| Summary: | 碩士 === 慈濟科技大學 === 放射醫學科學研究所 === 105 === Abstract
Proton therapy has different dose characteristics than photons used in conventional radiation therapy. After a short build-up region, conventional radiation shows an exponentially decreasing energy deposition with increasing depth in tissue. By contrast, protons show an increasing energy deposition with penetration distance leading to a maximum near the end of range of the proton beam, called the Bragg peak. The Bragg peak is too sharp to cover a target of any reasonable size. Incorporating the range modulator wheel, the Bragg peak can be transformed into a uniform depth dose region called the Spread-out Bragg Peak (SOBP). This study investigates the dose characteristics of protons in homogeneous and heterogeneous media using Monte Carlo simulation with code MCNPX. The purpose is to find the optimal modulation materials combination for heterogeneous lungs, and create the SOBP using optimized modulation materials combinations and weights. The validation results demonstrate that MCNPX is capable of simulating the transport of protons in media, obtaining the proton mass stopping power, range and dose results in media. In the heterogeneous lung region, the best modulation materials combination is plexiglass and aluminum. Based on the optimal modulation materials combination, the beam weighting regulation, the flat SOBPs in the water and heterogeneous lungs were successfully derived. Further studies investigating the effects of changing parameters, such as energy of protons, material of phantoms and geometry of structures, on the dose distribution will add to the literature. The mathematical methods of creating SOBP in this study may be applied to related studies.
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