目的：因應AAPM TG-51議定書之施行，核能研究所(INER)研製圓餅型石墨游離腔，作為60Co水吸收劑量原級校正標準。此外，INER亦與國內13家醫院合作，分別依據AAPM TG-21及TG-51議定書，量測6MV與10MV高能光子之水吸收劑量，並分析在不同議定書之條件下，量測水吸收劑量之差異值，以作為醫院在轉換不同議定書時之品保參考。材料與方法：INER完成圓餅型石墨游離腔各項物理參數與修正因子之評估、量測，再以三支NE 2571游離腔作為傳遞件，與澳洲ABPANSA國家實驗室進行60Co水吸收劑量標準之雙邊比對。同時，本實驗也以60Co照射器進行醫用游離腔之空氣克馬與水吸收劑量校正，而醫用加速器水吸收劑量之量測比對條件係參考TG-51議定書，選擇100 cm之SSD或SAD、10 cm×10 cm照野及10 cm校正深度。結果：INER與ARPANSA 60Co水吸收劑量標準之比對差異在0.3%以內，小於組合標準不確定度(k=1)，顯示雙方量測結果具有良好的一致性。針對國內13家比對單位所使用的5種規格游離腔而言，60Co水吸收劑量與空氣克馬校正係數之比值範圍為1.094至1.105，所有游離腔之校正係數比值與平均道的差異也維持在0.6%以內。同時，對於參加比對之醫院，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15%。結論：醫用加速器與游離腔之廠牌、構造，並不會影響使用TG-51與TG-21測量水吸收劑量所造成的差異。當醫院量測的TG-51/TG-21劑量差異超過15%時，就必須重新驗證在兩種劑量議定書所使用的各項計算參數與量測數據之正確性。

Purpose: In response to the AAPM TG-51 protocol, the Institute of Nuclear Energy Research (INER) fabricated a pancake graphite ionization chamber as the primary standard for the absorbed dose to water in 60Co. Meanwhile, INER cooperated with 13 domestic hospitals in measurements of the absorbed dose to water of 6 MV and 10 MV high-energy photon beams respectively based on the AAPM TG-21 and TG-51 protocols to analyze the dosimetry differences under the two protocols. A quality assurance guide for institutions switching from the TG-21 to TG-51 protocol was suggested based on the comparison results of these two dosimetry protocols. Material and Methods: INER completed the assessment and measurement of the physical parameters and correction factors of the pancake graphite ionization chamber and then made a bilateral comparison with ARPANSA/Australia for the standards of absorbed dose to water in 60Co by using three NE 2571 ionization chambers as transfer items. Also, INER used a 60Co irradiator in performing chamber calibrations in terms of the air kerma and absorbed dose to water. As to the measurement conditions of the absorbed dose to water for the 6 MV and 10 MV high-energy photon beams, they were based on the AAPM TG-51 protocol at 100 cm SSD (or SAD), 10 cm×10 cm irradiation field and 10 cm calibratoin depth. Results: The difference of the INER-ARPANSA bilateral comparison of the standards of the absorbed dose to water in 60Co was within 0.3%, which was less than the combined standard uncertainty (k=1). It showed that both laboratories had good agreement in measurement results. For the 5 types of cavity ionization chambers used by the 13 dosmetic hospitals, the ratios of the absorbed dose to water calibration coefficient against the air kerma calibration coefficient in 60Co were between 1.094 and 1.105, showing similar results within tight limits (envelop of ±0.6%), At the same time, for all the 13 participants, the measurement differences of the absorbed dose to water for 6 MV and 10 MV high-energy photon beams based on the TG-21 and TG-51 dosimetry protocols were less than 1.5%. Conclusions: The manufacturers and structures of medical accelerators and ionization chambers will not cause differences in dose measurement either it was TG-51 or TG-21 protocol that had been used. If an institution determines TG-5 or TG-21 ratios measurably different (>1.5%) than the values presented in this paper, it should recheck the correctness of calculation parameters and measurement data used in both protocols.