Shepard D M, Earl M A, Li X A, Naqvi S, Yu C
University of Maryland School of Medicine, Department of Radiation Oncology, Baltimore 21201-1595, USA.
Med Phys. 2002 Jun;29(6):1007-18. doi: 10.1118/1.1477415.
IMRT treatment plans for step-and-shoot delivery have traditionally been produced through the optimization of intensity distributions (or maps) for each beam angle. The optimization step is followed by the application of a leaf-sequencing algorithm that translates each intensity map into a set of deliverable aperture shapes. In this article, we introduce an automated planning system in which we bypass the traditional intensity optimization, and instead directly optimize the shapes and the weights of the apertures. We call this approach "direct aperture optimization." This technique allows the user to specify the maximum number of apertures per beam direction, and hence provides significant control over the complexity of the treatment delivery. This is possible because the machine dependent delivery constraints imposed by the MLC are enforced within the aperture optimization algorithm rather than in a separate leaf-sequencing step. The leaf settings and the aperture intensities are optimized simultaneously using a simulated annealing algorithm. We have tested direct aperture optimization on a variety of patient cases using the EGS4/BEAM Monte Carlo package for our dose calculation engine. The results demonstrate that direct aperture optimization can produce highly conformal step-and-shoot treatment plans using only three to five apertures per beam direction. As compared with traditional optimization strategies, our studies demonstrate that direct aperture optimization can result in a significant reduction in both the number of beam segments and the number of monitor units. Direct aperture optimization therefore produces highly efficient treatment deliveries that maintain the full dosimetric benefits of IMRT.
传统上，用于步进式射野放疗的调强放疗（IMRT）治疗计划是通过优化每个射野角度的强度分布（或剂量图）来生成的。优化步骤之后是应用叶序算法，该算法将每个强度图转换为一组可交付的射野形状。在本文中，我们介绍一种自动计划系统，在该系统中我们绕过传统的强度优化，而是直接优化射野的形状和权重。我们将这种方法称为“直接射野优化”。该技术允许用户指定每个射野方向的最大射野数，从而对治疗实施的复杂性提供显著控制。这是可行的，因为由多叶准直器（MLC）施加的与机器相关的实施约束在射野优化算法内得到执行，而不是在单独的叶序步骤中。使用模拟退火算法同时优化叶片设置和射野强度。我们使用EGS4/BEAM蒙特卡罗软件包作为剂量计算引擎，在各种患者病例上测试了直接射野优化。结果表明，直接射野优化每个射野方向仅使用三到五个射野就能产生高度适形的步进式射野治疗计划。与传统优化策略相比，我们的研究表明直接射野优化可显著减少射野段数和监测单位数。因此，直接射野优化可产生高效的治疗实施，同时保持IMRT的全部剂量学优势。
