Swedish Cancer Institute, Seattle, WA 98104, USA.
Med Phys. 2011 Nov;38(11):6106-18. doi: 10.1118/1.3651698.
Intensity modulated arc therapy (IMAT) is a radiation therapy delivery technique that combines the efficiency of arc based delivery with the dose painting capabilities of intensity modulated radiation therapy (IMRT). A key challenge in developing robust inverse planning solutions for IMAT is the need to account for the connectivity of the beam shapes as the gantry rotates from one beam angle to the next. To overcome this challenge, inverse planning solutions typically impose a leaf motion constraint that defines the maximum distance a multileaf collimator (MLC) leaf can travel between adjacent control points. The leaf motion constraint ensures the deliverability of the optimized plan, but it also impacts the plan quality, the delivery accuracy, and the delivery efficiency. In this work, the authors have studied leaf motion constraints in detail and have developed recommendations for optimizing the balance between plan quality and delivery efficiency.
Two steps were used to generate optimized IMAT treatment plans. The first was the direct machine parameter optimization (DMPO) inverse planning module in the Pinnacle(3) planning system. Then, a home-grown arc sequencer was applied to convert the optimized intensity maps into deliverable IMAT arcs. IMAT leaf motion constraints were imposed using limits of between 1 and 30 mm∕deg. Dose distributions were calculated using the convolution∕superposition algorithm in the Pinnacle(3) planning system. The IMAT plan dose calculation accuracy was examined using a finer sampling calculation and the quality assurance verification. All plans were delivered on an Elekta Synergy with an 80-leaf MLC and were verified using an IBA MatriXX 2D ion chamber array inserted in a MultiCube solid water phantom.
The use of a more restrictive leaf motion constraint (less than 1-2 mm∕deg) results in inferior plan quality. A less restrictive leaf motion constraint (greater than 5 mm∕deg) results in improved plan quality but can lead to less accurate dose distribution as evidenced by increasing discrepancies between the planned and the delivered doses. For example, the results from our patient-specific quality assurance measurements demonstrated that the average gamma analysis passing rate decreased from 98% to 80% when the allowable leaf motion increased from 3 to 20 mm∕deg. Larger leaf motion constraints also led to longer treatment delivery times (2 to 4 folds) due to the additional MLC leaf motion.
Leaf motion constraints significantly impact IMAT plans in terms of plan quality, delivery accuracy, and delivery efficiency with the impact magnified for more complex cases. Our studies indicate that a leaf motion constraint of 2 to 3 mm∕deg of gantry rotation can provide an optimal balance between plan quality, delivery accuracy, and efficiency.
调强弧形治疗（IMAT）是一种放射治疗输送技术，它结合了基于弧形的输送效率和强度调制放射治疗（IMRT）的剂量绘制能力。在为 IMAT 开发强大的逆规划解决方案时，一个关键挑战是需要考虑在机架从一个射束角度旋转到下一个射束角度时射束形状的连通性。为了克服这一挑战，逆规划解决方案通常采用叶片运动约束，该约束定义了多叶准直器（MLC）叶片在相邻控制点之间能够移动的最大距离。叶片运动约束确保了优化计划的可交付性，但它也会影响计划质量、交付准确性和交付效率。在这项工作中，作者详细研究了叶片运动约束，并提出了优化计划质量和交付效率之间平衡的建议。
采用两步法生成优化的 IMAT 治疗计划。第一步是 Pinnacle(3) 计划系统中的直接机器参数优化（DMPO）逆规划模块。然后，应用内部开发的弧形序列器将优化的强度图转换为可交付的 IMAT 弧形。采用 1 至 30 毫米/度的限制施加 IMAT 叶片运动约束。使用 Pinnacle(3) 计划系统中的卷积/叠加算法计算剂量分布。通过更精细的抽样计算和质量保证验证检查 IMAT 计划剂量计算的准确性。所有计划均在 Elekta Synergy 上使用 80 叶 MLC 进行交付，并使用插入 MultiCube 固体水模体中的 IBA MatriXX 2D 离子室阵列进行验证。
使用更严格的叶片运动约束（小于 1-2 毫米/度）会导致计划质量下降。使用较不严格的叶片运动约束（大于 5 毫米/度）会改善计划质量，但会导致剂量分布的准确性降低，这体现在计划剂量和交付剂量之间的差异增加。例如，我们的患者特定质量保证测量结果表明，当允许的叶片运动从 3 毫米增加到 20 毫米时，平均伽马分析通过率从 98%下降到 80%。较大的叶片运动约束还会由于额外的 MLC 叶片运动而导致更长的治疗输送时间（2 到 4 倍）。
叶片运动约束会显著影响 IMAT 计划的计划质量、交付准确性和交付效率，对于更复杂的情况，影响会更大。我们的研究表明，机架旋转 2 至 3 毫米/度的叶片运动约束可以在计划质量、交付准确性和效率之间提供最佳平衡。
