Investigation into the water-equivalence of plastic materials in high-energy clinical proton beams

In the last decades, cancer treatment modalities have been moved towards high technological developments that indicate in the external beam radiotherapy the primacy of delivering a highly conformal radiation dose to the target volume. In this scenario, proton therapy at Paul Scherrer Institute (PSI) occupies a relevant role because it represents the first clinic capable of applying proton radiotherapy with Spot-Scanning technique by means of dedicated beam delivery system mounted over a Gantry, worldwide. In this field, a study focused on determining how novel materials behave when traversed by protons has been carried out. The plastic candidates would be an extremely useful advancement in dosimetry for high energy proton beams, due to the lack of experimental or theoretical investigations on this aspect.The advantages of using plastic materials instead of water are better positioning accuracy, less time-consuming work flow in pre-treatment stages of the quality assurance and easier managing with respect to the reference material (water). Hence, this thesis is aimed to explore the properties of several commercial plastic materials when irradiated by protons. The possibility of using them as substitute of water in patient specific verifications as well as in patient treatments as range compensator, or more in general in dosimetric applications will be investigated. The explored water equivalence is assessed in terms of energy deposition, non-elastic nuclear interactions and scattering.The final goal is to identify the material, the combination of materials or the chemical composition of a new material, which is closely water-like. Some experimental measurements have been performed at the Center for Proton Therapy (CPT) to calculate the Relative Proton Stopping Power (RPSP) for each candidate material. Results are then compared with theoretical expectations. The remaining investigations regarding fluence reduction and multiple Coulomb scattering have been explored on a theoretical level. In conclusion, the analysis showed that a three-layers composed material is the best configuration to minimize the total discrepancy from water. The three layers are Polyethylene (PE), Plexiglass (PMMA) and Tecason PMT XRO. In this way, the energy deposition and inelastic nuclear interaction equivalences with water are always satisfied over the restrict clinical range (70 MeV - 200 MeV) whereas the scattering is estimated to be -40% with respect to water.