Department of Radiation Physics, The University of Texas MD Anderson Cancer Center, Houston, TX, USA.
The University of Texas MD Anderson Cancer Center UTHealth Graduate School of Biomedical Sciences, Houston, TX, USA.
J Appl Clin Med Phys. 2019 Aug;20(8):47-55. doi: 10.1002/acm2.12677. Epub 2019 Jul 11.
The purpose of this study is to investigate the dosimetric impact of multi-leaf collimator (MLC) positioning errors on a Varian Halcyon for both random and systematic errors, and to evaluate the effectiveness of portal dosimetry quality assurance in catching clinically significant changes caused by these errors. Both random and systematic errors were purposely added to 11 physician-approved head and neck volumetric modulated arc therapy (VMAT) treatment plans, yielding a total of 99 unique plans. Plans were then delivered on a preclinical Varian Halcyon linear accelerator and the fluence was captured by an opposed portal dosimeter. When comparing dose-volume histogram (DVH) values of plans with introduced MLC errors to known good plans, clinically significant changes to target structures quickly emerged for plans with systematic errors, while random errors caused less change. For both error types, the magnitude of clinically significant changes increased as error size increased. Portal dosimetry was able to detect all systematic errors, while random errors of ±5 mm or less were unlikely to be detected. Best detection of clinically significant errors, while minimizing false positives, was achieved by following the recommendations of AAPM TG-218. Furthermore, high- to moderate correlation was found between dose DVH metrics for normal tissues surrounding the target and portal dosimetry pass rates. Therefore, it may be concluded that portal dosimetry on the Halcyon is robust enough to detect errors in MLC positioning before they introduce clinically significant changes to VMAT treatment plans.
本研究旨在调查多叶准直器（MLC）定位误差对瓦里安 Halcyon 系统的随机和系统误差的剂量学影响，并评估门控剂量学质量保证在捕捉这些误差引起的临床显著变化方面的有效性。我们有目的地向 11 位经过医生批准的头颈部容积调强弧形治疗（VMAT）治疗计划中添加了随机和系统误差，共生成了 99 个独特的计划。然后，在临床前的瓦里安 Halcyon 线性加速器上交付了这些计划，并使用对置的门控剂量仪捕获了剂量。在将引入 MLC 误差的计划与已知的良好计划的剂量-体积直方图（DVH）值进行比较时，对于具有系统误差的计划，靶结构的临床显著变化很快出现，而随机误差则导致变化较小。对于这两种误差类型，随着误差大小的增加，临床显著变化的幅度也会增加。门控剂量学能够检测到所有的系统误差，而±5 毫米或更小的随机误差则不太可能被检测到。为了最小化假阳性，同时最好地检测到临床显著误差，我们遵循了 AAPM TG-218 的建议。此外，还发现靶周围正常组织的剂量 DVH 指标与门控剂量学通过率之间存在高度到中度相关性。因此，可以得出结论，在多叶准直器定位误差导致 VMAT 治疗计划出现临床显著变化之前，Halcyon 上的门控剂量学足以检测到这些误差。
