Department of Radiation Oncology, College of Medicine, University of Florida, Gainesville, FL, USA.
Department of Industrial & Systems Engineering, College of Engineering, University of Florida, Gainesville, FL, USA.
J Appl Clin Med Phys. 2020 Mar;21(3):142-152. doi: 10.1002/acm2.12837.
Flattening filter free (FFF) linear accelerators produce a fluence distribution that is forward peaked. Various dosimetric benefits, such as increased dose rate, reduced leakage and out of field dose has led to the growth of FFF technology in the clinic. The literature has suggested the idea of vendors offering dedicated FFF units where the flattening filter (FF) is removed completely and manipulating the beam to deliver conventional flat radiotherapy treatments. This work aims to develop an effective way to deliver modulated flat beam treatments, rather than utilizing a physical FF. This novel optimization model is an extension of the direct leaf trajectory optimization (DLTO) previously developed for volumetric modulated radiation therapy (VMAT) and is capable of accounting for all machine and multileaf collimator (MLC) dynamic delivery constraints, using a combination of linear constraints and a convex objective function. Furthermore, the tongue and groove (T&G) effect was also incorporated directly into our model without introducing nonlinearity to the constraints, nor nonconvexity to the objective function. The overall beam flatness, machine deliverability, and treatment time efficiency were assessed. Regular square fields, including field sizes of 10 × 10 cm to 40 × 40 cm were analyzed, as well as three clinical fields, and three arbitrary contours with "concave" features. Quantitative flatness was measured for all modulated FFF fields, and the results were comparable or better than their open FF counterparts, with the majority having a quantitative flatness of less than 3.0%. The modulated FFF beams, due to the included efficiency constraint, were able to achieve acceptable delivery time compared to their open FF counterpart. The results indicated that the dose uniformity and flatness for the modulated FFF beams optimized with the DLTO model can successfully match the uniformity and flatness of their conventional FF counterparts, and may even provide further benefit by taking advantage of the unique FFF beam characteristics.
无均整过滤器（FFF）线性加速器产生前向峰值的剂量分布。剂量学方面的各种优势，如提高剂量率、减少漏射线和野外剂量，导致 FFF 技术在临床上的发展。文献提出了供应商提供专用 FFF 单元的想法，在该单元中完全去除均整器（FF）并操纵射束以提供常规的平坦放射治疗。本工作旨在开发一种有效的方法来提供调制平坦射束治疗，而不是利用物理 FF。这种新颖的优化模型是以前为容积调制放射治疗（VMAT）开发的直接叶片轨迹优化（DLTO）的扩展，能够考虑到所有机器和多叶准直器（MLC）动态传递限制，使用线性约束和凸目标函数的组合。此外，还直接将榫槽（T&G）效应纳入我们的模型中，而不会对约束引入非线性，也不会对目标函数引入非凸性。评估了整体射束平坦度、机器可传递性和治疗时间效率。分析了常规方形野，包括 10×10cm 至 40×40cm 的射野尺寸，以及三个临床射野和三个具有“凹”特征的任意轮廓。对所有调制 FFF 射野进行了定量平坦度测量，结果与开放 FF 射野相当或更好，大多数定量平坦度小于 3.0%。由于包含效率约束，调制 FFF 射束能够与开放 FF 射束相比实现可接受的传递时间。结果表明，使用 DLTO 模型优化的调制 FFF 射束的剂量均匀性和平坦度可以成功匹配其常规 FF 射束的均匀性和平坦度，甚至可以通过利用 FFF 射束的独特特性提供进一步的好处。
