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題 名 | 原級標準鈷60水中劑量之量測=Ionometric Determination of Absorbed Dose to Water for Cobalt-60 Gamma Rays |
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作 者 | 蕭安成; 陳為立; 林芳仁; 黃文松; 蘇水華; | 書刊名 | 中華放射線醫學雜誌 |
卷 期 | 27:1 2002.02[民91.02] |
頁 次 | 頁9-16 |
分類號 | 414.93 |
關鍵詞 | 吸收劑量; 游離腔; 鈷60; 原級標準; Absorbed dose; Cavity ionization chamber; Co-60; Primary standards; |
語 文 | 中文(Chinese) |
中文摘要 | AAPM TG 51 工作群在1999年發表了專為臨床體外放射治療劑量校驗使用的TG-51 議定書,此議定書中使用的校正因子;(方程式略),必須追溯至原級標準(primary standard)。本研究在核能研究所國家游離輻射標準實驗室(NRSL)進行,以建立鈷60水中劑量的原級標準校驗程序,藉此提昇NRSL本身的校驗能力及對外的服務品質。本研究以石墨材料自行設計製作的平行板式圓盤狀游離腔來作為鈷60水中劑量校驗的原級標準件。其型式規格主要是參考設立在法國的國際度量標準局(The Bureau International des Poids et Measures; BIPM)針對相同目的所發展的游離腔。在意義上,其修正因子的求取可提供比對的參考,並且在測量元件的型式上可以提升至與國際比對的源頭BIPM相似。研究的過程包括:水假體(water phantom)的設計及製作,游離腔及其水密封裝置的設計及製作,游離腔空腔體積的測量,游離腔組裝及其本性能測試,原級游離腔各項修正因子的量測,鈷60空氣克馬(air Kerma)值測定及比對(comparison),水中測量系統組合及防水測試,鈷60水中劑量值測定及比對,劑量測量系統的不確定度分析等。在鈷60空氣克馬值的測定方面,與NRSL的原級標準件比對,其差異在0.5%之內。在水中劑量方面,與NRSL建立的次級標準件做比較,其差異在0.4%之內。在不確定度分析方面,鈷60空氣克馬值及鈷60水中劑量值的組合標準不確定度皆約為0.65%。綜合而言,本研究結果已充分能符合放射治療業界的需求,也符合TG-51議定書所提到簡化醫療界劑量測的步驟及降低潛在人為計算失誤的目的。 |
英文摘要 | For reference dosimetry of clinical external beam radiation therapy, a protocol written by AAPM TG-51 suggests to use ion chamber with absorbed-dose-to-water calibration factor, which is traceable to national primary standards. This study was a collaboration with national standards laboratory (INER) for developing an ionometric dosimetry system for the determination of absorbed dose to water for Co-60 γ rays, which will be regarded as the national primary standard in the future. This system consists of a cubic perspex water phantom and a flat graphite cylindrical ionization chamber. The diameters of the cavity, the thickness of the walls and the collecting plate were measured with a very sensitive comparator. The effective collecting volume is 8.4717 cm3. The dosimetry system is placed in a collimated beam of Co-60 γ rays. The wall of the water phantom facing the beam has a thickness of 4.0mm (0.479g/cm2). The chamber, inserted into a perspex support (0.912 mm thick front wall), is positioned into the water phantom at a depth equivalent to 5 g cm-2. Measurements for air Kerma and water dose are performed and compared with those obtained by the primary standard device, for air Kerma, and the secondary standard device, for water dose, in INER. Comparison for air Kerma is within 0.2%, for water dose is within 0.4%. TG-51 protocol represents a major simplification compared to the AAPM’s TG-21 protocol. This study makes an accurate Co-60 absorbed-dose-to-water calibration factor and will be supported by INER and may improve the accuracy of calibrating photon and electron beams for radiation therapy. |
本系統中英文摘要資訊取自各篇刊載內容。