Department of Medical Physics, Liverpool and Macarthur Cancer Therapy Centre, NSW, 2170, Australia.
Phys Med Biol. 2009 Dec 7;54(23):7151-69. doi: 10.1088/0031-9155/54/23/008. Epub 2009 Nov 11.
The routine use of electronic portal imaging devices (EPIDs) as dosimeters for radiotherapy quality assurance is complicated by the non-water equivalence of the EPID's dose response. A commercial EPID modified to a direct-detection configuration was previously demonstrated to provide water-equivalent dose response with d(max) solid water build-up and 10 cm solid water backscatter. Clinical implementation of the direct EPID (dEPID) requires a design that maintains the water-equivalent dose response, can be incorporated onto existing EPID support arms and maintains sufficient image quality for clinical imaging. This study investigated the dEPID dose response with different configurations of build-up and backscatter using varying thickness of solid water and copper. Field size output factors and beam profiles measured with the dEPID were compared with ionization chamber measurements of dose in water for both 6 MV and 18 MV. The dEPID configured with d(max) solid water build-up and no backscatter (except for the support arm) was within 1.5% of dose in water data for both energies. The dEPID was maintained in this configuration for clinical dosimetry and image quality studies. Close agreement between the dEPID and treatment planning system was obtained for an IMRT field with 98.4% of pixels within the field meeting a gamma criterion of 3% and 3 mm. The reduced sensitivity of the dEPID resulted in a poorer image quality based on quantitative (contrast-to-noise ratio) and qualitative (anthropomorphic phantom) studies. However, clinically useful images were obtained with the dEPID using typical treatment field doses. The dEPID is a water-equivalent dosimeter that can be implemented with minimal modifications to the standard commercial EPID design. The proposed dEPID design greatly simplifies the verification of IMRT dose delivery.
电子射野影像装置（EPID）常规用于放射治疗质量保证的剂量测定，但由于 EPID 的剂量响应与水并不等效，因此使用起来较为复杂。先前有研究证明，一种经过改装的、采用直接探测技术的商业 EPID 可以在使用最大剂量点固体水堆积和 10cm 固体水背散射的情况下，提供与水等效的剂量响应。直接 EPID（dEPID）的临床应用需要设计一种既能维持与水等效的剂量响应，又能集成到现有的 EPID 支撑臂上，并保持足够的临床成像质量的设备。本研究采用不同厚度的固体水和铜，研究了不同的堆积和背散射配置下 dEPID 的剂量响应。用 dEPID 测量的射野输出因子和射束轮廓与水腔电离室测量的 6MV 和 18MV 剂量进行了比较。对于这两种能量，最大剂量点固体水堆积且无背散射（除支撑臂外）的 dEPID 与水腔中的剂量数据相差在 1.5%以内。该 dEPID 在此配置下用于临床剂量测定和图像质量研究。对于一个 98.4%的像素都符合 3%和 3mm 伽玛标准的调强放疗射野，dEPID 与治疗计划系统之间的吻合度非常好。dEPID 的灵敏度降低导致图像质量较差，这是基于定量（对比度噪声比）和定性（人体模型）研究得出的。然而，对于典型的治疗场剂量，dEPID 仍可以获得临床有用的图像。dEPID 是一种与水等效的剂量计，可以在不改变标准商用 EPID 设计的情况下进行最小的修改。所提出的 dEPID 设计极大地简化了调强放疗剂量交付的验证。
