本研究使用Elekta的影像引導放射治療(lmage-guided radiation therapy, IGRT)，IGRT是一精密度很高的儀器，但仍受其治療床(Table)誤差的影響，且系統誤差(Systematic error)可能在患者治療前就已存在。本研究藉由所設兩種不同切面厚度、三種不同的移動誤差去探討其系統準確度，進而發展出品保(Quality assurance)程序。應用錐狀射束電腦斷層(Cone-beam computed tomography, CBCT)，評估其系統位置準確度，在2.5mm和5mm CT切面厚度的侖道假體(Rando phantom)上設計移動誤差(Translational errors)爲0.5cm、1cm、1.5cm，分別在一維(x、y、z)、二維(xy、yz、xz)、三維(xyz)空間，應用XVI system的兩種影像比對系統Bone registration和Grey value registration，計算各軸之誤差，實驗重複三次，並取其誤差平均值(Mean)、標準差(SD)及準確度(Accuracy)，以得知IGRT system之系統準確度和其穩定性。結果BE及GE的準確度並無顯著差異(p=0.48)且BE準確度爲0.038±0.02cm、GE準確度爲0.041±0.02cm，可知GE準確度並無較BE好，與理論不同；BE在各軸之SD範圍從0~0.1cm，GE爲0~0.1cm，其穩定度相當。治療床本身具有0.1cm誤差，且現今法規並無規定IGRT的品保程序，藉由這實驗我們可設計出每月或每日治療床位移的品保程序，進而得知系統準確度的大小，在未來有可能可納入臨床品保程序。

The accuracy of IGRT which has high precision instrument is affected by the table errors, and the systematic errors may be existed before the treatment. This study discusses the positioning accuracy in two different slice thicknesses and three translational errors and that will develop to quality assurance protocol. The objective of this study is evaluated the positioning accuracy using the XVI system (Elekta) for patients' set-up in IGRT. The positioning accuracy is provided by XVI system that is used bone and gray value registration methods. The positional errors of translational axes are set in 0.5 cm, 1 cm and 1.5 cm at 2.5 mm and 5 mm CT slice thicknesses with Rando phantom are evaluated and rectified. Each process was repeat for three times to obtain mean, SD and accuracy of positioning errors. There are no significant difference was found in accuracy between bone and gray value registration methods (p=0.48). The accuracy of bone registration is 0.038�0.02 cm and gray value registration is 041�0.02 cm, those revealed the gray value registration was not more accurate than the bone registration. The SD in translational axis is 0 to 0.1 cm for bone registration and 0 to 0.1 cm for gray value it is stable respectively. However, the table error had 0.1 cm already and there are no standard quality assurance protocol in 1GRT. In this study, we will obtain the quality assurance protocol in table accuracy, it has potential to be adopted for clinical usage in the future.