Machine Performance Check束流均匀性改变对Portal Dosimetry计划剂量验证的影响

Effects of beam uniformity changes of Machine Performance Check on Portal Dosimetry plan dose verification

目的:探究Machine Performance Check(MPC)系统束流均匀性变化对Portal Dosimetry(PD)计划验证的影响,为临床MPC均匀性的阈值设定和电子影像系统(EPID)的校准频率提供参考。方法:选取本中心EDGE加速器上首次治疗患者26例和10 cm×10 cm方野1例,制定治疗计划和验证计划。在MPC束流均匀性偏差增大的情况下,分别在EPID校准前和校准后执行验证计划,并在计划系统PD模块中分析,统计对比图像剂量和γ通过率。本研究还列出EDGE加速器一年间MPC束流均匀性的结果。结果:MPC 1年的统计结果显示束流均匀性偏差的升高趋势明显并且速度加快,表明EPID存在设备老化现象。EPID校准前后验证计划的图像剂量和γ通过率的对比结果表明不同能量方野计划在影像板中心附近的剂量差异为1%~2%,临床射野计划由于复杂性提高,剂量差异最大可以达到10%。EPID校准后的γ通过率高于校准前。结论:EPID探测器的一致性改变对PD计划剂量验证有一定影响,提示临床MPC均匀性阈值为2%时能够对PD计划剂量验证起到预警作用,EPID应在MPC重新采集基线之前校准,以保证验证计划的质量,保证患者放疗的安全性。

Objective To explore the effects of beam uniformity changes of Machine Performance Check(MPC) on Portal Dosimetry(PD) plan verification, thereby providing references for the threshold setting of MPC uniformity and the calibration frequency of electronic portal imaging device(EPID). Methods Twenty-six patients treated on the EDGE accelerator for the first time, and a case of 10 cm×10 cm square field were selected to make treatment and verification plans. With the increasing deviation of MPC beam uniformity, verification plans were executed before and after EPID calibration. The image dose and gamma passing rate were analyzed and compared in PD module. The MPC beam uniformity of the EDGE accelerator in one year was also listed. Results MPC statistical results for one year showed that beam uniformity deviation had a clear upward trend and the speed was accelerating,which indicated that there was equipment aging in EPID. The comparison of image doses and gamma passing rates of verification plans before and after EPID calibration revealed that the dose difference near the center of image plate was from 1% to 2% for the square field plans of different energies. Due to the increased complexity of the clinical plans, the maximum dose difference reached 10%. Gamma passing rate after EPID calibration was higher than that before calibration. Conclusion Uniformity change of EPID detector has a certain effect on PD plan dose verification, which suggests that when the clinical MPC uniformity threshold is 2%, it can play an early warning role for plan dose verification. EPID should be calibrated before the MPC baseline is re-acquired,thereby ensuring the quality of verification plans and guaranteeing the safety of radiotherapy.