Wiersma R D, Xing L
Department of Radiation Oncology, Stanford University School of Medicine, Stanford, California 94305-5847, USA.
Med Phys. 2007 Oct;34(10):3962-70. doi: 10.1118/1.2776671.
Due to the complicated technical nature of gated radiation therapy, electronic and mechanical limitations may affect the precision of delivery. The purpose of this study is to investigate the geometric and dosimetric accuracies of gated step-and-shoot intensity modulated radiation treatments (SS-IMRT). Unique segmental MLC plans are designed, which allow quantitative testing of the gating process. Both ungated and gated deliveries are investigated for different dose sizes, dose rates, and gating window times using a commercial treatment system (Varian Trilogy) together with a respiratory gating system [Varian Real-Time Position Management system]. Radiographic film measurements are used to study the geometric accuracy, where it is found that with both ungated and gated SS-IMRT deliveries the MLC leaf divergence away from planned is less than or equal to the MLC specified leaf tolerance value for all leafs (leaf tolerance being settable from 0.5-5 mm). Nevertheless, due to the MLC controller design, failure to define a specific leaf tolerance value suitable to the SS-IMRT plan can lead to undesired geometric effects, such as leaf motion of up to the maximum 5 mm leaf tolerance value occurring after the beam is turned on. In this case, gating may be advantageous over the ungated case, as it allows more time for the MLC to reach the intended leaf configuration. The dosimetric precision of gated SS-IMRT is investigated using ionization chamber methods. Compared with the ungated case, it is found that gating generally leads to increased dosimetric errors due to the interruption of the "overshoot phenomena." With gating the average timing deviation for intermediate segments is found to be 27 ms, compared to 18 ms for the ungated case. For a plan delivered at 600 MU/min this would correspond to an average segment dose error of approximately 0.27 MU and approximately 0.18 MU for gated and ungated deliveries, respectively. The maximum dosimetric errors for individual intermediate segments are found to deviate by up to approximately 0.64 MU from their planned value when delivered at 600 MU/min using gating, this compares to only approximately 0.32 MU for the ungated case.
由于门控放射治疗技术复杂，电子和机械限制可能会影响治疗的精确性。本研究旨在探讨门控步进式调强放射治疗（SS-IMRT）的几何和剂量学准确性。设计了独特的分段多叶准直器（MLC）计划，可对门控过程进行定量测试。使用商业治疗系统（瓦里安Trilogy）和呼吸门控系统[瓦里安实时位置管理系统]，针对不同的剂量大小、剂量率和门控窗口时间，研究了非门控和门控照射。通过射线照相胶片测量来研究几何准确性，结果发现，对于非门控和门控的SS-IMRT照射，所有叶片的MLC叶片偏离计划的程度均小于或等于MLC规定的叶片公差值（叶片公差可设置为0.5 - 5毫米）。然而，由于MLC控制器的设计，如果未能定义适合SS-IMRT计划的特定叶片公差值，可能会导致不良的几何效应，例如在射束开启后出现高达最大5毫米叶片公差值的叶片运动。在这种情况下，门控可能比非门控情况更具优势，因为它为MLC达到预期叶片配置留出了更多时间。使用电离室方法研究了门控SS-IMRT的剂量学精度。与非门控情况相比，发现门控通常会由于“过冲现象”的中断而导致剂量学误差增加。门控时，中间段的平均定时偏差为27毫秒，而非门控情况为18毫秒。对于以600 MU/分钟的速度输送的计划，这分别对应于门控和非门控照射时中间段平均剂量误差约为0.27 MU和0.18 MU。当以600 MU/分钟的速度使用门控输送时，发现单个中间段的最大剂量学误差与其计划值的偏差高达约0.64 MU，相比之下，非门控情况仅约为0.32 MU。
