Department of Applied Physics and Applied Mathematics, Columbia University, New York, New York 10027, USA.
Med Phys. 2010 Jun;37(6):2425-34. doi: 10.1118/1.3400043.
Portal dose images acquired with an amorphous silicon electronic portal imaging device (EPID) suffer from artifacts related to backscattered radiation. The backscatter signal varies as a function of field size (FS) and location on the EPID. Most current portal dosimetry algorithms fail to account for the FS dependence. The ramifications of this omission are investigated and solutions for correcting the measured dose images for FS specific backscatter are proposed.
A series of open field dose images were obtained for field sizes ranging from 2 x 2 to 30 x 40 cm2. Each image was analyzed to determine the amount of backscatter present. Two methods to account for the relationship between FS and backscatter are offered. These include the use of discrete FS specific correction matrices and the use of a single generalized equation. The efficacy of each approach was tested on the clinical dosimetric images for ten patients, 49 treatment fields. The fields were evaluated to determine whether there was an improvement in the dosimetric result over the commercial vendor's current algorithm.
It was found that backscatter manifests itself as an asymmetry in the measured signal primarily in the inplane direction. The maximum error is approximately 3.6% for 10 x 10 and 12.5 x 12.5 cm2 field sizes. The asymmetry decreased with increasing FS to approximately 0.6% for fields larger than 30 x 30 cm2. The dosimetric comparison between the measured and predicted dose images was significantly improved (p << .001) when a FS specific backscatter correction was applied. The average percentage of points passing a 2%, 2 mm gamma criteria increased from 90.6% to between 96.7% and 97.2% after the proposed methods were employed.
The error observed in a measured portal dose image depends on how much its FS differs from the 30 x 40 cm2 calibration conditions. The proposed methods for correcting for FS specific backscatter effectively improved the ability of the EPID to perform dosimetric measurements. Correcting for FS specific backscatter is important for accurate EPID dosimetry and can be carried out using the methods presented within this investigation.
使用非晶硅电子射野影像装置（EPID）获取的射野影像会受到与背散射辐射相关的伪影的影响。背散射信号随射野大小（FS）和 EPID 上的位置而变化。目前大多数的电子射野影像剂量学算法都无法考虑 FS 的依赖性。本文研究了这种忽略的后果，并提出了针对特定 FS 的背散射校正测量剂量图像的解决方案。
对 2×2 至 30×40cm2 的射野大小范围内的一系列开放野剂量图像进行了采集。分析了每张图像以确定存在的背散射量。提出了两种考虑 FS 与背散射之间关系的方法。这包括使用离散的 FS 特定校正矩阵和使用单个通用方程。在十位患者的 49 个治疗野的临床剂量图像上测试了每种方法的效果。评估了这些场，以确定与商业供应商当前算法相比，在剂量学结果上是否有所改善。
发现背散射表现为测量信号的不对称性，主要出现在平面内方向。对于 10×10 和 12.5×12.5cm2 的射野大小，最大误差约为 3.6%。随着 FS 的增加，不对称性减小，对于大于 30×30cm2 的射野，约为 0.6%。当应用 FS 特定的背散射校正时，测量和预测剂量图像之间的剂量比较显著改善（p<<0.001）。在采用提出的方法后，通过 2%、2mmγ标准的点的平均百分比从 90.6%增加到 96.7%至 97.2%之间。
在测量的电子射野影像中观察到的误差取决于其 FS 与 30×40cm2 校准条件的差异程度。提出的针对特定 FS 的背散射校正方法有效地提高了 EPID 进行剂量测量的能力。针对特定 FS 的背散射校正对于准确的 EPID 剂量学非常重要，可以使用本研究中提出的方法进行校正。
