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The fluence correction factor, which accounts for the loss of primary protons and the production of secondary particles due to different non-elastic nuclear interactions at water equivalent depths in different phantom materials compared to water is an important parameter for the dose conversion in clinical proton dosimetry between any phantom material and water. Non-elastic nuclear interactions introduce uncertainties in the standard absorbed dose-to-water in radiotherapy. This thesis is part of an ongoing project at the UK National Physical Laboratory (NPL) focussed on the development of graphite calorimeters for proton dosimetry. The fluence correction factor was investigated to give accurate dose conversions from dose-to-graphite in a graphite phantom to dose-to-water in a water phantom. The fluence correction factors at water equivalent depths have been studied for various dosimetric materials including A-150 tissue equivalent, polymethyl methacrylate (PMMA), aluminium and copper with respect to water and with respect to graphite. The water equivalence of materials such as Plastic Water (PW), Plastic Water Diagnostic Therapy (PWDT) and solid water (WT1) phantoms was evaluated using a 60 MeV proton beam at the Clatterbridge Centre for Oncology. Plastic-water phantoms are widely used in radiotherapy as a substitute for water, in particular for non-reference dosimetry. However, while they are usually made ‘water equivalent’ for a particular beam type, they are not universally water equivalent due to their different elemental composition and associated different proton interaction cross sections (compared to water). Numerous studies of the water equivalence of plastic-water phantoms have been reported for photon and electron beams, but none with clinical proton beams. In the latter, non-elastic nuclear interactions take place which could potentially influence the water equivalence. This thesis evaluates the fluence correction factor at equivalent depths for proton energies of 60 MeV and 200 MeV, with respect to both water and graphite. This work was performed using analytical model calculations (which incorporate the ICRU-49 (1993) stopping power data tables and ICRU-63 (2000) for the total nuclear interaction cross sections); Monte Carlo simulations using the FLUKA 2008. 3 code; and also experimental work at the Clatterbridge Centre for Oncology (CCO) 60 MeV with both modulated and unmodulated proton beams. The analytical calculations for primary protons indicate an increase in the fluence correction at both low and high energies compared to the Monte Carlo simulations. When the secondary charged particle were considered in the calculation, the fluence correction factor with respect to water was in general close to the unity for graphite, A-150, PMMA, aluminium and plastic-water materials in 60 MeV mono-energetic beam. For proton energies of 200 MeV, the fluence correction was found to increase to a the order of a few percent. The experimental finding for modulated and un-modulated 60 MeV protons showed that the fluence correction factor with respect to water is close to unity for graphite and PWDT with an uncertainty of 0. 2% at 1o. The derived fluence correction with respect to graphite was also close to the unity for A-150 and plastic-water materials, however, it was found to increase with depth to approximately 4% and 6% for aluminium and copper respectively (in modulated beam). In general, the experimental results for modulated and unmodulated 60 MeV proton beams show good agreement with the Monte Carlo simulations for modulated and un-modulated beams, yielding small fluence corrections, within the statistical uncertainty. For 200 MeV protons, the Monte Carlo simulations showed that the correction with respect to water increased with penetration depth giving values of up to 4% for graphite and 1. 5% for A-150, PMMA, aluminium, copper and plastic-water materials. The fluence correction with respect to graphite was found to vary with penetration depth and hence it can be concluded that fluence correction factors need to be applied to ensure accurate dosimetry for all of the materials used in the current work with a 200 MeV proton beam.