Modellizzazione di sistemi di trattamento per la radioterapia con elettroni ad alte energie: studio del contributo delle particelle secondarie ​

Electrons with kinetic energy up to a few hundred MeV, also called Very High Energy Electrons (VHEE), are currently studied in medical physics as a promising technique for the future of radiation therapy. They combine the potential to reach deep-seated tumours with the ability to be delivered at ultra-high dose rates thanks to high gradient electron laser accelerators or modified linacs as a lower cost alternative. In this context, this thesis proposes to study the two typical setups for clinical electron treatments: a dual scattering foil system adapted for VHEE and a pencil beam scanning design, also comparing the two techniques in terms of the secondary particles generated. In the first part of the work, an analytic model based on Fermi-Eyges multiple Coulomb scattering theory has been implemented for a fast calculation of the fluence profile of the primary electrons at the surface of a water phantom. This algorithm, firstly proposed by Green et al. (1991), allowed the automatic optimisation of the electron energy, the profile flatness and the penumbra of the beam. Moreover, following the extension of the model proposed by Carver et al. (2014), the relative off-axis photon contribution to the total dose has been calculated at the maximum range depth. The interaction of the electron beam with the double scattering setup has been simulated with Monte Carlo numerical methods and different configurations have been analysed. The analytic results have been verified through a comparison with simulations and literature results in the conventional energy range, then the model was extended to higher energies. A discussion on the magnitude of the neutron contribution has been presented and a parameterisation for the calculation of the photon dose has been proposed. The results for two treatment fields of 5x5 cm2 and 10x10 cm2 have been compared and ten initial beam energies between 20 and 200 MeV have been simulated. The homogeneous electron fluence profiles with an optimised penumbra have been obtained from the analytical model with 5% agreement with the Monte Carlo simulated data. It was found that for a beamline with two scatterers and an initial monoenergetic source of 200 MeV electrons, a beam with a mean energy of 150 MeV at the isocentre could be obtained with a treatment field of 10x10 cm2. In this configuration, the photon contribution to the total dose at the maximum range depth was estimated to be around 20% for the 5x5 cm2 field and 25% for 10x10 cm2 field. For the pencil beam scanning technique, the Monte Carlo results indicated a photon dose at the maximum range around 10% of the total dose for a 10x10 cm2 field. The bremsstrahlung contamination arising from the scattering components was significantly higher for the double scattering system respect to the pencil beam scanning technique. The provided tools and the simulated data lays the foundation for future radiation protection studies, especially in the context of a possible installation of a VHEE medical accelerator within clinical centres.