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Some open questions on the mechanisms underlying radiation-induced cell death were addressed by a biophysical model, focusing on DNA damage clustering and its consequences. DNA "cluster lesions" (CLs) were assumed to produce independent chromosome fragments that, if created within a micrometer-scale threshold distance (d), can lead to chromosome aberrations following mis-rejoining; in turn, certain aberrations (dicentrics, rings and large deletions) were assumed to lead to clonogenic cell death. The CL yield and d were the only adjustable parameters. The model, implemented as a Monte Carlo code called BIophysical ANalysis of Cell death and chromosome Aberrations (BIANCA), provided simulated survival curves that were directly compared with experimental data on human and hamster cells exposed to photons, protons, α-particles and heavier ions including carbon and iron. d = 5 μm, independent of radiation quality, and CL yields in the range ~2-20 CLs Gy(-1) cell(-1), depending on particle type and energy, led to good agreement between simulations and data. This supports the hypothesis of a pivotal role of DNA cluster damage at sub-micrometric scale, modulated by chromosome fragment mis-rejoining at micrometric scale. To investigate the features of such critical damage, the CL yields were compared with experimental or theoretical yields of DNA fragments of different sizes, focusing on the base-pair scale (related to the so-called local clustering), the kbp scale ("regional clustering") and the Mbp scale, corresponding to chromatin loops. Interestingly, the CL yields showed better agreement with kbp fragments rather than bp fragments or Mbp fragments; this suggests that also regional clustering, in addition to other clustering levels, may play an important role, possibly due to its relationship with nucleosome organization in the chromatin fiber.

A mechanism-based, two-parameter biophysical model of cell killing was developed with the aim of elucidating the mechanisms underlying radiation-induced cell death and predicting cell killing by different radiation types, including protons and carbon ions at energies and doses of interest for cancer therapy. The model assumed that certain chromosome aberrations (dicentrics, rings and large deletions, called "lethal aberrations") lead to clonogenic inactivation, and that aberrations derive from μm-scale misrejoining of chromatin fragments, which in turn are produced by "dirty" double-strand breaks called "cluster lesions" (CLs). The average numbers of CLs per Gy per cell were left as a semi-free parameter and the threshold distance for chromatin-fragment rejoining was defined the second parameter. The model was "translated" into Monte Carlo code and provided simulated survival curves, which were compared with survival data on V79 cells exposed to protons, carbon ions and X rays. The agreement was good between simulations and survival data and supported the assumptions of the model at least for doses up to a few Gy. Dicentrics, rings and large deletions were found to be lethal not only for AG1522 cells exposed to X rays, as already reported by others, but also for V79 cells exposed to protons and carbon ions of different energies. Furthermore, the derived CL yields suggest that the critical DNA lesions leading to clonogenic inactivation are more complex than "clean" DSBs. After initial validation, the model was applied to characterize the particle and LET dependence of proton and carbon cell killing. Consistent with the proton data, the predicted fraction of inactivated cells after 2 Gy protons was 40-50% below 7.7 keV/μm, increased by a factor ∼1.6 between 7.7-30.5 keV/μm, and decreased by a factor ∼1.1 between 30.5-34.6 keV/μm. These LET values correspond to proton energies below a few MeV, which are always present in the distal region of hadron therapy spread-out Bragg peaks (SOBP). Consistent with the carbon data, the predicted fraction of inactivated cells after 2 Gy carbon was 40-50% between 13.7-32.4 keV/μm, it increased by a factor ∼1.7 between 32.4-153.5 keV/μm, and decreased by a factor ∼1.1 between 153.5-339.1 keV/μm. Finally, we applied the model to predict cell death at different depths along a carbon SOBP used for preclinical experiments at HIMAC in Chiba, Japan. The predicted fraction of inactivated cells was found to be roughly constant (less than 10%) along the SOBP, suggesting that this approach may be applied to predict cell killing of therapeutic carbon beams and that, more generally, dicentrics, rings and deletions at the first mitosis may be regarded as a biological dose for these beams. This study advanced our understanding of the mechanisms of radiation-induced cell death and characterized the particle and LET dependence of proton and carbon cell killing along a carbon SOBP. The model does not use RBE values, which can be a source of uncertainty. More generally, this model is a mechanism-based tool that in minutes can predict cell inactivation by protons or carbon ions of a given energy and dose, based on an experimental photon curve and in principle, a single (experimental) survival point for the considered ion type and energy.

