Mihaylov I B, Lerma F A, Wu Y, Siebers J V
Department of Radiation Oncology, Virginia Commonwealth University, PO. Box 980058, Richmond, Virginia 23298, USA.
Med Phys. 2006 Apr;33(4):828-39. doi: 10.1118/1.2178449.
A hybrid dose-computation method is designed which accurately accounts for multileaf collimator (MLC)-induced intensity modulation in intensity modulated radiation therapy (IMRT) dose calculations. The method employs Monte Carlo (MC) modeling to determine the fluence modulation caused by the delivery of dynamic or multisegmental (step-and-shoot) MLC fields, and a conventional dose-computation algorithm to estimate the delivered dose to a phantom or a patient. Thus, it determines the IMRT fluence prediction accuracy achievable by analytic methods in the limit that the analytic method includes all details of the MLC leaf transport and scatter. The hybrid method is validated and benchmarked by comparison with in-phantom film dose measurements, as well as dose calculations from two in-house, and two commercial treatment planning system analytic fluence estimation methods. All computation methods utilize the same dose algorithm to calculate dose to a phantom, varying only in the estimation of the MLC modulation of the incident photon energy fluence. Gamma analysis, with respect to measured two-dimensional (2D) dose planes, is used to benchmark each algorithm's performance. The analyzed fields include static and dynamic test patterns, as well as fields from ten DMLC IMRT treatment plans (79 fields) and five SMLC treatment plans (29 fields). The test fields (fully closed MLC, picket fence, sliding windows of different size, and leaf-tip profiles) cover the extremes of MLC usage during IMRT, while the patient fields represent realistic clinical conditions. Of the methods tested, the hybrid method most accurately reproduces measurements. For the hybrid method, 79 of 79 DMLC field calculations have gamma < 1 (3%/3 mm) for more than 95% of the points (per field) while for SMLC fields, 27 of 29 pass the same criteria. The analytic energy fluence estimation methods show inferior pass rates, with 76 of 79 DMLC and 24 of 29 SMLC fields having more than 95% of the test points with gamma < or = 1 (3%/3 mm). Paired one-way ANOVA tests of the gamma analysis results found that the hybrid method better predicts measurements in terms of both the fraction of points with gamma < or = 1 and the average gamma for both 2%/2 mm and 3%/3 mm criteria. These results quantify the enhancement in accuracy in IMRT dose calculations when MC is used to model the MLC field modulation.
设计了一种混合剂量计算方法，该方法在调强放射治疗（IMRT）剂量计算中能准确考虑多叶准直器（MLC）引起的强度调制。该方法采用蒙特卡罗（MC）建模来确定动态或多段式（步进-射野）MLC射野传输所引起的注量调制，并使用传统剂量计算算法来估计模体或患者所接受的剂量。因此，它确定了在解析方法包含MLC叶片传输和散射所有细节的极限情况下，解析方法所能达到的IMRT注量预测精度。通过与模体内胶片剂量测量结果以及两种内部和两种商业治疗计划系统解析注量估计方法的剂量计算结果进行比较，对该混合方法进行了验证和基准测试。所有计算方法都使用相同的剂量算法来计算模体剂量，仅在入射光子能量注量的MLC调制估计方面有所不同。针对测量的二维（2D）剂量平面进行伽马分析，以评估每种算法的性能。分析的射野包括静态和动态测试模式，以及来自十个动态MLC IMRT治疗计划（79个射野）和五个静态MLC治疗计划（29个射野）的射野。测试射野（完全关闭的MLC、栅栏式、不同大小的滑动窗口以及叶尖轮廓）涵盖了IMRT期间MLC使用的极端情况，而患者射野代表了实际临床情况。在测试的方法中，混合方法最能准确地重现测量结果。对于混合方法，79个动态MLC射野计算中的79个在超过95%的点（每个射野）处伽马值<1（3%/3 mm），而对于静态MLC射野，29个中的27个通过了相同标准。解析能量注量估计方法的通过率较低，79个动态MLC射野中的76个和29个静态MLC射野中的24个在超过95%的测试点处伽马值≤1（3%/3 mm）。对伽马分析结果进行的配对单因素方差分析测试发现，就伽马值≤1的点的比例以及2%/2 mm和3%/3 mm标准下的平均伽马值而言，混合方法能更好地预测测量结果。这些结果量化了在IMRT剂量计算中使用MC对MLC射野调制进行建模时精度的提高。
