Department of Radiation Oncology, Stanford University School of Medicine, Stanford, CA 94305, USA.
Med Phys. 2012 Oct;39(10):6360-71. doi: 10.1118/1.4752444.
The dosimetric leaf gap (DLG) in the Varian Eclipse treatment planning system is determined during commissioning and is used to model the effect of the rounded leaf-end of the multileaf collimator (MLC). This parameter attempts to model the physical difference between the radiation and light field and account for inherent leakage between leaf tips. With the increased use of single fraction high dose treatments requiring larger monitor units comes an enhanced concern in the accuracy of leakage calculations, as it accounts for much of the patient dose. This study serves to verify the dosimetric accuracy of the algorithm used to model the rounded leaf effect for the TrueBeam STx, and describes a methodology for determining best-practice parameter values, given the novel capabilities of the linear accelerator such as flattening filter free (FFF) treatments and a high definition MLC (HDMLC).
During commissioning, the nominal MLC position was verified and the DLG parameter was determined using MLC-defined field sizes and moving gap tests, as is common in clinical testing. Treatment plans were created, and the DLG was optimized to achieve less than 1% difference between measured and calculated dose. The DLG value found was tested on treatment plans for all energies (6 MV, 10 MV, 15 MV, 6 MV FFF, 10 MV FFF) and modalities (3D conventional, IMRT, conformal arc, VMAT) available on the TrueBeam STx.
The DLG parameter found during the initial MLC testing did not match the leaf gap modeling parameter that provided the most accurate dose delivery in clinical treatment plans. Using the physical leaf gap size as the DLG for the HDMLC can lead to 5% differences in measured and calculated doses.
Separate optimization of the DLG parameter using end-to-end tests must be performed to ensure dosimetric accuracy in the modeling of the rounded leaf ends for the Eclipse treatment planning system. The difference in leaf gap modeling versus physical leaf gap dimensions is more pronounced in the more recent versions of Eclipse for both the HDMLC and the Millennium MLC. Once properly commissioned and tested using a methodology based on treatment plan verification, Eclipse is able to accurately model radiation dose delivered for SBRT treatments using the TrueBeam STx.
瓦里安 Eclipse 治疗计划系统中的剂量学叶间隙（DLG）在调试过程中确定，用于模拟多叶准直器（MLC）的圆形叶端的影响。该参数试图模拟辐射和光场之间的物理差异，并考虑到叶尖之间的固有泄漏。随着单次大剂量分割治疗的使用增加，需要更多的监测单位，因此对泄漏计算的准确性更加关注，因为它占患者剂量的很大一部分。本研究旨在验证用于模拟 TrueBeam STx 圆形叶效应的算法的剂量学准确性，并描述一种确定最佳实践参数值的方法，考虑到线性加速器的新功能，如无均整器（FFF）治疗和高分辨率 MLC（HDMLC）。
在调试过程中，验证了标称 MLC 位置，并使用 MLC 定义的射野尺寸和移动间隙测试确定了 DLG 参数，这在临床测试中很常见。创建了治疗计划，并优化了 DLG，以实现测量剂量与计算剂量之间的差异小于 1%。在 TrueBeam STx 上可用的所有能量（6 MV、10 MV、15 MV、6 MV FFF、10 MV FFF）和模式（3D 常规、IMRT、适形弧、VMAT）的治疗计划中测试了找到的 DLG 值。
在初始 MLC 测试中找到的 DLG 参数与在临床治疗计划中提供最准确剂量输送的叶片间隙建模参数不匹配。使用物理叶片间隙尺寸作为 HDMLC 的 DLG 可能导致测量和计算剂量之间出现 5%的差异。
必须使用端到端测试对 DLG 参数进行单独优化，以确保 Eclipse 治疗计划系统中圆形叶片末端建模的剂量学准确性。对于 Eclipse 的较新版本，无论是对于 HDMLC 还是 Millennium MLC，叶片间隙建模与物理叶片间隙尺寸之间的差异都更加明显。一旦使用基于治疗计划验证的方法进行适当的调试和测试，Eclipse 就能够准确地为 TrueBeam STx 的 SBRT 治疗建模辐射剂量。
