Treatment of low-grade meningiomas with protons and helium ions

Particle therapy for cancer patient treatments has gained in popularity over the last decades. The physical characteristics of ion beams over conventional radiotherapy with photons, mainly the inverted depth dose profile, with sharp longitudinal and lateral dose gradient, make ion therapy an interesting modality for treatment of deep-seated tumors near critical structures. Nowadays, only protons and carbon ions are used in clinical routine. The introduction of other ions species in clinical practice is foreseen as potential asset for improving clinical outcomes. Helium ions, with physical and biological properties in between protons and carbon ions, have been pointed out as potential candidates. At the Heidelberg ion beam therapy center (HIT), helium and oxygen ions, which are currently only used for experiments, are foreseen to be employed clinically in the near future in addition to the established protons and carbon ions. The introduction of ion species in the clinical practice needs to be supported by a thorough assessment of their dosimetric and biological properties, and subsequently by treatment planning comparisons with clinically used proton and carbon ion beams. This thesis aimed at comparing the treatment of low-grade meningiomas with protons and helium ions. To achieve this goal, an extensive study of the basic dosimetric features of the different ions available at the HIT was performed in the entire therapeutic energy range, comparing depth dose distributions and lateral profiles in water. Subsequently, thorough comparisons and validations of the Monte Carlo (MC) code FLUKA for helium ion beams was conducted against in-depth dosimetric measurements, after an experimentally-driven tuning of the less known initial beam characteristics in vacuum. After integration of the detailed beamline model into a simulation framework with a new original phase space approach, a dosimetric verification of the FLUKA-based Monte Carlo Treatment Planning tool (MCTP) for spread-out Bragg peaks of different sizes and at different depths in water was performed for all ion species available at HIT. Using this engine coupled to a validated biological model, plan comparisons between protons and helium ions were investigated for meningioma patient treatments. The experimental results of the dosimetric characterization indicate that helium ions could afford a more conformal treatment, due to a smaller lateral scattering and longitudinal straggling than with protons, leading to better lateral and distal fall-off, as well as a lower fragmentation tail compared to carbon and oxygen ions. Overall, good agreements were found between MC FLUKA predictions and measurements of helium ions, confirming the suitability of the code for beam modeling in preparation of clinical establishment at HIT. Subsequently, the MCTP tool was validated against dosimetric measurements for challenging targets, to provide a solid research engine for treatment planning comparisons at HIT, supporting the usage of multiple state-of-the-art models of relative biological effectiveness. The final results of the thesis for treatment plan comparisons under different treatment conditions indicate than helium ions could be an alternative to proton therapy due to their favorable physical and biological characteristics, and thus could provide promising treatment for low-grade meningioma in the head. Future treatment planning studies and biological investigations with helium ions are foreseen to proceed toward clinical implementation.