School of Mathematical and Physical Sciences, University of Newcastle, Newcastle, NSW 2308, Australia.
J Appl Clin Med Phys. 2012 Nov 8;13(6):4024. doi: 10.1120/jacmp.v13i6.4024.
Amorphous silicon electronic portal imaging devices (EPIDs) have been investigated and used for dosimetry in radiotherapy for several years. The presence of a phosphor scintillator layer in the structure of these EPIDs has made them sensitive to low-energy scattered and backscattered radiation. In this study, the backscattered radiation from the walls, ceiling, and floor of a linac bunker has been investigated as a possible source of inaccuracy in EPID dosimetry. EPID images acquired in integrated mode at discrete gantry angles and cine images taken during arcs were used with different field setups (18 × 18 and 10 × 10 cm2 open square fields at 150 and 105 cm source-to-detector distances) to compare the EPID response at different gantry angles. A sliding gap and a dynamic head-and-neck IMRT field and a square field with a 15 cm thick cylindrical phantom in the beam were also investigated using integrated EPID images at several gantry angles. The contribution of linac output variations at different angles was evaluated using a 2D array of ion chambers. In addition, a portable brick wall was moved to different distances from the EPID to check the effect at a single angle. The results showed an agreement of within 0.1% between the arc mode and gantry-static mode measurements, and the variation of EPID response during gantry rotation was about 1% in all measurement conditions.
非晶硅电子射野影像装置（EPID）已经被研究并用于放射治疗中的剂量测量多年。这些 EPID 结构中磷光体闪烁层的存在使其对低能散射和背散射辐射敏感。在这项研究中，研究了直线加速器机房墙壁、天花板和地板的背散射辐射，因为它们可能是 EPID 剂量测量不准确的一个原因。使用不同的射野设置（150 和 105 cm 源-探测器距离处的 18×18 和 10×10 cm2 开放正方形射野），在离散的机架角度下获取集成模式的 EPID 图像和弧形采集的电影图像，以比较不同机架角度下 EPID 的响应。还使用集成的 EPID 图像在几个机架角度下研究了滑动间隙和动态头颈部调强放疗射野以及带有 15 cm 厚圆柱形模体的方形射野。使用二维电离室阵列评估了不同角度下直线加速器输出变化的贡献。此外，还将一个便携式砖墙移动到离 EPID 的不同距离，以检查单一角度的影响。结果表明，弧形模式和机架静态模式测量之间的差异在 0.1%以内，在所有测量条件下，机架旋转过程中 EPID 的响应变化约为 1%。
