Peter MacCallum Cancer Centre, Melbourne, VIC, Australia.
Centre for Medical Radiation Physics, University of Wollongong, Wollongong, NSW, Australia.
J Appl Clin Med Phys. 2020 Aug;21(8):299-304. doi: 10.1002/acm2.12919. Epub 2020 May 29.
Peripheral lung lesions treated with a single fraction of stereotactic ablative body radiotherapy (SABR) utilizing volumetric modulated arc therapy (VMAT) delivery and flattening filter-free (FFF) beams represent a potentially high-risk scenario for clinically significant dose blurring effects due to interplay between the respiratory motion of the lesion and dynamic multi-leaf collimators (MLCs). The aim of this study was to determine an efficient means of developing low-modulation VMAT plans in the Eclipse treatment planning system (v15.5, Varian Medical Systems, Palo Alto, USA) in order to minimize this risk, while maintaining dosimetric quality. The study involved 19 patients where an internal target volume (ITV) was contoured to encompass the entire range of tumor motion, and a planning target volume (PTV) created using a 5-mm isotropic expansion of this contour. Each patient had seven plan variations created, with each rescaled to achieve the clinical planning goal for PTV coverage. All plan variations used the same field arrangement, and consisted of one dynamic conformal arc therapy (DCAT) plan, and six VMAT plans with varying degrees of modulation restriction, achieved through utilizing different combinations of the aperture shape controller (ASC) in the calculation parameters, and monitor unit (MU) objective during optimization. The dosimetric quality was assessed based on RTOG conformity indices (CI100/CI50), as well as adherence to dose-volume metrics used clinically at our institution. Plan complexity was assessed based on the modulation factor (MU/cGy) and the field edge metric. While VMAT plans with the least modulation restriction achieved the best dosimetry, it was found that there was no clinically significant trade-off in terms of dose to organs at risk and conformity by reducing complexity. Furthermore, it was found that utilizing the ASC and MU objective could reduce plan complexity to near-DCAT levels with improved dosimetry, which may be sufficiently robust to overcome the interplay effect.
采用容积旋转调强放疗（VMAT）技术和非均整滤过（FFF）射线进行单次分割立体定向消融放疗（SABR）治疗的周边肺部病变，由于病变的呼吸运动与动态多叶准直器（MLC）之间的相互作用，可能会导致临床上显著的剂量模糊效应，存在较高风险。本研究旨在确定一种在 Eclipse 治疗计划系统（v15.5，美国瓦里安医疗系统公司）中开发低调制 VMAT 计划的有效方法，以最大程度地降低这种风险，同时保持剂量学质量。本研究涉及 19 名患者，其中内部靶区（ITV）被勾画以包含肿瘤运动的整个范围，而计划靶区（PTV）则是通过对该轮廓进行 5mm 各向同性扩展来创建。为了实现 PTV 覆盖的临床计划目标，对每个患者创建了 7 种计划变化，每种变化都进行了缩放。所有计划变化都使用相同的射野排布，由一个动态适形弧治疗（DCAT）计划和六个不同程度调制限制的 VMAT 计划组成，通过在计算参数中使用不同的孔径形状控制器（ASC）组合以及在优化过程中使用不同的 MU 目标来实现。剂量学质量评估基于 RTOG 适形指数（CI100/CI50），以及在我们机构临床使用的剂量-体积指标的符合程度。计划复杂性评估基于调制因子（MU/cGy）和射野边缘指标。尽管调制限制最小的 VMAT 计划实现了最佳的剂量学效果，但研究发现，通过降低复杂性，在危及器官的剂量和适形性方面没有明显的临床权衡。此外，研究发现，使用 ASC 和 MU 目标可以将计划复杂性降低到接近 DCAT 的水平，同时改善剂量学，这可能足以克服相互作用效应。
