses the survival curve was exponential. Exposure of only 10% of all nuclei resulted in a similar radiation response in the low dose region. For higher doses up to 2.2 Gy no cell killing was observed. For 4.5 MeV alpha-particles an exponential survival curve was obtained. Irradiation of only 10% of all cell nuclei resulted in an survival curve as had been expected in the absence of any bystander effect.

CONCLUSION

The type and extent of bystander effects turned out to be dependent on the particles' LET and are likely to depend also on the cell line used and the techniques applied.

Applications of particle microbeams in space radiation research

Galactic cosmic radiation is acknowledged as one of the major barriers to human space exploration. In space, astronauts are exposed to charged particles from Z = 1 (H) up to Z = 28 (Ni), but the probability of a hit to a specific single cell in the human body is low. Particle microbeams can deliver single charged particles of different charge and energy to single cells from different tissues, and microbeam studies are therefore very useful for improving current risk estimates for long-term space travel. 2D in vitro cell cultures can be very useful for establishing basic molecular mechanisms, but they are not sufficient to extrapolate risk, given the substantial evidence proving tissue effects are key in determining the response to radiation insult. 3D tissue or animal systems represent a more promising target for space radiobiology using microbeams.

Targeted alpha-therapy: past, present, future?

Monoclonal antibodies have become a viable strategy for the delivery of therapeutic, particle emitting radionuclides specifically to tumor cells to either augment anti-tumor action of the native antibodies or to solely take advantage of their action as targeting vectors. Proper and rational selection of radionuclide and antibody combinations is critical to making radioimmunotherapy (RIT) a standard therapeutic modality due to the fundamental and significant differences in the emission of either alpha- and beta-particles. The alpha-particle has a short path length (50-80 microm) that is characterized by high linear energy transfer (100 keV microm(-1)). Actively targeted alpha-therapy potentially offers a more specific tumor cell killing action with less collateral damage to the surrounding normal tissues than beta-emitters. These properties make targeted alpha-therapy an appropriate therapy to eliminate minimal residual or micrometastatic disease. RIT using alpha-emitters such as (213)Bi, (211)At, (225)Ac, and others has demonstrated significant activity in both in vitro and in vivo model systems. Limited numbers of clinical trials have progressed to demonstrate safety, feasibility, and therapeutic activity of targeted alpha-therapy, despite having to traverse complex obstacles. Further advances may require more potent isotopes, additional sources and more efficient means of isotope production. Refinements in chelation and/or radiolabeling chemistry combined with rational improvements of isotope delivery, targeting vectors, molecular targets, and identification of appropriate clinical applications remain as active areas of research. Ultimately, randomized trials comparing targeted alpha-therapy combined with integration into existing standards of care treatment regimens will determine the clinical utility of this modality.

The average number of alpha-particle hits to the cell nucleus required to eradicate a tumour cell population

Alpha-particle emitters are currently being considered for the treatment of micrometastatic disease. Based on in vitro studies, it has been speculated that only a few alpha-particle hits to the cell nucleus are considered lethal. However, such estimates do not consider the stochastic variations in the number of alpha-particle hits, energy deposited, or in the cell survival process itself. Using a tumour control probability (TCP) model for alpha-particle emitters, we derive an estimate of the average number of hits to the cell nucleus required to provide a high probability of eradicating a tumour cell population. In simulation studies, our results demonstrate that the average number of hits required to achieve a 90% TCP for 10(4) clonogenic cells ranges from 18 to 108. Those cells that have large cell nuclei, high radiosensitivities and alpha-particle emissions occurring primarily in the nuclei tended to require more hits. As the clinical implementation of alpha-particle emitters is considered, this type of analysis may be useful in interpreting clinical results and in designing treatment strategies to achieve a favourable therapeutic outcome.

