应用提高分辨率的MatriXX检测等中心偏移的方法简介

目的:以检测等中心在X方向的偏移示例，介绍使用提高分辨率之后的MatriXX检测等中心偏移的方法。方法:在确保MLC的leaf bank关于collimator中心轴旋转对称，且MatriXX中心与等中心的偏差已知的基础上，将gantry和collimator的角度都设为0°，治疗床向X正方向每移动1 mm测量1次5 cm×5 cm照射野100 MU的剂量分布曲线，共7次移动治疗床，测量8组数据，然后将这8组数据叠加为一组复合数据，得到gantry和collimator角度为0°、5 cm×5 cm照射野100 MU时MatriXX在X方向分辨率为1 mm的剂量分布曲线。同样的方法测量得到将gantry角度设为180°时相对应的剂量分布曲线，然后使用OmniPro I’mRT软件对比分析这两个profile，得出等中心在X方向的偏移值。结果:等中心的偏移值为1.8 mm。结论:提高分辨率之后的MatriXX能够检测出等中心的偏移值；等中心的偏移会导致病人接受剂量出现偏差，而这种偏差可以通过调整Elekta Synergy MLC的leaf bank关于gantry旋转中心轴对称和计划设计中设置collimator与couch角度为0°来克服；等中心的偏差使得gantry角度在90°和270°附近照射野的平面剂量偏差非常大。因此，不建议计划设计中设置gantry角度在90°和270°附近的照射野，也不建议选用MatriXX或者其他平面探测器做照射野gantry角度集中在90°和270°附近的病人计划验证。

Brief introduction to isocenter deviation detection using MatriXX with improved resolution

Abstract: Objective To introduce a method for detecting the collimator isocenter deviation with high-resolution MatriXX by taking the detection of collimator isocenter deviation in X direction as an example. Methods Firstly, we ensured that the leaf bank of MLC was rotational symmetry by the central axis of collimator and that the distance of MatriXX center to isocenter was known, and then the angles of gantry and collimator were set to 0°. The dose distribution curve in an irradiation field of 5 cm×5 cm, 100 MU, was obtained after the linear accelerator was moved 1 mm at a time along X axis. The treatment couch was moved 7 times and a total of 8 measurements were obtained. All the measurements were superimposed to generate a "finer" profile, and the dose distribution curve with gantry 0°, collimator 0°, irradiation field of 5 cm×5 cm, 100 MU (G0C0) was obtained by MatriXX with an X-direction resolution of 1 mm. The same method was used to measure the dose distribution curve with gantry 180°, collimator 0°, irradiation field of 5 cm×5 cm, 100 MU (G180C0). Finally, we applied OmniPro I’mRT software to compare and analyze the profiles of G0C0 and G180C0 for obtaining the deviation of isocenter in X direction. Results The isocenter deviation was 1.8 mm. Conclusion MatriXX with an improved resolution is able to detect isocenter deviation. The isocenter deviation leads to the deviation of dose to patients, which can be avoided by adjusting the leaf bank of Elekta Synergy MLC to achieve the rotational symmetry by the central axis of gantry, and setting the angle of collimator and couch to 0°. The isocenter deviation results in a large planar dose deviation in irradiation fields with gantry angles closer to 90° and 270°, therefore, it’s not recommended to select the irradiation fields with gantry angles closer to 90° and 270°, and use MatriXX or other planar detectors to validate the patient plan with fields gantry angle centered around 90° and 270°.