Aloufi, KMH; (2016) Neutron Spectroscopy in Proton Therapy. Doctoral thesis , UCL (University College London).

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Abstract

Aim: During proton therapy, neutrons are generated through the interactions of a proton beam with the treatment head and the patient’s body. A minor neutron dose to healthy tissues could be significant because of the high radiation weighting factor of neutrons. The aim of this research was to conduct a Monte Carlo (MC) simulation assessment of the relative neutron dose (neutron equivalent dose/prescribed proton therapy dose) and dose distribution during the proton irradiation at Clatterbridge Hospital. Materials and methods: Due to the required criteria for a neutron detector in a proton therapy room, a prototype neutron detector based on EJ-331 (gadolinium-loaded liquid scintillator) was simulated using Geant4 and GAMOS.4.0.0 MC simulation codes. Then, the detector was constructed, calibrated and tested. Four pulse shape discrimination (PSD) methods were obtained and evaluated: charges ratio, charge to amplitude ratio, amplitude-fall time and fall time-amplitude. The proton beam line at Clatterbridge Hospital was simulated using Geant4 and GAMOS.4.0.0 MC simulation codes. Neutrons and gamma rays were tracked during the proton irradiation and their deposited energies (DEs) were scored in a voxelised water phantom (50 x 100 x 50cm^3). The simulated prototype neutron detector was located 15cm in front of and 30cm below the final collimator of the simulated proton therapy beam line. In addition, measurement was taken using the prototype neutron detector during the proton irradiation at Clatterbridge Hospital. The measurement geometry was adjusted so that it was the same as the MC simulation geometry to allow a comparison with the MC simulation results and to validate the MC results. Results: The measured prototype neutron detector energy resolution was the same as the simulated detector, which was 17% at 477keV (Cs^137 Compton edge). Using a Figure of Merit to evaluate the obtained PSD methods, the best PSD method performance was found to be the charges ratio. Thus, the charges ratio PSD method was applied to the collected data from the measurements at the proton therapy room in Clatterbridge Hospital. A good agreement was found (within 80%) between the measured and the MC results. Hence, the MC simulation of the relative DE distributions from the neutrons and the gamma rays in the voxelised water phantom were validated. The MC simulation results showed that the contribution of gamma rays to the integral equivalent radiation dose was 5.1%. In addition, the contributions of internal and thermal neutrons to the integral equivalent neutron dose were 4.1% and 1.2% respectively. Thus, fast external neutrons are the main source (89.6%) of the secondary radiation dose during proton irradiation at Clatterbridge Hospital. Most of the neutron DE was distributed in and around the target voxel. In contrast, the gamma-ray DE was widely distributed. The relative integral neutron equivalent dose, which was 1.48mSv/Gy, and its distribution, in the patient’s body (i.e. the voxelised water phantom), can be generalised for any prescribed proton therapy dose during proton therapy at Clatterbridge Hospital. Conclusion: Fast external neutrons are the main concern in terms of the additional unwanted secondary radiation dose during proton therapy at the Clatterbridge proton beam. Although the neutron dose was small compared to the prescribed proton therapy dose, it is not negligible and the dose distribution can be used as the basis of the risk estimation from radiation induced secondary cancers.

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