Department of Radiation Oncology, Stanford University, Stanford, California 94305-5847, USA.
Med Phys. 2013 May;40(5):050701. doi: 10.1118/1.4802748.
Conventional volumetric modulated arc therapy (VMAT) discretizes the angular space into equally spaced control points during planning and then optimizes the apertures and weights of the control points. The aperture at an angle in between two control points is obtained through interpolation. This approach tacitly ignores the differential need for intensity modulation of different angles. As such, multiple arcs are often required, which may oversample some angle(s) and undersample others. The purpose of this work is to develop a segmentally boosted VMAT scheme to eliminate the need for multiple arcs in VMAT treatment with improved dose distribution and∕or delivery efficiency.
The essence of the new treatment scheme is how to identify the need of individual angles for intensity modulation and to provide the necessary beam intensity modulation for those beam angles that need it. We introduce a "demand metric" at each control point to decide which station or control points need intensity modulation. To boost the modulation at selected stations, additional segments are added in the vicinity of the selected stations. The added segments are then optimized together with the original set of station or control points as a whole. The authors apply the segmentally boosted planning technique to four previously treated clinical cases: two head and neck (HN) cases, one prostate case, and one liver case. The proposed planning technique is compared with conventional one-arc and two-arc VMAT.
The proposed segmentally boosted VMAT technique achieves better critical structure sparing than one-arc VMAT with similar or better target coverage in all four clinical cases. The segmentally boosted VMAT also outperforms two-arc VMAT for the two complicated HN cases, yet with ∼30% reduction in the machine monitor units (MUs) relative to two-arc VMAT, which leads to less leakage∕scatter dose to the patient and can potentially translate into faster dose delivery. For the less challenging prostate and liver cases, similar critical structure sparing as the two-arc VMAT plans was obtained using the segmentally boosted VMAT. The benefit for the two simpler cases is the reduction of MUs and improvement of treatment delivery efficiency.
Segmentally boosted VMAT achieves better dose conformality and∕or reduced MUs through effective consideration of the need of individual beam angles for intensity modulation. Elimination of the need for multiple arcs in rotational arc therapy while improving the dose distribution should lead to improved workflow and treatment efficacy, thus may have significant implication to radiation oncology practice.
传统的容积调强弧形治疗（VMAT）在规划过程中将角空间离散化为等距的控制点，然后优化控制点的孔径和权重。两个控制点之间的角度的孔径通过插值获得。这种方法默认忽略了不同角度的强度调制的不同需求。因此，通常需要多个弧，这可能会对某些角度过度采样，而对其他角度欠采样。本工作的目的是开发分段增强的 VMAT 方案，以消除 VMAT 治疗中对多个弧的需求，同时改善剂量分布和/或提高治疗效率。
新治疗方案的本质是如何识别各个角度对强度调制的需求，并为需要的光束角度提供必要的光束强度调制。我们在每个控制点引入一个“需求指标”，以决定哪些站点或控制点需要强度调制。为了增强选定站点的调制，在选定站点附近添加附加段。然后，将添加的段与原始站点或控制点集一起进行整体优化。作者将分段增强的规划技术应用于四个先前治疗的临床病例：两个头颈部（HN）病例、一个前列腺病例和一个肝脏病例。所提出的规划技术与传统的单弧和双弧 VMAT 进行了比较。
所提出的分段增强 VMAT 技术在所有四个临床病例中都实现了比单弧 VMAT 更好的关键结构保护，同时具有相似或更好的靶区覆盖率。分段增强 VMAT 也优于两个 HN 复杂病例的双弧 VMAT，而相对于双弧 VMAT ，机器监测单位（MU）减少了约 30%，这导致对患者的泄漏/散射剂量减少，并且可以潜在地转化为更快的剂量输送。对于挑战性较小的前列腺和肝脏病例，使用分段增强 VMAT 获得了与双弧 VMAT 计划相似的关键结构保护。对于两个更简单的病例，其优势在于 MU 的减少和治疗输送效率的提高。
分段增强 VMAT 通过有效考虑各个光束角度对强度调制的需求，实现了更好的剂量适形性和/或减少 MU。在旋转弧形治疗中消除对多个弧的需求，同时改善剂量分布，应该会提高工作流程和治疗效果，因此可能对放射肿瘤学实践有重大意义。
