Davidson Scott E, Cui Jing, Kry Stephen, Deasy Joseph O, Ibbott Geoffrey S, Vicic Milos, White R Allen, Followill David S
Radiation Oncology, The University of Texas Medical Branch, Galveston, Texas 77555.
Radiation Oncology, University of Southern California, Los Angeles, California 90033.
Med Phys. 2016 Aug;43(8):4842. doi: 10.1118/1.4955434.
A dose calculation tool, which combines the accuracy of the dose planning method (DPM) Monte Carlo code and the versatility of a practical analytical multisource model, which was previously reported has been improved and validated for the Varian 6 and 10 MV linear accelerators (linacs). The calculation tool can be used to calculate doses in advanced clinical application studies. One shortcoming of current clinical trials that report dose from patient plans is the lack of a standardized dose calculation methodology. Because commercial treatment planning systems (TPSs) have their own dose calculation algorithms and the clinical trial participant who uses these systems is responsible for commissioning the beam model, variation exists in the reported calculated dose distributions. Today's modern linac is manufactured to tight specifications so that variability within a linac model is quite low. The expectation is that a single dose calculation tool for a specific linac model can be used to accurately recalculate dose from patient plans that have been submitted to the clinical trial community from any institution. The calculation tool would provide for a more meaningful outcome analysis.
The analytical source model was described by a primary point source, a secondary extra-focal source, and a contaminant electron source. Off-axis energy softening and fluence effects were also included. The additions of hyperbolic functions have been incorporated into the model to correct for the changes in output and in electron contamination with field size. A multileaf collimator (MLC) model is included to facilitate phantom and patient dose calculations. An offset to the MLC leaf positions was used to correct for the rudimentary assumed primary point source.
Dose calculations of the depth dose and profiles for field sizes 4 × 4 to 40 × 40 cm agree with measurement within 2% of the maximum dose or 2 mm distance to agreement (DTA) for 95% of the data points tested. The model was capable of predicting the depth of the maximum dose within 1 mm. Anthropomorphic phantom benchmark testing of modulated and patterned MLCs treatment plans showed agreement to measurement within 3% in target regions using thermoluminescent dosimeters (TLD). Using radiochromic film normalized to TLD, a gamma criteria of 3% of maximum dose and 2 mm DTA was applied with a pass rate of least 85% in the high dose, high gradient, and low dose regions. Finally, recalculations of patient plans using DPM showed good agreement relative to a commercial TPS when comparing dose volume histograms and 2D dose distributions.
A unique analytical source model coupled to the dose planning method Monte Carlo dose calculation code has been modified and validated using basic beam data and anthropomorphic phantom measurement. While this tool can be applied in general use for a particular linac model, specifically it was developed to provide a singular methodology to independently assess treatment plan dose distributions from those clinical institutions participating in National Cancer Institute trials.
一种剂量计算工具已得到改进并针对瓦里安6兆伏和10兆伏直线加速器进行了验证，该工具结合了剂量规划方法（DPM）蒙特卡罗代码的准确性和实用分析多源模型的通用性，此前已有报道。该计算工具可用于先进临床应用研究中的剂量计算。当前报告患者计划剂量的临床试验的一个缺点是缺乏标准化的剂量计算方法。由于商业治疗计划系统（TPS）有其自己的剂量计算算法，且使用这些系统的临床试验参与者负责调试射束模型，因此报告的计算剂量分布存在差异。如今的现代直线加速器是按照严格的规格制造的，因此同一型号直线加速器内的变异性相当低。期望针对特定直线加速器型号的单一剂量计算工具可用于准确重新计算已提交给来自任何机构的临床试验群体的患者计划的剂量。该计算工具将提供更有意义的结果分析。
分析源模型由一个主点源、一个次级焦外源和一个污染电子源描述。还包括离轴能量软化和注量效应。已将双曲线函数添加到模型中以校正输出和电子污染随射野大小的变化。包含多叶准直器（MLC）模型以方便体模和患者剂量计算。使用MLC叶片位置的偏移来校正基本假设的主点源。
对于4×4至40×40厘米射野大小的深度剂量和剖面的剂量计算，与测量结果在最大剂量的2%以内或95%测试数据点的2毫米距离一致性（DTA）范围内相符。该模型能够将最大剂量深度预测在1毫米以内。使用热释光剂量计（TLD）对调制和图案化MLC治疗计划进行的人体模型基准测试表明，在目标区域内与测量结果的一致性在3%以内。使用归一化到TLD的放射变色胶片，应用最大剂量的3%和2毫米DTA的伽马标准，在高剂量、高梯度和低剂量区域的通过率至少为85%。最后，当比较剂量体积直方图和二维剂量分布时，使用DPM对患者计划进行的重新计算相对于商业TPS显示出良好的一致性。
已使用基本射束数据和人体模型测量对与剂量规划方法蒙特卡罗剂量计算代码耦合的独特分析源模型进行了修改和验证。虽然该工具可普遍应用于特定直线加速器型号，但具体而言，它是为提供一种单一方法而开发的，以独立评估参与美国国立癌症研究所试验的那些临床机构的治疗计划剂量分布。
