Modelling radiation fields of ion beams in tissue-like materials

Fast nuclei are ionizing radiation which can cause deleterious effects to irradiated cells. The modelling of the interactions of such ions with matter and the related effects are very important to physics, radiobiology, medicine and space science and technology. A powerful method to study the interactions of ionizing radiation with biological systems was developed in the field of microdosimetry. Microdosimetry spectra characterize the energy deposition to objects of cellular size, i.e., a few micrometers. In the present thesis the interaction of ions with tissue-like media was investigated using the Monte Carlo model for Heavy-Ion Therapy (MCHIT) developed at the Frankfurt Institute for Advanced Studies. MCHIT is a Geant4-based application intended to benchmark the physical models of Geant4 and investigate the physical properties of therapeutic ion beams. We have implemented new features in MCHIT in order to calculate microdosimetric quantities characterizing the radiation fields of accelerated nucleons and nuclei. The results of our Monte Carlo simulations were compared with recent experimental microdosimetry data. In addition to microdosimetry calculations with MCHIT, we also investigated the biological properties of ion beams, e.g. their relative biological effectiveness (RBE), by means of the modified Microdosimetric-Kinetic model (MKM). The MKM uses microdosimetry spectra in describing cell response to radiation. MCHIT+MKM allowed us to study the physical and biological properties of ion beams. The main results of the thesis are as follows: • MCHIT is able to describe the spatial distribution of the physical dose in tissue-like media and microdosimetry spectra for ions with energies relevant to space research and ion-beam cancer therapy. • MCHIT+MKM predicts a reduction of the biological effectiveness of ions propagating in extended medium due to nuclear fragmentation reactions. • We predicted favourable biological dose-depth profiles for monoenergetic helium and lithium beams similar to the one for carbon beam. Well-adjusted biological dose distributions for H, He, C and O with a very flat spreadout Bragg peak (SOBP) plateau were calculated with MCHIT+MKM. • MCHIT+MKM predicts less damage to healthy tissues in the entrance channel for SOBP He and C beams compared to H and O ones. No definitive advantages for oxygen ions with respect to carbon were found.