Department of Radiation Medicine, Medical Physics Graduate Program, University of Kentucky, Lexington, KY, USA.
Department of Radiation Medicine, University of Kentucky, Markey Cancer Center, Lexington, KY, USA.
J Appl Clin Med Phys. 2020 Jul;21(7):29-38. doi: 10.1002/acm2.12878. Epub 2020 Apr 19.
Volumetric modulated arc therapy (VMAT) is gaining popularity for stereotactic treatment of lung lesions for medically inoperable patients. Due to multiple beamlets in delivery of highly modulated VMAT plans, there are dose delivery uncertainties associated with small-field dosimetry error and interplay effects with small lesions. We describe and compare a clinically useful dynamic conformal arc (DCA)-based VMAT (d-VMAT) technique for lung SBRT using flattening filter free (FFF) beams to minimize these effects.
Ten solitary early-stage I-II non-small-cell lung cancer (NSCLC) patients were treated with a single dose of 30 Gy using 3-6 non-coplanar VMAT arcs (clinical VMAT) with 6X-FFF beams in our clinic. These clinically treated plans were re-optimized using a novel d-VMAT planning technique. For comparison, d-VMAT plans were recalculated using DCA with user-controlled field aperture shape before VMAT optimization. Identical beam geometry, dose calculation algorithm, grid size, and planning objectives were used. The clinical VMAT and d-VMAT plans were compared via RTOG-0915 protocol compliances for conformity, gradient indices, and dose to organs at risk (OAR). Additionally, treatment delivery efficiency and accuracy were recorded.
All plans met RTOG-0915 requirements. Comparing with clinical VMAT, d-VMAT plans gave similar target coverage with better target conformity, tighter radiosurgical dose distribution with lower gradient indices, and dose to OAR. Lower total number of monitor units and small beam modulation factor reduced beam-on time by 1.75 min (P < 0.001), on average (maximum up to 2.52 min). Beam delivery accuracy was improved by 2%, on average (P < 0.05) and maximum up to 6% in some cases for d-VMAT plans.
This simple d-VMAT technique provided excellent plan quality, reduced intermediate dose-spillage, and dose to OAR while providing faster treatment delivery by significantly reducing beam-on time. This novel treatment planning approach will improve patient compliance along with potentially reducing intrafraction motion error. Moreover, with less MLC modulation through the target, d-VMAT could potentially minimize small-field dosimetry errors and MLC interplay effects. If available, d-VMAT planning approach is recommended for future clinical lung SBRT plan optimization.
容积调强弧形治疗（VMAT）因其在无法手术的患者中对肺部病变进行立体定向治疗的优势而越来越受欢迎。由于在高度调制 VMAT 计划的传输中有多个射束，因此与小射野剂量误差和小病变相互作用有关的剂量传输不确定度。我们描述并比较了一种基于动态适形弧（DCA）的用于肺部 SBRT 的临床实用的 VMAT（d-VMAT）技术，该技术使用无均整滤波器（FFF）射线来最小化这些影响。
我们的临床科室对 10 例早期 I-II 期非小细胞肺癌（NSCLC）患者进行了单次 30 Gy 的治疗，使用 3-6 个非共面 VMAT 弧（临床 VMAT），共使用 6X-FFF 射线。在我们的临床科室中，对这些经过临床治疗的计划使用一种新的 d-VMAT 计划技术进行了重新优化。为了进行比较，在 VMAT 优化之前，使用用户控制的射野孔径形状的 DCA 重新计算了 d-VMAT 计划。使用了相同的射束几何形状、剂量计算算法、网格大小和计划目标。根据 RTOG-0915 协议，通过一致性、梯度指数和危及器官（OAR）的剂量对临床 VMAT 和 d-VMAT 计划进行了比较。此外，还记录了治疗的输送效率和准确性。
所有计划均符合 RTOG-0915 的要求。与临床 VMAT 相比，d-VMAT 计划在保证靶区覆盖的同时，提高了靶区适形度，降低了梯度指数，使放射性外科剂量分布更加紧密，降低了危及器官的剂量。平均（最大可达 2.52 分钟）减少了 1.75 分钟的总监测器单位数量和小射束调制因子，减少了束流照射时间（P<0.001）。d-VMAT 计划的束流输送精度平均提高了 2%（P<0.05），在某些情况下最大提高了 6%。
这种简单的 d-VMAT 技术提供了极好的计划质量，降低了中间剂量溢出和危及器官的剂量，同时通过显著减少束流照射时间，实现了更快的治疗输送。这种新的治疗计划方法将提高患者的依从性，同时有可能降低分次内运动误差。此外，由于通过靶区的 MLC 调制减少，d-VMAT 可以潜在地最小化小射野剂量误差和 MLC 相互作用效应。如果可用，建议在未来的肺部 SBRT 计划优化中使用 d-VMAT 计划方法。
