Berbeco Ross I, Pope Cynthia J, Jiang Steve B
Department of Radiation Oncology, Massachusetts General Hospital and Harvard Medical School, Boston, Massachusetts, USA.
J Appl Clin Med Phys. 2006 Nov 28;7(4):33-42. doi: 10.1120/jacmp.v7i4.2222.
Intra-fraction organ motion during the dynamic delivery of IMRT treatment of lung tumors may cause unexpected hot/cold spots even within the target volume, due to the interplay effect between tumor motion and multi-leaf collimator (MLC) leaf motion. In the past, this has been investigated through theoretical analysis, computer simulation, and experimental measurement using an ionization chamber dosimeter. In the work presented here, the interplay effect was studied experimentally in 2D, using Kodak EDR2 films. A five-field lung IMRT plan was delivered to a solid water phantom with embedded film. The phantom was placed on a motor-driven platform with a sinusoidal motion to simulate the respiration induced tumor motion. The delivery of each field began at one of eight equally spaced initial breathing phases. The dose distribution for each treatment fraction was estimated by combining the dose distributions for all fields with randomly sampled initial breathing phases. The dose variation caused by the interplay effect was estimated by looking at the dose values from 1000 trials of 30 fractions. It was found that, on a day-to-day basis, the standard deviation of the dose to a given pixel in the high dose region could be as high as 2-3% due to the motion interplay effect. After thirty fractions, the standard deviation in the dose to each pixel is reduced to about 0.3-0.5%. However, compared to the static delivery, the dose distribution from a thirty-fraction case in the presence of motion shows some under-dosing in the region of interest. Our results indicate that the interplay effect is of no significant consequence to tumor dose coverage in lung IMRT delivery over the entire course of a thirty-fraction treatment.
在肺癌调强放射治疗（IMRT）动态照射过程中，由于肿瘤运动与多叶准直器（MLC）叶片运动之间的相互作用，分次治疗期间器官内部运动甚至可能在靶区内导致意外的热点或冷点。过去，人们通过理论分析、计算机模拟以及使用电离室剂量仪进行实验测量来对此进行研究。在本文所展示的工作中，使用柯达EDR2胶片在二维空间对相互作用效应进行了进行了实验研究。将一个五野肺癌IMRT计划照射到嵌入胶片的固体水模体上。该模体放置在一个带有正弦运动的电动平台上，以模拟呼吸引起的肿瘤运动。每个射野的照射从八个等间隔的初始呼吸相位之一开始。通过将所有射野在随机采样的初始呼吸相位下的剂量分布相结合，来估计每个治疗分次的剂量分布。通过查看30个分次的1000次试验的剂量值，来估计由相互作用效应引起的剂量变化。结果发现，在日常情况下，由于运动相互作用效应，高剂量区域中给定像素的剂量标准差可能高达2% - 3%。经过30个分次后，每个像素剂量的标准差降至约0.3% - 0.5%。然而，与静态照射相比，存在运动情况下30个分次病例的剂量分布在感兴趣区域显示出一些剂量不足。我们的结果表明，在30个分次的整个治疗过程中，相互作用效应对于肺癌IMRT照射中肿瘤剂量覆盖没有显著影响。