In parallel to boron measurements and animal studies, investigations on radiation-induced cell death are also in progress in Pavia, with the aim of better characterisation of the effects of a BNCT treatment down to the cellular level. Such studies are being carried out not only experimentally but also theoretically, based on a mechanistic model and a Monte Carlo code. Such model assumes that: (1) only clustered DNA strand breaks can lead to chromosome aberrations; (2) only chromosome fragments within a certain threshold distance can undergo misrejoining; (3) the so-called "lethal aberrations" (dicentrics, rings and large deletions) lead to cell death. After applying the model to normal cells exposed to monochromatic fields of different radiation types, the irradiation section of the code was purposely extended to mimic the cell exposure to a mixed radiation field produced by the (10)B(n,α) (7)Li reaction, which gives rise to alpha particles and Li ions of short range and high biological effectiveness, and by the (14)N(n,p)(14)C reaction, which produces 0.58 MeV protons. Very good agreement between model predictions and literature data was found for human and animal cells exposed to X- or gamma-rays, protons and alpha particles, thus allowing to validate the model for cell death induced by monochromatic radiation fields. The model predictions showed good agreement also with experimental data obtained by our group exposing DHD cells to thermal neutrons in the TRIGA Mark II reactor of the University of Pavia; this allowed to validate the model also for a BNCT exposure scenario, providing a useful predictive tool to bridge the gap between irradiation and cell death., (Copyright © 2011 Elsevier Ltd. All rights reserved.)

The role played by DNA cluster damage and chromosome aberrations in radiation-induced cell killing was investigated, assuming that certain chromosome aberrations (dicentrics, rings and large deletions,'lethal aberrations') lead to clonogenic inactivation and that chromosome aberrations are due to micrometre-scale rejoining of chromosome fragments derived from DNA cluster lesions (CLs). The CL yield and the threshold distance governing fragment rejoining were left as model parameters. The model, implemented as a Monte Carlo code called BIANCA (BIophysical ANalysis of Cell death and chromosome Aberrations), provided simulated survival curves that were compared with survival data on AG1522 and V79 cells exposed to different radiation types, including heavy ions. The agreement between simulation outcomes and experimental data suggests that lethal aberrations are likely to play an important role in cell killing not only for AG1522 cells exposed to X rays, as already reported by others, but alsofor other radiation types and other cells. Furthermore, the results are consistent with the hypothesis that the critical DNA lesions leading to cell death and chromosome aberrations are double-strand break clusters ( possibly involving the ∼1000-10 000 bp scale) and that the effects of such clusters are modulated by micrometre-scale proximity effects during DNA damage processing. [ABSTRACT FROM AUTHOR]

Cell death is a crucial endpoint in radiation-induced biological damage: on one side, cell death is a reference endpoint to characterise the action of radiation in biological targets; on the other side, any cancer therapy aims to kill tumour cells. Starting from Lea's target theory, many models have been proposed to interpret radiation-induced cell killing; after briefly discussing some of these models, in this paper, a mechanistic approach based on an experimentally observed link between chromosome aberrations and cell death was presented. More specifically, a model and a Monte Carlo code originally developed for chromosome aberrations were extended to simulate radiation-induced cell death applying an experimentally observed one-to-one relationship between the average number of ‘lethal aberrations’ (dicentrics, rings and deletions) per cell and –ln S, S being the fraction of surviving cells. Although such observation was related to X rays, in the present work, the approach was also applied to protons and alpha particles. A good agreement between simulation outcomes and literature data provided a model validation for different radiation types. The same approach was then successfully applied to simulate the survival of cells enriched with boron and irradiated with thermal neutrons at the Triga Mark II reactor in Pavia, to mimic a typical treatment for boron neutron capture therapy. [ABSTRACT FROM PUBLISHER]