Chui Chen-Shou, Yorke Ellen, Hong Linda
Department of Medical Physics, Memorial Sloan-Kettering Cancer Center, 1275 York Ave., New York, New York 10021, USA.
Med Phys. 2003 Jul;30(7):1736-46. doi: 10.1118/1.1578771.
Intensity-modulated radiation therapy can be conveniently delivered with a multileaf collimator. With this method, the entire field is not delivered at once, but rather it is composed of many subfields defined by the leaf positions as a function of beam on time. At any given instant, only these subfields are delivered. During treatment, if the organ moves, part of the volume may move in or out of these subfields. Due to this interplay between organ motion and leaf motion the delivered dose may be different from what was planned. In this work, we present a method that calculates the effects of organ motion on delivered dose. The direction of organ motion may be parallel or perpendicular to the leaf motion, and the effect can be calculated for a single fraction or for multiple fractions. Three breast patients and four lung patients were included in this study,with the amplitude of the organ motion varying from +/- 3.5 mm to +/- 10 mm, and the period varying from 4 to 8 seconds. Calculations were made for these patients with and without organ motion, and results were examined in terms of isodose distribution and dose volume histograms. Each calculation was repeated ten times in order to estimate the statistical uncertainties. For selected patients, calculations were also made with conventional treatment technique. The effects of organ motion on conventional techniques were compared relative to that on IMRT techniques. For breast treatment, the effect of organ motion primarily broadened the penumbra at the posterior field edge. The dose in the rest of the treatment volume was not significantly affected. For lung treatment, the effect also broadened the penumbra and degraded the coverage of the planning target volume (PTV). However, the coverage of the clinical target volume (CTV) was not much affected, provided the PTV margin was adequate. The same effects were observed for both IMRT and conventional treatment techniques. For the IMRT technique, the standard deviations of ten samples of a 30-fraction calculation were very small for all patients, implying that over a typical treatment course of 30 fractions, the delivered dose was very close to the expected value. Hence, under typical clinical conditions, the effect of organ motion on delivered dose can be calculated without considering the interplay between the organ motion and the leaf motion. It can be calculated as the weighted average of the dose distribution without organ motion with the distribution of organ motion. Since the effects of organ motion on dose were comparable for both IMRT and conventional techniques, the PTV margin should remain the same for both techniques.
调强放射治疗可以通过多叶准直器方便地实施。采用这种方法时，整个射野并非一次性照射，而是由许多子野组成，这些子野由叶片位置根据束流开启时间来定义。在任何给定时刻，仅照射这些子野。在治疗过程中，如果器官移动，部分体积可能移入或移出这些子野。由于器官运动和叶片运动之间的这种相互作用，实际给予的剂量可能与计划剂量不同。在这项工作中，我们提出了一种计算器官运动对实际给予剂量影响的方法。器官运动的方向可能与叶片运动平行或垂直，并且可以针对单次分割或多次分割计算其影响。本研究纳入了3例乳腺癌患者和4例肺癌患者，器官运动的幅度在±3.5毫米至±10毫米之间变化，周期在4至8秒之间变化。对这些患者在有和没有器官运动的情况下进行了计算，并根据等剂量分布和剂量体积直方图检查结果。为了估计统计不确定性，每个计算重复了十次。对于选定的患者，还采用传统治疗技术进行了计算。将器官运动对传统技术的影响与对调强放疗技术的影响进行了比较。对于乳腺癌治疗，器官运动的影响主要是使后野边缘的半影变宽。治疗体积其余部分的剂量未受到显著影响。对于肺癌治疗，这种影响同样使半影变宽并降低了计划靶区（PTV）的覆盖度。然而，只要PTV边界足够，临床靶区（CTV）的覆盖度受影响不大。调强放疗技术和传统治疗技术都观察到了相同的影响。对于调强放疗技术，所有患者30次分割计算的十个样本的标准差都非常小，这意味着在典型的30次分割治疗疗程中，实际给予的剂量非常接近预期值。因此，在典型的临床条件下，可以在不考虑器官运动和叶片运动之间相互作用的情况下计算器官运动对实际给予剂量的影响。它可以作为无器官运动时的剂量分布与器官运动分布的加权平均值来计算。由于器官运动对剂量的影响在调强放疗技术和传统技术中相当，两种技术的PTV边界应该保持相同。
