Department of Medical Physics, Tom Baker Cancer Centre, University of Calgary, Calgary, Alberta, Canada.
J Appl Clin Med Phys. 2010 Jun 8;11(3):3213. doi: 10.1120/jacmp.v11i3.3213.
The accuracy of dose calculations at extended SSD is of significant importance in the dosimetric planning of total body irradiation (TBI). In a first step toward the implementation of electronic, multi-leaf collimator compensation for dose inhomogeneities and surface contour in TBI, we have evaluated the ability of the Eclipse AAA to accurately predict dose distributions in water at extended SSD. For this purpose, we use the Eclipse AAA algorithm, commissioned with machine-specific beam data for a 6 MV photon beam, at standard SSD (100 cm). The model was then used for dose distribution calculations at extended SSD (179.5 cm). Two sets of measurements were acquired for a 6 MV beam (from a Varian linear accelerator) in a water tank at extended SSD: i) open beam for 5 x 5, 10 x 10, 20 x 20 and 40 x 40 cm2 field sizes (defined at 179.5 cm SSD), and ii) identical field sizes but with a 1.3 cm thick acrylic spoiler placed 10 cm above the water surface. Dose profiles were acquired at 5 cm, 10 cm and 20 cm depths. Dose distributions for the two setups were calculated using the AAA algorithm in Eclipse. Confidence limits for comparisons between measured and calculated absolute depth dose curves and normalized dose profiles were determined as suggested by Venselaar et al. The confidence limits were within 2% and 2 mm for both setups. Extended SSD calculations were also performed using Eclipse AAA, commissioned with Varian Golden beam data at standard SSD. No significant difference between the custom commissioned and Golden Eclipse AAA was observed. In conclusion, Eclipse AAA commissioned at standard SSD can be used to accurately predict dose distributions in water at extended SSD for 6 MV open beams.
在全身照射（TBI）的剂量规划中，扩展源轴距（SSD）下剂量计算的准确性非常重要。在为 TBI 中的剂量不均匀性和表面轮廓实施电子、多叶准直器补偿的第一步中，我们评估了 Eclipse AAA 准确预测扩展 SSD 下水体中剂量分布的能力。为此，我们使用 Eclipse AAA 算法，该算法针对特定机器的 6 MV 光子束进行了配置，并在标准 SSD（100 cm）下运行。然后，我们使用该模型在扩展 SSD（179.5 cm）下进行剂量分布计算。我们在扩展 SSD（179.5 cm）下的水箱中对 6 MV 射线（来自瓦里安直线加速器）进行了两组测量：i）开放束，用于 5 x 5、10 x 10、20 x 20 和 40 x 40 cm2 射野尺寸（在 179.5 cm SSD 下定义），以及 ii）相同的射野尺寸，但在水面上方 10 cm 处放置了 1.3 cm 厚的亚克力遮挡板。在 5 cm、10 cm 和 20 cm 深度处获取剂量分布。使用 Eclipse 中的 AAA 算法计算这两种设置的剂量分布。根据 Venselaar 等人的建议，确定了测量和计算的绝对深度剂量曲线和归一化剂量分布之间比较的置信限。置信限在两种设置下均为 2%和 2 mm。还使用 Eclipse AAA 对扩展 SSD 进行了计算，该算法针对标准 SSD 下的瓦里安 Golden 射线数据进行了配置。未观察到定制委托和 Golden Eclipse AAA 之间存在显著差异。总之，在标准 SSD 下委托的 Eclipse AAA 可用于准确预测 6 MV 开放束在扩展 SSD 下水体中的剂量分布。
