半导体矩阵在HT加速器同步性测试中应用

摘要

目的 探讨如何使用ArcCHECK完成HT同步性测试.方法 根据AAPM 148号报告测试HT同步性参数,即机架角度一致性、治疗床匀速运动、治疗床运动和机架旋转同步性.(1)机架角度一致性:将ArcCHECK通过虚拟等中心摆位,使其长轴垂直于旋转平面;照射序列使用25mm的铅门宽度,螺距为0.1的40圈旋转照射;叶片控制正弦图设置中央两片叶片(第32、33片)于机架投射角度为0°、120°和240°时打开.(2)治疗床匀速运动:将ArcCHECK固定于治疗床上,长轴沿床运动方向摆位;编辑照射序列,使用临床常用的床运动速度(0.3~0.5mm/s)运动200mm,机架角度固定于0°、铅门宽度10mm、叶片全部打开进行照射.(3)治疗床运动和机架旋转同步性:使用10mm铅门宽度、螺距为1的13圈旋转照射;叶片控制正弦图第2、7、12圈照射时,叶片于半周全开,其他时刻均为关闭状态.最初采集数据后作为基线,后续测量与其进行比较,采用点对点剂量差别分析方法,1%标准评价差异水平.结果 机架角度一致性可以检测治疗开始时刻机架初始角度准确性及每圈旋转同一角度重复投照能力.ArcCHECK软件界面内将获得6条平行的直性高剂量区域.床匀速测试中软件内高剂量区域内,长轴方向相对剂量变化<2%.治疗床运动和机架旋转同步性测量结果在软件界面中获得间距相等的3条平行高剂量区域.第2、7、12圈照射时28次同步性测试通过率分别为93.2%±1.5%、93.7%±1.1%、93.5%±1.3%.结论 ArcCHECK1次摆位即可完成所有测试,与使用胶片相比降低工作量的同时节约成本,可作为除胶片外HT同步性测试的另一选择.%Objective To investigate how to use the ArcCHECK to complete synchronization test in helical tomotherapy(HT). Methods According to the synchronization test suggested by AAPM TG148, three aspects should be tested.(1)Gantry angle consistency:ArcCHECK device in the long axis was perpendicular to the rotation plane using virtual isocenter positioning. A delivery sequence was defined with a slice width of 2.5 cm,a pitch of 0.1,and a minimum of 40 rotations. The control sinogram was set to open the middle two leaves(No. 32 and 33)at projections centered at 0°, 120°, and 240°.(2)Couch speed uniformity:The ArcCHECK was fixed on the treatment couch by positioning the long axis in the movement direction of couch. An irradiation was done with a static gantry in the 0° position, the collimation set to 1 cm,and all MLC leaves open. A couch travel distance of 20 cm was programmed using a clinically common couch speed(0.3-0.5 mm/s for 2.5 cm treatment slice width)in the course of irradiation.(3) Synchronization of couch translation and gantry rotation:In this test,a rotational irradiation was used with the nominal 1.0 cm beam and a pitch of 1 for 13 rotations. The control sonogram was set to open all the leaves for a half rotation on the second,seventh,and twelfth rotations. The initial collected data were used as the baseline data of parameters. The subsequent measurements were compared with the baseline data using the point to point absolute dose pass rates of DD(dose difference,1%). Results The gantry angle consistency could check the correct initial angles at the beginning of the treatment and the ability to reproduce the projection every rotation.There were six parallel and straight high-dose regions in ArcCHECK software.In the couch speed uniformity test,the relative dose in the long axis in the high-dose regions was changed less than 2%. In the synchronization test of couch translation and gantry rotation,three parallel high-dose regions with the same separation spacing were present in the ArcCHECK software. The synchronization tests were done twenty-eight times in the second,seventh, and twelfth irradiation rotations and the average pass rates were 93.2%±1.5%,93.7%±1.1%,and 93.5%±1.3%,respectively. Conclusions The ArcCHECK device is a suitable alternative method for performing the synchronization test in HT due to its lower workload and costs compared with the film.