Department of Radiation Oncology, Massachusetts General Hospital, and Harvard Medical School, Boston, MA 02114, USA.
Phys Med Biol. 2010 Mar 7;55(5):1475-90. doi: 10.1088/0031-9155/55/5/014. Epub 2010 Feb 16.
The purpose of this work was to create a computational platform for studying motion in intensity modulated radiotherapy (IMRT). Specifically, the non-uniform rational B-spline (NURB) cardiac and torso (NCAT) phantom was modified for use in a four-dimensional Monte Carlo (4D-MC) simulation system to investigate the effect of respiratory-induced intra-fraction organ motion on IMRT dose distributions as a function of diaphragm motion, lesion size and lung density. Treatment plans for four clinical scenarios were designed: diaphragm peak-to-peak amplitude of 1 cm and 3 cm, and two lesion sizes--2 cm and 4 cm diameter placed in the lower lobe of the right lung. Lung density was changed for each phase using a conservation of mass calculation. Further, a new heterogeneous lung model was implemented and tested. Each lesion had an internal target volume (ITV) subsequently expanded by 15 mm isotropically to give the planning target volume (PTV). The PTV was prescribed to receive 72 Gy in 40 fractions. The MLC leaf sequence defined by the planning system for each patient was exported and used as input into the MC system. MC simulations using the dose planning method (DPM) code together with deformable image registration based on the NCAT deformation field were used to find a composite dose distribution for each phantom. These composite distributions were subsequently analyzed using information from the dose volume histograms (DVH). Lesion motion amplitude has the largest effect on the dose distribution. Tumor size was found to have a smaller effect and can be mitigated by ensuring the planning constraints are optimized for the tumor size. The use of a dynamic or heterogeneous lung density model over a respiratory cycle does not appear to be an important factor with a <or=0.6% change in the mean dose received by the ITV, PTV and right lung. The heterogeneous model increases the realism of the NCAT phantom and may provide more accurate simulations in radiation therapy investigations that use the phantom. This work further evaluates the NCAT phantom for use as a tool in radiation therapy research in addition to its extensive use in diagnostic imaging and nuclear medicine research. Our results indicate that the NCAT phantom, combined with 4D-MC simulations, is a useful tool in radiation therapy investigations and may allow the study of relative effects in many clinically relevant situations.
这项工作的目的是创建一个用于研究强度调制放射治疗（IMRT）中运动的计算平台。具体来说，对非均匀有理 B 样条（NURB）心脏和躯干（NCAT）体模进行了修改，以便在四维蒙特卡罗（4D-MC）模拟系统中使用，以研究呼吸诱导的分次内器官运动对 IMRT 剂量分布的影响，其影响因素包括膈膜运动、病变大小和肺密度。为四个临床场景设计了治疗计划：膈膜峰值到峰值幅度为 1 厘米和 3 厘米，以及两个病变大小-2 厘米和 4 厘米直径，放置在右肺下叶。使用质量守恒计算为每个相位改变肺密度。此外，还实现并测试了一种新的不均匀肺模型。每个病变都有一个内部靶区（ITV），随后向各向同性扩展 15 毫米以给出计划靶区（PTV）。PTV 规定接受 72 Gy 的 40 个分次。为每个患者导出并用作 MC 系统输入的规划系统定义的 MLC 叶片序列。使用剂量规划方法（DPM）代码与基于 NCAT 变形场的变形图像配准相结合的 MC 模拟，为每个体模找到复合剂量分布。随后使用剂量体积直方图（DVH）中的信息对这些复合分布进行分析。病变运动幅度对剂量分布的影响最大。发现肿瘤大小的影响较小，通过确保为肿瘤大小优化规划约束，可以减轻其影响。在呼吸周期中使用动态或不均匀的肺密度模型似乎不是一个重要因素，对于 ITV、PTV 和右肺的平均剂量，其变化小于或等于 0.6%。不均匀模型增加了 NCAT 体模的现实性，并可能为使用体模进行的放射治疗研究提供更准确的模拟。除了在诊断成像和核医学研究中广泛使用外，这项工作还进一步评估了 NCAT 体模在放射治疗研究中的用途。我们的结果表明，NCAT 体模与 4D-MC 模拟相结合，是放射治疗研究的有用工具，并且可能允许在许多临床相关情况下研究相对影响。