Number of Fe ion traversals through a cell nucleus for mammalian cell inactivation near the bragg peak

HeLa and CHO-K1 cells were irradiated with Fe ions (1.14 MeV/nucleon) near the Bragg peak to determine how many ion traversals through a cell nucleus are necessary to induce cell inactivation. The ion traversals through a cell nucleus were visualized by immunostaining the phosphorylated histone H2AX (gamma-H2AX), as an indicator of DNA double strand breaks (DSBs), to confirm that DSBs are actually induced along every Fe ion traversal through the nucleus. The survival curves after irradiation with Fe ions decreased exponentially with the ion fluence without a shoulder. The inactivation cross sections calculated from the slope of the survival curves and the standard errors were 96.9 +/- 1.8 and 57.9 +/- 5.4 microm2 for HeLa and CHO-K1 cells, respectively, corresponding to 0.442 and 0.456 of the mean value of each cell nucleus area. Taking the distribution of the cell nucleus area into consideration with an equation proposed by Goodhead et al. (1980), which calculates the average number of lesions per single ion track through the average area of a sensitive organelle (mainly nucleus), these two ratios were converted to 0.705 and 0.659 for HeLa and CHO-K1 cells, respectively. These ratios were less than one, suggesting that the average numbers of lethal hits per cell produced by a single ion traversal were less than one. We thus considered two possible explanations for ion traversals of more than one, necessary for cell inactivation.

Simulation of binary methods for the microdosimetric analysis of cell survival after alpha-particle irradiation: ability to distinguish between different models

Analysis of cell survival after alpha-particle irradiation must account for the distribution in the amounts of energy deposited in each cell nucleus. Microdosimetric computations are usually used to determine these distributions. Irradiation with microbeams and other modern techniques has made these computations unnecessary for certain cell geometries. These techniques allow the survival of individual cells to be correlated with the amount of radiation delivered to individual cell nuclei. However, to maintain the individuality of data generated for each cell, new methods of analysis are required. In this study, we propose the use of binary methods. Each cell is regarded as a Bernoulli trial with a different probability for success (colony formation). Parameter values of the survival model are chosen to maximize the likelihood of the observed outcome. To evaluate this method, simulated data for 500, 5000 and 50,000 cells irradiated by alpha particles are analyzed along with the associated outcome for four different cell survival models. Each survival model has a different dependence on the radius of the cell nucleus. These results indicate that the model that was simulated has the highest likelihood value in all cases. However, the ability to distinguish between competing models is present only for a larger numbers of cells.

The influence of radiation quality on radiation-induced hemolysis and hemoglobin oxidation of human erythrocytes

Human erythrocytes were exposed to gamma-rays and alpha-particles to assess radiation-induced membrane damage and hemoglobin oxidation and denaturation. With all parameters measured, the alpha-particles proved to be less efficient than the gamma-rays. The time-dependence of hemolysis showed also clear differences: with the gamma-rays the process was faster, reaching saturation after 40-90 min (depending on dose), but with the alpha-particles the final level was attained only after about 3-7 h. Hemoglobin oxidation and denaturation could be measured only after gamma-exposure, but they were negligible with the alpha-particles when comparable doses were applied. These results are interpreted by proposing that OH-radicals, whose yields are smaller with densely ionizing radiation, play a crucial role in the induction of the processes for radiation-induced erythrocyte damage.

Low-dose hypersensitivity in Chinese hamster V79 cells targeted with counted protons using a charged-particle microbeam

The Gray Laboratory charged-particle microbeam has been used to assess the clonogenic ability of Chinese hamster V79 cells after irradiation of their nuclei with a precisely defined number of protons with energies of 1.0 and 3.2 MeV. The microbeam uses a 1-microm silica capillary collimator to deliver protons to subcellular targets with high accuracy. The detection system is based on a miniature photomultiplier tube positioned above the cell dish, which detects the photons generated by the passage of the charged particles through an 18-microm-thick scintillator placed below the cells. With this system, a detection efficiency of greater than 99% is achieved. The cells are plated on specially designed dishes (3-microm-thick Mylar base), and the nuclei are identified by fluorescence microscopy. After an incubation period of 3 days, the cells are revisited individually to assess the formation of colonies from the surviving cells. For each energy investigated, the survival curve obtained for the microbeam shows a significant deviation below 1 Gy from a response extrapolated using the LQ model for the survival data above 1 Gy. The data are well fitted by a model that supports the hypothesis that radioresistance is induced by low-dose hypersensitivity. These studies demonstrate the potential of the microbeam for performing studies of the effects of single charged particles on cells in vitro. The hypersensitive responses observed are comparable with those reported by others using different radiations and techniques.

