基于非晶硅电子射野影像装置的剂量响应研究

摘要

To investigate the dose-response characteristics of amorphous silicon-based electronic portal imaging device (a-Si EPID) under clinical conditions. Methods: PTW ion chambers and different copper plates were used into the measurement. EPID measurements were performed using Elekta commercial a-Si detectors on a linear accelerator with 6 MV and 10 MV X-ray beams. First, the EPID signal and ionization chamber measurements in a mini-phantom were compared to determine the amount of build-up required for EPID dosimetry. Subsequently, with different phantom,EPID signal characteristics were studied as a function of dose per pulse, pulse repetition frequency (PRf )and total dose. Results: There was an over-response of the EPID signal compared to the ionization chamber of up to 11 .6%, with no additional buildup layer over an air gap range of 10 cm to 60 cm. The addition of a 3 mm thick copper plate sufficiently reduced this over-response to within 1 % at clinically relevant patient-detector air gaps (40 cm). The response of the EPIDs varied from up to 3% over a large range of dose per pulse values, PRF values and number of monitor units. The EPID response showed an under-response at shorter beam times due to ghosting and image lag, which depended on the number of exposure frames for a fixed frame acquisition rate. Conclusions: With an appropriate build-up layer and corrections for dose per pulse, PRF and ghosting, the variation of dose response in the a-Si EPID can be decreased to well within 1 %.%目的:临床条件下研究探讨非晶硅电子射野影像装置(a-Si EPID)的剂量响应特性.方法:本实验在Elekta Precise直线加速器上X射线能量分别为6 MV和10 MV,采用PTW电离室、等效固体水和不同厚度铜板条件下实施测量.首先,通过EPID信号和模体中电离室的测量比较,确定出EPID剂量响应的建成厚度.其次,临床条件下利用模体的不同厚度测量分析有关剂量、每脉冲剂量和脉冲重复频率(PRF)函数的EPID信号响应情况.结果:在不增加建成材料、10 cm～60cm空气间隙条件下EPID显示了最大11.6％的过响应信号变化.临床上额外将3mm铜建成区置于EPID上方,空气间隙大于40 cm条件下EPID响应变化将会降至1％以内.在测量范围内随MU数、PRF和每脉冲剂量变化的EPID信号响应是非线性的,最大信号变化接近于3％.因假峰和图像滞后效应等影响,短时间照射EPID会明显地产生出低剂量响应.结论:采用合适的建成层和实施对每脉冲剂量、PRF等校正,非晶硅EPID剂量响应变化可控制在1％以内,从而建立起较为理想的剂量响应曲线.