Dempsey J F, Low D A, Mutic S, Markman J, Kirov A S, Nussbaum G H, Williamson J F
Radiation Oncology Center, Mallinckrodt Institute of Radiology, Washington University School of Medicine, St. Louis, Missouri 63110, USA.
Med Phys. 2000 Oct;27(10):2462-75. doi: 10.1118/1.1290488.
We present an evaluation of the precision and accuracy of image-based radiochromic film (RCF) dosimetry performed using a commercial RCF product (Gafchromic MD-55-2, Nuclear Associates, Inc.) and a commercial high-spatial resolution (100 microm pixel size) He-Ne scanning-laser film-digitizer (Personal Densitometer, Molecular Dynamics, Inc.) as an optical density (OD) imaging system. The precision and accuracy of this dosimetry system are evaluated by performing RCF imaging dosimetry in well characterized conformal external beam and brachytherapy high dose-rate (HDR) radiation fields. Benchmarking of image-based RCF dosimetry is necessary due to many potential errors inherent to RCF dosimetry including: a temperature-dependent time evolution of RCF dose response; nonuniform response of RCF; and optical-polarization artifacts. In addition, laser-densitometer imaging artifacts can produce systematic OD measurement errors as large as 35% in the presence of high OD gradients. We present a RCF exposure and readout protocol that was developed for the accurate dosimetry of high dose rate (HDR) radiation sources. This protocol follows and expands upon the guidelines set forth by the American Association of Physicists in Medicine (AAPM) Task Group 55 report. Particular attention is focused on the OD imaging system, a scanning-laser film digitizer, modified to eliminate OD artifacts that were not addressed in the AAPM Task Group 55 report. RCF precision using this technique was evaluated with films given uniform 6 MV x-ray doses between 1 and 200 Gy. RCF absolute dose accuracy using this technique was evaluated by comparing RCF measurements to small volume ionization chamber measurements for conformal external-beam sources and an experimentally validated Monte Carlo photon-transport simulation code for a 192Ir brachytherapy source. Pixel-to-pixel standard deviations of uniformly irradiated films were less than 1% for doses between 10 and 150 Gy; between 1% and 5% for lower doses down to 1 Gy and 1% and 1.5% for higher doses up to 200 Gy. Pixel averaging to form 200-800 microm pixels reduces these standard deviations by a factor of 2 to 5. Comparisons of absolute dose show agreement within 1.5%-4% of dose benchmarks, consistent with a highly accurate dosimeter limited by its observed precision and the precision of the dose standards to which it is compared. These results provide a comprehensive benchmarking of RCF, enabling its use in the commissioning of novel HDR therapy sources.
我们展示了对基于图像的放射变色胶片（RCF）剂量测定法的精度和准确性的评估，该方法使用一种商用RCF产品（Gafchromic MD - 55 - 2，Nuclear Associates公司）和一种商用高空间分辨率（像素尺寸为100微米）的氦氖扫描激光胶片数字化仪（Personal Densitometer，Molecular Dynamics公司）作为光密度（OD）成像系统。通过在特征明确的适形外照射束和近距离治疗高剂量率（HDR）辐射场中进行RCF成像剂量测定，来评估该剂量测定系统的精度和准确性。由于RCF剂量测定法存在许多潜在误差，包括：RCF剂量响应的温度依赖性时间演变；RCF的不均匀响应；以及光偏振伪影，因此基于图像的RCF剂量测定法的基准测试是必要的。此外，在存在高OD梯度的情况下，激光密度计成像伪影会产生高达35%的系统OD测量误差。我们提出了一种为高剂量率（HDR）辐射源的精确剂量测定而开发的RCF曝光和读出方案。该方案遵循并扩展了美国医学物理学家协会（AAPM）任务组55报告中提出的指南。特别关注的是OD成像系统，即一种经过改进的扫描激光胶片数字化仪，以消除AAPM任务组55报告中未涉及的OD伪影。使用该技术对RCF精度进行评估时，对胶片给予1至200 Gy之间均匀的6 MV X射线剂量。通过将RCF测量结果与适形外照射束源的小体积电离室测量结果以及针对192Ir近距离治疗源的经过实验验证的蒙特卡罗光子传输模拟代码进行比较，来评估使用该技术的RCF绝对剂量准确性。对于10至150 Gy之间的剂量，均匀辐照胶片的逐像素标准偏差小于1%；对于低至1 Gy的较低剂量，标准偏差在1%至5%之间；对于高达200 Gy的较高剂量，标准偏差在1%至1.5%之间。通过像素平均形成200 - 800微米的像素可将这些标准偏差降低2至5倍。绝对剂量的比较显示与剂量基准的一致性在剂量基准的1.5% - 4%以内，这与一个受其观测精度和与之比较的剂量标准精度限制的高精度剂量计一致。这些结果为RCF提供了全面的基准测试，使其能够用于新型HDR治疗源的调试。
