Hardcastle Nicholas, Tome Wolfgang A
Departments of Human Oncology and Medical Physics, University of Wisconsin-Madison, Madison, Wisconsin, USA.
Department of Physical Sciences, Peter MacCallum Cancer Centre, East Melbourne, Victoria, Australia.
J Radiosurg SBRT. 2012;2(1):1-9.
Volumetric modulated arc therapy (VMAT) is a new technique for efficient delivery of intensity modulated dose distributions. This study investigates a single isocenter VMAT technique to treat multiple brain metastases to 15 - 24 Gy. The Pinnacle SmartArc VMAT optimization plugin was used for all VMAT plans. A non-coplanar arc technique using five 100° arcs and one isocenter was compared with a conformal arc technique which used anywhere from 5 to 9 arcs with at least one isocenter per target. Comparison was done using the Conformality Number (CN), Prescription Isodose to Target Volume (PITV), Homogeneity Index (HI), Conformity-Gradient Index (CGI) as well as the 12 Gy isodose volume in the normal brain from which the risk of symptomatic necrosis (S-NEC) was calculated. The VMAT technique resulted in plans with a maximum delivery of 15 minutes, regardless of the number of targets. The VMAT technique provided superior conformity for large targets but for small targets the conformal arc technique resulted in superior conformity. For all targets, the conformal arc technique resulted in superior dose fall off outside of the target. The VMAT technique resulted in an increase in the 12 Gy volume over the conformal arc technique, with an accompanying increase in risk of S-NEC. While the 12 Gy volume was still within an acceptable clinical range, 4 out 20 patients showed a significant increase (15-20%) in absolute risk of S-NEC. Thus the VMAT technique resulted in clinically acceptable plans with vast reductions in treatment time.
容积调强弧形放疗（VMAT）是一种高效输送调强剂量分布的新技术。本研究探讨了一种单等中心VMAT技术，用于将多发脑转移瘤的剂量治疗至15 - 24 Gy。所有VMAT计划均使用了Pinnacle SmartArc VMAT优化插件。将使用五条100°弧和一个等中心的非共面弧形技术与使用5至9条弧且每个靶区至少有一个等中心的适形弧形技术进行了比较。使用适形数（CN）、靶区处方等剂量线（PITV）、均匀性指数（HI）、适形梯度指数（CGI）以及正常脑内12 Gy等剂量体积（据此计算有症状坏死（S - NEC）的风险）进行比较。VMAT技术生成的计划无论靶区数量多少，最大照射时间为15分钟。VMAT技术对大靶区提供了更好的适形性，但对小靶区而言，适形弧形技术的适形性更好。对于所有靶区，适形弧形技术在靶区外的剂量下降更好。与适形弧形技术相比，VMAT技术导致12 Gy体积增加，同时S - NEC风险也随之增加。虽然12 Gy体积仍在可接受的临床范围内，但20例患者中有4例显示S - NEC的绝对风险显著增加（15 - 20%）。因此，VMAT技术生成的计划在临床上是可接受的，且治疗时间大幅缩短。
