Schneider Frank, Polednik Martin, Wolff Dirk, Steil Volker, Delana Anna, Wenz Frederik, Menegotti Loris
Department of Radiation Oncology of the University Medical Center Mannheim, Mannheim, Germany.
Z Med Phys. 2009;19(1):29-37. doi: 10.1016/j.zemedi.2008.09.001.
Quality assurance of external beam (radio)therapy (EBT) requires tools with specific characteristics. A radiochromic film dubbed "Gafchromic EBT" (G-EBT) that is particularly suited for external beam therapy because of its features was introduced in 2004. Its characteristics, especially the high spatial resolution, make it suitable for measurement of dose distributions in radiotherapy, especially intensity-modulated radiation therapy (IMRT). While several aspects of the film characteristics have been previously reported separately, we present a comprehensive evaluation centered on practical IMRT verification, leading to an optimized protocol. Therefore the constancy within one batch, the relationship between optical density (OD) and dose (dose range between 1.4 Gy and 8.4 Gy) and the dose rate dependence for four dose rates (55, 108, 217, 441 MU/min) were investigated. In addition to these characteristics, energy dependence between two energies (50kV and 6 MV), tissue equivalency, post irradiation coloration over one month, pressure and temperature sensitivity were evaluated. We then optimized the protocol using the G-EBT films, in combination with an EPSON-Expression 1680 pro flatbed scanner, for IMRT QA, while either striving to keep the compound error as small as possible or trying to reduce evaluation time. As a basis for this protocol optimization, the characteristics of the scanner (such as inhomogeneity of the scanning field) and its software (such as consequences of extracting only the red color channel) had to be determined first. The interaction of film and scanner (variation of the OD depending on the scanning direction or the scanning resolution) was assessed as well. Using the optimized protocol for IMRT QA, the compound error could be reduced to approximately 2% for a quality-driven approach and maximum 5.5% for an approach attempting to reduce procedure time. While the quality-driven approach provides appropriate accuracy for individual patient QA, the procedure-time driven approach can only be used for preliminary measurements.
外照射（放射）治疗（EBT）的质量保证需要具备特定特性的工具。一种名为“Gafchromic EBT”（G-EBT）的放射变色薄膜于2004年被引入，因其特性特别适合外照射治疗。它的特性，尤其是高空间分辨率，使其适用于放射治疗中剂量分布的测量，特别是适用于调强放射治疗（IMRT）。虽然此前已分别报道过该薄膜特性的几个方面，但我们以实际的IMRT验证为中心进行了全面评估，得出了一个优化方案。因此，研究了同一批次内的稳定性、光密度（OD）与剂量（剂量范围在1.4 Gy至8.4 Gy之间）的关系以及四种剂量率（55、108、217、441 MU/min）下的剂量率依赖性。除了这些特性外，还评估了两种能量（50kV和6 MV）之间的能量依赖性、组织等效性、照射后一个月内的颜色变化、压力和温度敏感性。然后，我们将G-EBT薄膜与爱普生Expression 1680 pro平板扫描仪结合使用，针对IMRT质量保证对方案进行了优化，同时努力使复合误差尽可能小或缩短评估时间。作为该方案优化的基础，必须首先确定扫描仪的特性（如扫描场的不均匀性）及其软件（如仅提取红色通道的后果）。还评估了薄膜与扫描仪的相互作用（OD随扫描方向或扫描分辨率的变化）。使用针对IMRT质量保证的优化方案，对于以质量为导向的方法，复合误差可降至约2%，对于试图缩短程序时间的方法，最大误差为5.5%。虽然以质量为导向的方法为个体患者的质量保证提供了适当的准确性，但以程序时间为导向的方法仅可用于初步测量。
