| Summary: | 碩士 === 長庚大學 === 醫學影像暨放射科學系 === 99 === A proton therapy center is expected to operate in Taiwan about three years later. Microdosimetric study is of particular importance in proton therapy that converts the physical dose into the biological dose. Accordingly, it is imperative to study microdosimetry for proton therapy as a pioneer research in Taiwan. In experimental microdosimetry, the effects of ionizing radiation on biological targets are studied by investigating the statistical distribution of single energy-deposition event at a microscopic level employing a Tissue Equivalent Proportional Counter (TEPC). TEPC is a proportional counter with a cavity and a central anode wire operating with a tissue-equivalent counting gas. But in high dose-rate/fluence radiation fields, such as found in clinical application of linear accelerators or proton therapy facilities, a normal size TEPC cannot be employed because of pile-up of the electronic signals. For the purpose of proton radiotherapy of microdosimetric study, it is desirable to use such a counter which is able to detect and record single energy-deposition event of proton on the chamber. The physical dimensions of a TEPC are an important instrument-design parameter in reducing the effects of dead time and spectrum distortion on account of pulse pile-up. In the past few years, several groups have developed various types of mini-TEPCs. In experience of those literatures, this is usually done by miniaturizing the dimensions of all components of a conventional TEPC. Furthermore, for our design, cylindrical geometry rather than spherical geometry was chosen which results in considerable simplification of the construction of a counter with smaller dimensions. A cylindrical mini-TEPC of 1 mm sensitive volume has been constructed. With such a mini counter, the pulse-height measurements of 30 MeV proton beam have been performed successfully at INER. These results show that promising properties for application of this new type of TEPC, which can be used for the therapeutic proton beam facility of Chang-Gung Memorial Hospital in terms of microdosimetric measurements in the near future.
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