School of Mathematical and Physical Sciences, University of Newcastle, Newcastle, New South Wales 2308, Australia.
Med Phys. 2010 May;37(5):2269-78. doi: 10.1118/1.3369445.
Amorphous silicon EPIDs have been used for planar dose verification in IMRT treatments for many years. The support arm used to attach some types of EPIDs to linear accelerators can introduce inaccuracies to dosimetry measurements due to the presence of metallic parts in their structures. It is demonstrated that this uncertainty may be as large as approximately 6% of maximum image signal for large fields. In this study, a method has been described to quantify, model and correct for the effect of backscattered radiation from the EPID support arm (E-Arm type, Varian Medical Systems).
Measurements of a support arm backscatter kernel were made using several 1 x 1 cm2 6 MV pencil beam irradiations at a sample of positions over the sensitive area of the EPID in standard clinical setup and repeated with the EPID removed from the support arm but at the same positions. A curve-fit to the subtraction of EPID response obtained on and off the arm was used to define the backscatter kernel. The measured kernel was compared with a backscatter kernel obtained by Monte Carlo simulations with EGS/BEAM code. A backscatter dose prediction using the measured backscatter kernel was added to an existing EPID dose prediction model. The improvement in the agreement of the modified model predictions with EPID measurements for a number of open fields and IMRT beams were investigated by comparison to the original model results.
Considering all functions tested to find the best functional fit to the data points, a broad Gaussian curve proved to be the optimum fit to the backscatter data. The best fit through the Monte Carlo simulated backscatter kernel was also found to be a Gaussian curve. The maximum decrease in normalized root mean squared deviation of the measured and modeled EPID image profiles for open fields was 13.7% for a 15 x 15 cm2 field with no decrease observed for a 3 x 3 cm2 (the smallest) field as it was not affected by the arm backscatter. Gamma evaluation (2%, 2 mm criteria) showed the improvement in agreement between the model and measurement results when the backscatter was incorporated. The average increase in Gamma pass rate was 2% for head and neck and 1.3% for prostate IMRT fields investigated in this study.
The application of the backscatter kernel determined in this study improved the accuracy of dosimetry using a Varian EPID with E-arm for open fields of different sizes: Eight head and neck and seven prostate IMRT fields. Further improvement in the agreement between the model predictions and EPID measurements requires more sophisticated modeling of the backscatter.
多年来，非晶硅 EPID 已被用于 IMRT 治疗中的平面剂量验证。由于其结构中存在金属部件，某些类型的 EPID 连接到直线加速器的支撑臂会给剂量测量带来不准确性。研究表明，这种不确定性对于大野可能高达最大图像信号的 6%左右。在这项研究中，描述了一种量化、建模和校正 EPID 支撑臂（Varian Medical Systems 的 E-Arm 型）反向散射辐射影响的方法。
在标准临床设置中，使用几个 1x1cm2 的 6MV 铅笔束照射，在 EPID 的敏感区域的样本位置上进行了支撑臂反向散射核的测量，并在 EPID 从支撑臂上取下但处于相同位置的情况下重复了这些测量。通过对 EPID 响应的减法进行曲线拟合，定义了反向散射核。将测量的核与 EGS/BEAM 代码的蒙特卡罗模拟得到的反向散射核进行了比较。使用测量的反向散射核进行反向散射剂量预测，并添加到现有的 EPID 剂量预测模型中。通过与原始模型结果的比较，研究了对许多开放野和 IMRT 射束进行修改后的模型预测与 EPID 测量之间的一致性的改进。
考虑到为找到数据点的最佳函数拟合而测试的所有函数，宽高斯曲线被证明是反向散射数据的最佳拟合。通过蒙特卡罗模拟反向散射核的最佳拟合也是高斯曲线。对于 15x15cm2 的野，归一化均方根偏差的测量和建模的 EPID 图像轮廓的最大减少量为 13.7%，而对于 3x3cm2（最小）的野，由于不受臂反向散射的影响，没有观察到减少。伽马评估（2%，2mm 标准）表明，当纳入反向散射时，模型和测量结果之间的一致性得到了改善。在这项研究中，对头颈和前列腺 IMRT 进行的调查，伽马通过率平均提高了 2%。
本研究确定的反向散射核的应用提高了使用带有 E 臂的瓦里安 EPID 进行不同大小的开放野剂量测量的准确性：8 个头颈和 7 个前列腺 IMRT 场。要进一步提高模型预测与 EPID 测量之间的一致性，需要更复杂的反向散射建模。
