Dahlgren Christina Vallhagen, Eilertsen Karsten, Jørgensen Trude Dahl, Ahnesjö Anders
Department of Oncology, Radiology and Clinical Immunology, Uppsala University, Akademiska Sjukhuset, SE-751 85 Uppsala, Sweden.
Phys Med Biol. 2006 Jan 21;51(2):335-49. doi: 10.1088/0031-9155/51/2/010. Epub 2006 Jan 4.
Two different commercial electronic portal imaging devices (EPIDs), one based on a liquid ion chamber matrix and the other based on a fluoroscopic CCD camera, were used to acquire readings that, through a calibration procedure, provided images proportional to the absolute dose to a virtual water slab located at the EPID plane. The transformation of the matrix ion chamber image into a portal dose image (PDI) was based on a published relationship between dose rate and ionization current. For the fluoroscopic CCD-camera-based system, the transformation was based on a deconvolution with a radial light scatter kernel. Local response variations were corrected in the images from both systems using open field fluence maps. The acquired PDIs were compared with PDIs calculated with the collapsed cone superposition method for a three-dimensional detector model in water equivalent buildup material. The calculation model was based on the beam modelling and geometrical description of the treatment unit and energy used for treatment planning in a kernel-based system. The validity of the calculation method was evaluated for several field shapes and thicknesses of patient phantoms for the matrix ion chamber at 6 MV x-rays and for the camera-based EPID at 6 and 15 MV x-rays. The agreement between predicted and measured PDIs was evaluated with dose comparisons at points of interest and gamma index calculations. The average area failing the passing criteria in dose and position deviation was analysed to validate the performance of the method. For the matrix ion chamber on average an area less than 1% fails the passing criteria of 3 mm and 3%. For the camera-based EPID, the average area is 7% and 1% for 6 and 15 MV, respectively. The overall agreement centrally in the fields was 0.1 +/- 1.6% (1 sd) for the camera-based EPID and -0.1 +/- 1.6% (1 sd) for the matrix ion chamber. Thus, an absolute dose calibrated EPID could validate the delivered dose to the patient by comparing a calculated and a measured PDI.
使用了两种不同的商用电子射野影像装置（EPID），一种基于液体电离室矩阵，另一种基于荧光透视电荷耦合器件（CCD）相机，来获取读数。通过校准程序，这些读数可提供与位于EPID平面的虚拟水模体的绝对剂量成比例的图像。矩阵电离室图像到射野剂量图像（PDI）的转换基于已发表的剂量率与电离电流之间的关系。对于基于荧光透视CCD相机的系统，转换基于与径向光散射核的去卷积。使用开放野注量图对两个系统图像中的局部响应变化进行校正。将获取的PDI与在水等效建成材料中的三维探测器模型用坍缩锥叠加法计算得到的PDI进行比较。计算模型基于治疗单元的射束建模和几何描述以及在基于核的系统中用于治疗计划的能量。针对6兆伏X射线的矩阵电离室以及6和15兆伏X射线的基于相机的EPID，评估了几种野形状和患者体模厚度下计算方法的有效性。通过感兴趣点的剂量比较和伽马指数计算来评估预测PDI与测量PDI之间的一致性。分析未通过剂量和位置偏差通过标准的平均面积，以验证该方法的性能。对于矩阵电离室，平均面积小于1%未通过3毫米和3%的通过标准。对于基于相机的EPID，6兆伏和15兆伏时的平均面积分别为7%和1%。基于相机的EPID在射野中心的总体一致性为0.1±1.6%（1标准差），矩阵电离室为-0.1±1.6%（1标准差）。因此，通过比较计算得到的和测量得到的PDI，绝对剂量校准的EPID可以验证给予患者的剂量。
