Wild Esther, Bangert Mark, Nill Simeon, Oelfke Uwe
Department of Medical Physics in Radiation Oncology, German Cancer Research Center, Im Neuenheimer Feld 280, D-69120 Heidelberg, Germany.
Joint Department of Physics at The Institute of Cancer Research and The Royal Marsden NHS Foundation Trust, London SM2 5NG, United Kingdom.
Med Phys. 2015 May;42(5):2157-68. doi: 10.1118/1.4914863.
The authors investigated the potential of optimized noncoplanar irradiation trajectories for volumetric modulated arc therapy (VMAT) treatments of nasopharyngeal patients and studied the trade-off between treatment plan quality and delivery time in radiation therapy.
For three nasopharyngeal patients, the authors generated treatment plans for nine different delivery scenarios using dedicated optimization methods. They compared these scenarios according to dose characteristics, number of beam directions, and estimated delivery times. In particular, the authors generated the following treatment plans: (1) a 4π plan, which is a not sequenced, fluence optimized plan that uses beam directions from approximately 1400 noncoplanar directions and marks a theoretical upper limit of the treatment plan quality, (2) a coplanar 2π plan with 72 coplanar beam directions as pendant to the noncoplanar 4π plan, (3) a coplanar VMAT plan, (4) a coplanar step and shoot (SnS) plan, (5) a beam angle optimized (BAO) coplanar SnS IMRT plan, (6) a noncoplanar BAO SnS plan, (7) a VMAT plan with rotated treatment couch, (8) a noncoplanar VMAT plan with an optimized great circle around the patient, and (9) a noncoplanar BAO VMAT plan with an arbitrary trajectory around the patient.
VMAT using optimized noncoplanar irradiation trajectories reduced the mean and maximum doses in organs at risk compared to coplanar VMAT plans by 19% on average while the target coverage remains constant. A coplanar BAO SnS plan was superior to coplanar SnS or VMAT; however, noncoplanar plans like a noncoplanar BAO SnS plan or noncoplanar VMAT yielded a better plan quality than the best coplanar 2π plan. The treatment plan quality of VMAT plans depended on the length of the trajectory. The delivery times of noncoplanar VMAT plans were estimated to be 6.5 min in average; 1.6 min longer than a coplanar plan but on average 2.8 min faster than a noncoplanar SnS plan with comparable treatment plan quality.
The authors' study reconfirms the dosimetric benefits of noncoplanar irradiation of nasopharyngeal tumors. Both SnS using optimized noncoplanar beam ensembles and VMAT using an optimized, arbitrary, noncoplanar trajectory enabled dose reductions in organs at risk compared to coplanar SnS and VMAT. Using great circles or simple couch rotations to implement noncoplanar VMAT, however, was not sufficient to yield meaningful improvements in treatment plan quality. The authors estimate that noncoplanar VMAT using arbitrary optimized irradiation trajectories comes at an increased delivery time compared to coplanar VMAT yet at a decreased delivery time compared to noncoplanar SnS IMRT.
作者研究了优化的非共面照射轨迹用于鼻咽癌患者容积调强弧形治疗（VMAT）的潜力，并研究了放射治疗中治疗计划质量与照射时间之间的权衡。
对于三名鼻咽癌患者，作者使用专用优化方法针对九种不同的照射方案生成了治疗计划。他们根据剂量特征、射束方向数量和估计的照射时间对这些方案进行了比较。具体而言，作者生成了以下治疗计划：（1）一个4π计划，这是一个未排序的、通量优化的计划，使用来自大约1400个非共面方向的射束方向，标志着治疗计划质量的理论上限；（2）一个具有72个共面射束方向的共面2π计划，作为非共面4π计划的对应方案；（3）一个共面VMAT计划；（4）一个共面步进式射野（SnS）计划；（5）一个射束角度优化（BAO）的共面SnS调强放疗（IMRT）计划；（6）一个非共面BAO SnS计划；（7）一个治疗床旋转的VMAT计划；（8）一个围绕患者优化大圆的非共面VMAT计划；（9）一个围绕患者任意轨迹的非共面BAO VMAT计划。
与共面VMAT计划相比，使用优化的非共面照射轨迹的VMAT使危及器官的平均剂量和最大剂量平均降低了19%，而靶区覆盖保持不变。一个共面BAO SnS计划优于共面SnS或VMAT计划；然而，像非共面BAO SnS计划或非共面VMAT这样的非共面计划产生的计划质量比最佳共面2π计划更好。VMAT计划的治疗计划质量取决于轨迹的长度。非共面VMAT计划的照射时间估计平均为6.5分钟；比共面计划长1.6分钟，但与具有可比治疗计划质量的非共面SnS计划相比平均快2.8分钟。
作者的研究再次证实了鼻咽癌肿瘤非共面照射在剂量学方面的益处。与共面SnS和VMAT相比，使用优化非共面射束组合的SnS以及使用优化的、任意的、非共面轨迹的VMAT均能降低危及器官的剂量。然而，使用大圆或简单的治疗床旋转来实施非共面VMAT不足以在治疗计划质量上产生有意义的改善。作者估计，与共面VMAT相比，使用任意优化照射轨迹的非共面VMAT照射时间会增加，但与非共面SnS IMRT相比照射时间会减少。
