MatriXX角度修正因子对VMAT剂量验证影响及分析

目的 探讨MatriXX电离室矩阵角度修正因子对VMAT计划剂量学验证的影响。方法 将MatriXX与MultiCube组成的测量装置进行CT扫描并导入到Raystation计划系统中。设置一组28×28,100 MU,6 MV X射线的照射野,从0°~180°开始每隔5°(在85°~95°方向每隔1°)用MatriXX进行测量。将MatriXX矩阵测量所得剂量平面与计划系统计算值比较得到一组角度修正因子CF (θ) cor并镜像到对侧,与出厂时所给出的角度修正因子CF (θ) def进行比较,分析MatriXX角度修正因子对剂量学验证影响。配对t检验差异。结果 CF (θ) cor与CF (θ) def存在一定差异,在85°~95°差异较大,最大可达7.5%(P<0.05)。CF (θ) cor修正γ通过率比CF (θ) def修正的高,最大可达17%(P<0.05)。结论 MatriXX电离室矩阵具有角度依赖性,应对其角度修正。角度修正因子具有个体化特征且MatriXX中每一单元电离室修正因子均有所不同,因此在使用MatriXX+Cube进行验证时应对所有电离室进行角度修正。在修正之前要考虑各种因素引起的误差,可提高计划验证准确率及通过率。

Effect of angle correction factors of MatriXX on VMAT dose verification

Objective To investigate the effect of the angle correction factors of ionization chamber array Matrixx on the dosimetric verification of volumetric modulated arc therapy plan. Methods The Matrixx (IBA) was put in the middle of the MultiCube Phantom. Computed tomography scan was performed and the results were sent to the Raystation treatment planning system (TPS). All data calculated from TPS and actually measured using MatriXX were gained under the following conditions:6 MV, 100 MU, 28 cm×28 cm radiation field. And the directions of beam were from 0°to 180°with an interval of 5°(except an interval of 1° from 85° to 95°). The angle correction factors (CF)(θ) cor of MatriXX were calculated by the calculated dose values from TPS and the measured values using MatriXX and then mirrored to the opposite side. CF (θ) cor values were compared with CF (θ) def values that were given by manufacturer. The comparison was made by paired t-test. Results CF (θ) cor and CF (θ) def were different, and the difference was significant between 85° and 95°, with a maximum of 7.5%(P<0.05). The CF (θ) cor correction method had a higher gamma pass rate than the CF (θ) def correction method, with a maximum of 17%(P<0.05). Conclusions The ionization chamber array MatriXX has an angular dependence and the angle correction should be performed. The angle correction factors have the individual characteristics and are different in every unit ionization chamber of the MatriXX. Therefore, it is necessary to do angle correction for every unit ionization chamber of the MatriXX and the possible errors caused by various factors should be considered in order to improve the accuracy rate and gamma pass rate of the plan verification.