Ding George X, Duggan Dennis M, Coffey Charles W, Shokrani Parvaneh, Cygler Joanna E
Department of Radiation Oncology, Vanderbilt University Medical Center, Nashville, USA.
Phys Med Biol. 2006 Jun 7;51(11):2781-99. doi: 10.1088/0031-9155/51/11/007. Epub 2006 May 17.
The purpose of this study is to present our experience of commissioning, testing and use of the first commercial macro Monte Carlo based dose calculation algorithm for electron beam treatment planning and to investigate new issues regarding dose reporting (dose-to-water versus dose-to-medium) as well as statistical uncertainties for the calculations arising when Monte Carlo based systems are used in patient dose calculations. All phantoms studied were obtained by CT scan. The calculated dose distributions and monitor units were validated against measurements with film and ionization chambers in phantoms containing two-dimensional (2D) and three-dimensional (3D) type low- and high-density inhomogeneities at different source-to-surface distances. Beam energies ranged from 6 to 18 MeV. New required experimental input data for commissioning are presented. The result of validation shows an excellent agreement between calculated and measured dose distributions. The calculated monitor units were within 2% of measured values except in the case of a 6 MeV beam and small cutout fields at extended SSDs (>110 cm). The investigation on the new issue of dose reporting demonstrates the differences up to 4% for lung and 12% for bone when 'dose-to-medium' is calculated and reported instead of 'dose-to-water' as done in a conventional system. The accuracy of the Monte Carlo calculation is shown to be clinically acceptable even for very complex 3D-type inhomogeneities. As Monte Carlo based treatment planning systems begin to enter clinical practice, new issues, such as dose reporting and statistical variations, may be clinically significant. Therefore it is imperative that a consistent approach to dose reporting is used.
本研究的目的是介绍我们对用于电子束治疗计划的首个基于宏观蒙特卡罗的商业剂量计算算法进行调试、测试和使用的经验，并研究与剂量报告（水的剂量与介质的剂量）相关的新问题，以及在患者剂量计算中使用基于蒙特卡罗的系统时计算产生的统计不确定性。所有研究的体模均通过CT扫描获得。针对在不同源皮距下包含二维（2D）和三维（3D）类型低密度和高密度不均匀性的体模中使用胶片和电离室进行的测量，对计算得到的剂量分布和监测单位进行了验证。束能量范围为6至18 MeV。给出了调试所需的新实验输入数据。验证结果表明计算得到的剂量分布与测量值之间具有极好的一致性。计算得到的监测单位除了在6 MeV束和扩展源皮距（>110 cm）下的小射野情况下，均在测量值的2%以内。对剂量报告新问题的研究表明，当计算并报告“介质的剂量”而非传统系统中所做的“水的剂量”时，肺部的差异高达4%，骨骼的差异高达12%。即使对于非常复杂的3D型不均匀性，蒙特卡罗计算的准确性在临床上也是可接受的。随着基于蒙特卡罗的治疗计划系统开始进入临床实践，诸如剂量报告和统计变化等新问题在临床上可能具有重要意义。因此，必须采用一致的剂量报告方法。
