| Summary: | 碩士 === 元培科學技術學院 === 影像醫學研究所 === 93 === The photon beam produced by high energy linear accelerator used regularly in the radiotherapy. As the photon beam passes through tungsten target and flattening filter and other materials in the path, the photonuclear reactions might happen. Energy range of medical linear accelerator in clinical is from 6 to 18 MV. Effects of neutron contamination produced by high energy photons, therefore, become the high-light in photon therapy. The goal of this research is to estimate thermal neutron dose distribution in phantom and in room space as performing high energy photon therapy.
By the 3-plane placements of paired TLDs (TLD-600 and TLD-700) in the space of treatment room and the tool of extrapolation code, the special thermal neutron dose distributions of the treatment room when performed a single clinical irradiation were estimated. Phantom thermal neutron dose distributions were estimated by means of placing paired TLDs in the target and lung tissue in Rando phantom when delivering 15MV high energy photon radiotherapy. Thermal neutron dose can be figured out by the factor of flux-to-dose.
The range of the special thermal neutron dose distribution in the treatment room is 0.5~14μGy when executing high energy radiotherapy single time with linear accelerator output of 200 MU. The average and maximum thermal neutron doses in Rando phantom in the irradiated conditions of 0° gantry degree and 238MU are 118μGy and 1187μGy, the ratio of maximum thermal neutron dose to maximum photon dose in the target is 0.069 %. The average and maximum thermal neutron doses in Rando phantom in the irradiated conditions of 0°、90°、180°、270° gantry degree and total prescript output: 244MU are 89μGy and 603μGy, the ratio of maximum thermal neutron dose to maximum photon dose in the target is 0.035 %.
Paired TLD techniques were introduced in this research to estimate the dose distribution of thermal neutron and the dose ratio between thermal neutron and photon. Results of this research can be the reference in the clinical photon radiotherapy.
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