Charged particle interaction with biological materialsmodelling and application to ion beam cancer therapy

Resumen

This thesis presents a theoretical physics study about the interaction of swift charged particles with materials of radiotherapeutic interest, with applications in ion beam cancer therapy. The work focuses on the calculation of probabilities (cross sections) for the electronic interaction (excitation and ionisation) of charged particles (swift ion and electron beams) with condensed materials of interest (including inorganic and organic-biological), and their use in programs for simulating the ion propagation and interaction with biological materials. After a general introduction (chapter 1), the first part of this thesis (chapters 2 to 4) deals with the calculation of the cross sections. The dielectric formalism has been used to obtain basic electronic slowing down quantities, such as stopping power, energy-loss straggling, mean free path, and others, for ion and electron beams. A methodology has been developed to apply the dielectric formalism to also obtain electron production cross sections, including the calculation of energy and angular distributions of secondary electrons. A methodology has been implemented to extend these calculations to arbitrary biological materials, including complex targets such as cortical bone, DNA and DNA components, proteins, or cell compartments. Once the cross sections have been obtained, they have been used to feed the simulation code SEICS for ion beam propagation in condensed matter (chapters 5 and 6). The code has been extended to account for the effects of relativistic protons (relativistic corrections and nuclear fragmentation reactions have been implemented), and then it has been applied to simulate situations of interest in ion beam cancer therapy. A set of experiments of irradiation of micrometric cylindrical targets has been reproduced, to benchmark the performance of SEICS and to assess the stopping power of protons in liquid water (the main constituent of living tissues). Also, the SEICS code has been used to obtain quantities such as depth-dose curves, lateral dose profiles, and related quantities, useful for treatment planning. Finally, analytical techniques have been implemented, in contrast with the simulation techniques (chapter 7). A pencil beam algorithm for fast calculation of depth-dose curves and lateral dose profiles has been programmed, taking advantage of the results obtained with the SEICS code. Also, an anlytical model for ion beam interaction with realistic sub-cellular compartments has been developed. This model is very useful for assessing the energy deposition and number of ionisations produced in the cell nucleus and cytoplasm. Such quantities are very relevant in radiobiology, since the energy deposited in the cell nucleus can be related to cell death, and since other biological effects have been reported regarding energy deposition outside the cell nucleus. All the results presented in this thesis explore different physical processes involved in the mechanisms underlying ion beam cancer therapy.