基于EPID三维剂量验证系统的物理模型测试及临床应用的初步研究

目的 使用EPID三维剂量验证系统进行物理建模和物理参数优化,并行临床应用前的初步研究。方法 通过EPID采集3、5、10、15、20、25 cm的方野图像建立物理模型,比较在均匀水模体中系统重建的百分深度剂量、射野总散因子及10 cm深度处的离轴比曲线,优化物理模型参数。采用指型电离室和免冲洗胶片,在均匀模体和仿真人模体中测量单野、组合野及IMRT计划点剂量和平面剂量,并与系统重建结果比较。在仿真人模体和 10例不同部位IMRT计划中,比较系统重建和TPS计算的5%/3 mm、3%/3 mm标准下的γ通过率,并对临床病例进行靶区和OAR剂量体积分析。结果 对于单野、组合野以及IMRT计划,系统重建剂量和电离室测量及TPS计算的点剂量平均偏差分别<0.5%和2.0%;在均匀或仿真人模体中以及临床病例中其平面或三维剂量的5%/3 mm、3%/3 mm平均γ通过率均>95%;但临床病例中体现小体积的OAR有较大剂量偏差。结论 通过一系列临床应用前测试,明确了该三维剂量验证系统可有效应用于临床剂量验证,并有较好的临床应用价值。

A preliminary study of test and clinical application of a physical model based on the three-dimensional dose verification system using electronic portal imaging device

Objective To establish a physical model and optimize its physical parameters using the electronic portable imaging device ( EPID) three-dimensional dose verification system, and to prepare it for clinical application. Methods EPID was used to acquire images of 3, 5, 10, 15, 20, and 25 cm square fields for construction of a physical model. The parameters of the physical model were optimized based on the percentage depth dose, total scatter factor, and off-axis ratio at a depth of 10 cm in a homogeneous water phantom. A thimble ionization chamber and radiochromic films were used to measure the point and planar doses for single fields, combined fields, and IMRT plans in a homogenous phantom and a human phantom. The results were compared with those in the three-dimensional reconstruction. In the human phantom and 10 intensity-modulated radiotherapy ( IMRT) plans for tumors in different sites, the passing rates under the criteria of 5%/3 mm and 3%/3 mm were compared between three-dimensional reconstruction and treatment planning system ( TPS) calculation. The dose and volume analyses were performed on target volume and organs at risk ( OARs) in patients. Results For the single fields, combined fields, and IMRT plans, the mean deviations of point dose were less than 0. 5% between three-dimensional reconstruction and ionization chamber measurement, and less than 2. 0% between three-dimensional reconstruction and TPS calculation. In the homogenous phantom, the human phantom, and patients, the mean passing rates of both two-and three-dimensional doses were higher than 95% under the criteria of 5%/3 mm and 3%/3 mm. In patients, however, OARs with small volume had relatively large dose deviations. Conclusions A series of pre-clinical tests show that the three-dimensional dose verification system is an effective approach for clinical dose verification and holds promise for clinical application.