Direct evidence for the spatial correlation between individual particle traversals and localized CDKN1A (p21) response induced by high-LET radiation

The spatial correlation between individual particle traversals and the nuclear CDKN1A (p21) response after high-LET irradiation of human fibroblasts was investigated. The experiments were based on a technique for the retrospective detection of particle traversals by means of nuclear track detectors, which were used as the cell substratum. This technique requires the precise repositioning of a sample at different steps of the experimental procedure and uses a computerized microscope stage control. The precision of the spatial correlation is further enhanced by means of reference marks in the track etch material that are produced by preirradiation of the plates with charged-particle beams at low fluences. The pattern of the CDKN1A foci that were induced by charged-particle traversals at 1 h postirradiation was found to coincide extremely well with the pattern of particle tracks. This represents direct evidence that CDKN1A foci are located at the sites of particle traversals and thus provides further evidence that the radiation-induced accumulation of the CDKN1A protein takes place at the sites of the primary damage.

Induction of a bystander mutagenic effect of alpha particles in mammalian cells

Ever since the discovery of X-rays was made by Röntgen more than a hundred years ago, it has always been accepted that the deleterious effects of ionizing radiation such as mutation and carcinogenesis are attributable mainly to direct damage to DNA. Although evidence based on microdosimetric estimation in support of a bystander effect appears to be consistent, direct proof of such extranuclear/extracellular effects are limited. Using a precision charged particle microbeam, we show here that irradiation of 20% of randomly selected A(L) cells with 20 alpha particles each results in a mutant fraction that is 3-fold higher than expected, assuming no bystander modulation effect. Furthermore, analysis by multiplex PCR shows that the types of mutants induced are significantly different from those of spontaneous origin. Pretreatment of cells with the radical scavenger DMSO had no effect on the mutagenic incidence. In contrast, cells pretreated with a 40 microM dose of lindane, which inhibits cell-cell communication, significantly decreased the mutant yield. The doses of DMSO and lindane used in these experiments are nontoxic and nonmutagenic. We further examined the mutagenic yield when 5-10% of randomly selected cells were irradiated with 20 alpha particles each. Results showed, likewise, a higher mutant yield than expected assuming no bystander effects. Our studies provide clear evidence that irradiated cells can induce a bystander mutagenic response in neighboring cells not directly traversed by alpha particles and that cell-cell communication process play a critical role in mediating the bystander phenomenon.

A new alpha-particle irradiator with absolute dosimetric determination

A new experimental setup for uniform alpha-particle irradiation of cells in vitro is described. The alpha-particle irradiator is based on a radioactive (212)Pb/(212)Bi source. In the experimental setup proposed, cells are grown directly on a polylysine-coated track-etch material that forms the base of custom-made cell dishes. Alpha-particle irradiation is done through the base of the dish. Immediately prior to irradiation, the cell dish is scanned under a microscope, and images of cells with the corresponding coordinates are saved. After irradiation and after the biological end point under study has been determined, the cell dish is etched to develop alpha-particle tracks in the dish base. A microscope image series of alpha-particle track images is obtained by accurately revisiting every original (preirradiation) cell position in the track-etched dish. The number of alpha-particle traversals of each individual cell is scored by mapping images of alpha-particle tracks onto the images of cells recorded prior to irradiation. The uncertainty of the alpha-particle hit determination is 0.9 microm. The procedure described thus presents a method for radiobiological experiments with absolute, rather than statistical, cell dosimetry.

A Monte Carlo calculation of cell inactivation by light ions

This study simulates the exposure of V79 Chinese hamster fibroblasts to low-energy protons, deuterons and alpha-particles in the LET range 10-200 keV/microm. The starting assumption is that the induction of clustered lesions in DNA is a fundamental step for cell inactivation. A non-homogeneous cell population was simulated by a computer program, using as input measured morphological parameters reported in the literature. Variations in the number of traversals through each cell of the population and in the length of the traversal, depending on actual nuclear thickness and position of the traversal, the energy spread of the incident beam, and the change of LET along the tracks were included in the simulation. Microdosimetric spectra were computed and compared with spectra obtained neglecting particle slowing-down and stochastic aspects of cell morphology. Simulated cell survival was estimated under the assumption that surviving cells are those with no clustered DNA lesions or no passages. The main features of experimental RBE versus LET and particle type were reproduced by the simulations. The influence of stochastic aspects of target-cell morphology and of the energy of the incident particles on survival were investigated under different assumptions about the correlation between morphological parameters. Results support the hypothesis of a relevant role of clustered DNA damage in cell killing and point out the importance of target-cell morphology and its variability in beam dosimetry and computer simulations of low-energy particle radiation effects.