Ferreira De Almeida Lourenco, AM; (2017) Water-equivalence of phantom materials in proton and carbon-ion dosimetry. Doctoral thesis , UCL (University College London).

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Abstract

The growing interest in light-ion therapy in recent decades has led to a need for accurate dosimetry. At present, a calibration service based on a primary-standard calorimeter for the direct determination of absorbed dose to water for light-ion beams does not exist. Furthermore, the water-equivalent plastics currently used for light-ion beam dosimetry were designed to be used with high-energy photon and electron beams thus energy- and depth-dependent correction factors need to be applied to measurements of dose to water in plastic phantoms. The first portable graphite calorimeter for light-ion beams was built at the National Physical Laboratory (NPL), UK. In this work, fluence correction factors required to convert absorbed dose to graphite, measured by graphite calorimetry, to absorbed dose to water were determined experimentally and compared with Monte Carlo simulations in proton and carbon-ion beams. Fluence corrections in high-energy light-ion beams could amount to as much as 4% and therefore need to be considered. Novel water-equivalent plastics were specifically designed for light-ion beams and three test compositions were produced and experimentally characterised in proton and carbon-ion beams. Commercially available plastics were also simulated for comparison with the plastics tested experimentally. Experimental data showed that each of the novel water-equivalent plastics showed measurements of dose similar to water to within 1\% across all depths. Monte Carlo simulations showed that one of the novel plastics had superior water-equivalence to commercially available plastics in carbon-ion beams, with a maximum fluence correction of 0.5%. The accuracy of particle transport in the FLUKA Monte Carlo code for proton beams was assessed by performing a Fano cavity test. FLUKA was found to pass this test to within 0.1%. Ion-chamber perturbation factors were also computed for the PTW 34070 Bragg peak chamber typically used in clinical proton beams.

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