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PhD_thesis_LVerpoest.pdf
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- (en) Advanced radiotherapy modalities such as proton, carbon and helium ion therapy may offer advantages compared to conventional photon therapy due to their characteristic Bragg peak, which allows most of the dose to be deposited at a well-defined depth. In addition, light ion therapy may benefit from an increased biological effectiveness. Accurate dosimetry is therefore essential to ensure safe and effective treatment delivery. In clinical reference dosimetry, absorbed dose to water is typically measured using ionization chambers calibrated in a reference beam quality and corrected for differences in radiation quality using a beam quality correction factor, kQ. While reference dosimetry for high-energy photon beams is well established, uncertainties associated with absorbed dose measurements remain larger for proton, carbon and helium ion beams, mainly due to the limited availability of experimentally determined kQ-factors. The aim of this thesis was to reduce uncertainties in reference dosimetry for advanced radiotherapy modalities using water calorimetry, the most direct method for determining absorbed dose to water. Beam quality correction factors were experimentally determined for several ionization chamber types in high-energy photon, proton, carbon and helium ion beams. The measurements provide experimental validation of current recommendations in international dosimetry protocols for proton beams and substantially expand the available experimental database of kQ-factors for carbon ion beams. For helium ion beams, this work provides the first experimentally determined kQ-factors. In addition, a direct comparison between the water calorimeter and a graphite calorimeter developed at the National Physical Laboratory was performed in carbon ion beams, providing an important cross-validation between independent calorimetric techniques. The results of this thesis strengthen the experimental foundation of reference dosimetry for particle therapy and contribute to improving the accuracy and reliability of absorbed dose measurements in advanced radiotherapy modalities.
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