Danno al DNA da radiazioni cariche e neutre a diverse scale spaziali e temporali: integrazione di simulazioni Monte Carlo con esperimenti in vitro

The purpose of this Ph.D. thesis is to contribute to the investigation on the mechanisms underlying the induction of DNA damage by different radiation qualities, using both in vitro experimental measurements and modelling approaches to quantify and characterize the damage. Specific exposure scenarios are addressed, offering examples of the possible applications that could benefit from results and approaches developed in this study, such as clinical treatments, biodosimetry and space radiation protection. Monte Carlo simulations for radiation transport and tracks were carried out to investigate how radiation quality impacts the biological outcome, in terms of spatial distribution of the energy depositions and of the characteristics of the final DNA lesions, intimately related to the biological effectiveness. The characterization of DNA damage complexity as a function of the LET was addressed, and cluster lesions due to light and heavy charged particles was simulated with the PARTRAC code. The formalism behind the concept of LET was then investigated, starting from the definition of averaged macroscopic LET values, to its microdosimetric equivalent, used to describe the stochastic pattern of energy depositions at the microscopic scale. Moreover, a model for the neutron relative biological effectiveness (RBE) was developed, to trace back the variation of RBE as a function of neutron energy to initial physical events and clustered DNA damage induction. In another chapter the simulation of DNA damage was extended to the reproduction of DNA foci: γ-H2AX foci were considered, as they were demonstrated to be one of the early events following the induction of DSBs and to play a key role in the recruitment of repair factors by the DNA damage repair system. γ-H2AX foci measured at the earliest time-points can therefore be correlated with initial DNA damage. The aim of the modelling was to reproduce the observer's point of view, delivering the read-out of the experimental endpoint: to this aim, a clustering algorithm was developed that starts from initial damage at the nanometre level and takes account of both the physical extension of the genomic region interested by the phosporylation and of the experimental technique chosen to detect foci. This approach was applied to different exposure conditions, with X-rays, neutrons and 12C ions. Data were obtained from experiments on cultures of normal human lung fibroblasts and mouse breast cancer cells for the benchmark of the newly developed modelling approach. At the RARAF facility, Center for Radiological Research, USA, measurements were carried out with X-rays and an analogue of the neutron field generated by the Hiroshima bomb at 1.5 km from the hypocentre of the explosion. 12C ion irradiation was instead performed at the CNAO facility in Pavia, using a treatment plan for 12C ions in a water phantom. Foci induction was measured through immunocytochemistry and conventional and confocal microscopy were compared, to highlight differences in the quantification of foci (and of underlying initial DNA damage) due to technical limitations related to the read-out. Finally, applications to space radiation are presented, motivated by the great interest arisen by future manned missions in deep space. Results from the modelling of DNA clustered damages due to the neutron field expected at the surface of Mars are presented, including estimates on the RBE of Martian neutrons. To conclude, calculations were performed at the tissue/organ level, to evaluate the dose reduction achieved in organs that can display the onset of non-cancer short-term effects thanks to a personal radiation protection device, in form of a water-filled garment, designed and constructed in the framework of the PERSEO project, to mitigate the harmful effects of cosmic radiation during solar particle events. This set of results provides therefore an example of application of modelling radiation transport at the macroscopic scale.