中高能量光子輻射量測標準與劑量驗證之研究

Measurement standards and dosimetry verifications for medium and high energy photon radiations

李振弘(Jeng-Hung Lee)

指導教授 ： 王竹方

國立清華大學/原子科學院/生醫工程與環境科學系/博士(2009年)

https://doi.org/10.6843/NTHU.2009.00350

摘要

Photon radiation is widely used in industries, radiation diagnosis and treatment and radiation protection and is most related with the quality of daily life of the public. Hence, it is necessary to develop and improve photon measurement standards to establish complete domestic calibration and traceability systems to reduce the risk induced by radiation and improve healthcare and life quality of the public. The dissertation is primarily concerning measurement standard systems for photon radiations higher than 50 keV. It deals with methods that are sufficiently precise and well established to be incorporated into the ionimetric measurement as primary standards. The primary standards used in Taiwan and the international comparisons for photon dosimetry are discussed in this research. We also describe the reference conditions that are suitable for establishing primary standards and we provide the measurements for determining the air kerma and absorbed dose, including the evaluation of correction factors and uncertainty needed under conditions that were used to calibrate an instrument at the National Radiation Standard Laboratory (NRSL) of the Institute of Nuclear Energy Research (INER, Taiwan). The comparison results for the air kerma of medium-energy X-rays and absorbed dose to water of the INER’s primary standards showed satisfactory agreements in the measurements which were within the combined expanded uncertainties (k = 2).
Many aspects are relevant to the reference dosimetry of other therapeutic beams including the comparison of the absorbed dose to water calibration in medical accelerator photon beams traceable to primary standards following the recommendations given in the American Association of Physicists in Medicine (AAPM) Task Group 21 (TG-21) and Task Group (TG-51) dosimetry protocols and the accurate and convenient quality assurance program that should be included in the dosimetry system of the radiotherapy level radiation. Through the photon reference dosimetry analysis of the TG-51/TG-21 ratio, the measurement differences caused from using TG-21 and TG-51 protocols in the hospitals were evaluated. For all types of linear accelerators and cylindrical chambers at the 13 participating hospitals in Taiwan, the TG-51/TG-21 dose ratios were the same within ±1.5% using 6 MV and 10 MV high photon energies; the differences were less than the combined uncertainty irrespective of the chamber make and model for each photon included here. Thus, a draft guide for quality assurance to institutions changing their standard basis from the TG-21 to TG-51 protocol was suggested based on the comparison results of the two dosimetry protocols. Besides, this dissertation shows the results of the dosimetry characteristic comparison for environmental radiophotoluminescent glass dosemeter (RPLGD) and thermoluminescent dosemeter (TLD) systems employed at the INER. Environmental cumulative doses measured by a RPLGD and a TLD were compared at the same monitoring points of INER during two years. The sensitivity of TLD decreased gradually due to fading at higher temperatures. The differences between the results obtained by RPLGD and TLD are mainly caused by the ambient temperature. In addition, We referred to the American National Standard Institute (ANSI) Draft Standard N13.29 (1996) to organize the environmental dosimetry performance tests of the INER’s RPLGDs and the TLDs. The results mean that both kinds of dosemeters can meet the performance test criteria. An accreditation procedure for the institutes which provide the environmental dosimetry services in Taiwan was suggested based on the comparison results of these two dosimetry systems.

關鍵字

量測標準 ； 自由空氣游離腔 ； 石墨圓餅型游離腔 ； 空氣克馬 ； 水吸收劑量 ； 劑量驗證

並列摘要

光子輻射已廣泛應用於工業、放射診療與輻射防護等領域，並與民眾日常生活品質密切相關，實有必要研發與精進光子量測標準，以建立國內完整之校正追溯體系，降低輻射所造成之風險，增進國人之健康照護與生活品質。本論文旨在探討能量高於50 keV之光子輻射量測標準系統，與自由空氣游離腔與石墨游離腔等原級標準件之研製，本研究亦討論在台灣地區與國際比對所使用之光子劑量原級標準，也說明核能研究所國家游離輻射標準實驗室建立原級標準之參考條件，與進行空氣克馬及水吸收劑量之量測過程，並包含原級標準各項修正因子與不確定度之評估。由核能研究所中能量X射線空氣克馬與水吸收劑量原級標準之國際比對結果，顯示比對差異均小於擴充不確定度(k=2)，證明本研究建立之標準系統與國際一致。
為研究醫用高能光子參考劑量追溯至原級標準之一致性，我們也分別依據美國醫學物理學會(AAPM) TG-21 (1983)及TG-51 (1999)議定書(protocol)，進行醫用加速器參考劑量之量測比對實驗，透過分析TG-51/TG-21光子參考劑量比值數據，可評估國內醫院在使用TG-21與TG-51進行劑量量測時所造成的差異。實驗證明，對於國內參加比對之十三家醫院而言，在6 MV與10 MV兩種高能光子能量下，利用 TG-51及TG-21議定書所量測之加速器水吸收劑量差異均小於1.5%，且醫用加速器與游離腔之廠牌、構造，並不會影響TG-51與TG-21之劑量測定結果，研究成果可提供相關單位在轉換議定書時，量測臨床光子參考劑量的品保指引；同時本研究也建立一套簡便之劑量驗證系統，以執行醫用加速器輸出劑量之品保驗證。此外，本論文也研究核能研究所玻璃劑量計(RPLGD)與熱發光劑量計(TLD)等兩種環境劑量計之特性，比較在2005-2007年間，玻璃劑量計與熱發光劑量計在相同偵測位置所量測之環境累積劑量值，結果顯示TLD在高溫下的消光現象會造成量測訊號的減少，且RPLGD 與 TLD 的劑量評估差異主要是環境溫度所造成的影響，我們也參考美國國家標準協會(ANSI) N13.29 (1996 )規範，建立環境劑量計之能力試驗執行方法，相關研究成果將可作為台灣地區執行環境劑量評估實驗室認證評鑑之參考。

並列關鍵字

Measurement standard ； Free air ionization chamber ； Graphite pancake ionization chamber ； Air kerma ； Absorbed dose to water ； Dose verification

參考文獻

Lin, M.S., Yang, J.A., Yeh, S.H., 2000. The establishment of environmental dose evaluation model for Harshaw 8807 dosemeter. Institute of Nuclear Energy Research Technical Reports Series INER-2347, Longtan.

Lin, M.S., Yeh, S.H., Chiu, J.H., Hsu, S.M., 2004b. The study of the FGD-202 glass dosimetry system. Institute of Nuclear Energy Research Technical Reports Series INER-2404, Longtan.

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Lee. J.H., Hwang, W.S., Kotler, L.H., Webb, D.V., Büermann, L., Burns, D.T., Takeyeddin, M., Shaha, V.V., Srimanoroth, S., Meghzifene, A., Hah, S.H., Chun, K.J., Kadni, B., Takata, N., Msimang, Z., 2008. APMP/TCRI key comparison report of measurement of air kerma for medium-energy X-Rays (APMP.RI(I)-K3). Metrologia 45, Tech. Suppl., 06012.

AAPM TG-21, 1983. A protocol for the determination of absorbed dose from high-energy photon and electron beams. Med. Phys.10, 741–771.