Zhu Lei, Lee Louis, Ma Yunzhi, Ye Yinyu, Mazzeo Rafe, Xing Lei
Department of Radiation Oncology, Stanford University, Stanford, CA 94305, USA.
Phys Med Biol. 2008 Dec 7;53(23):6653-72. doi: 10.1088/0031-9155/53/23/002. Epub 2008 Nov 7.
Currently, there are two types of treatment planning algorithms for intensity modulated radiation therapy (IMRT). The beamlet-based algorithm generates beamlet intensity maps with high complexity, resulting in large numbers of segments in the delivery after a leaf-sequencing algorithm is applied. The segment-based direct aperture optimization (DAO) algorithm includes the physical constraints of the deliverable apertures in the calculation, and achieves a conformal dose distribution using a small number of segments. However, the number of segments is pre-fixed in most of the DAO approaches, and the typical random search scheme in the optimization is computationally intensive. A regularization-based algorithm is proposed to overcome the drawbacks of the DAO method. Instead of smoothing the beamlet intensity maps as in many existing methods, we include a total-variation term in the optimization objective function to reduce the number of signal levels of the beam intensity maps. An aperture rectification algorithm is then applied to generate a significantly reduced number of deliverable apertures. As compared to the DAO algorithm, our method has an efficient form of quadratic optimization, with an additional advantage of optimizing field-specific numbers of segments based on the modulation complexity. The proposed approach is evaluated using two clinical cases. Under the condition that the clinical acceptance criteria of the treatment plan are satisfied, for the prostate patient, the total number of segments for five fields is reduced from 61 using the Eclipse planning system to 35 using the proposed algorithm; for the head and neck patient, the total number of segments for seven fields is reduced from 107 to 28. The head and neck result is also compared to that using an equal number of four segments for each field. The comparison shows that using field-specific numbers of segments achieves a much improved dose distribution.
目前，调强放射治疗（IMRT）有两种治疗计划算法。基于子野的算法生成复杂度高的子野强度图，在应用叶序算法后，导致射野中有大量子段。基于子段的直接孔径优化（DAO）算法在计算中纳入了可交付孔径的物理约束，并使用少量子段实现了适形剂量分布。然而，在大多数DAO方法中，子段数量是预先固定的，并且优化中典型的随机搜索方案计算量很大。提出了一种基于正则化的算法来克服DAO方法的缺点。与许多现有方法中对子野强度图进行平滑处理不同，我们在优化目标函数中纳入了一个全变差项，以减少射束强度图的信号电平数量。然后应用孔径校正算法来生成数量大幅减少的可交付孔径。与DAO算法相比，我们的方法具有高效的二次优化形式，还有一个额外的优点，即能根据调制复杂度优化特定射野的子段数量。使用两个临床病例对所提出的方法进行了评估。在满足治疗计划的临床验收标准的条件下，对于前列腺患者，五个射野的子段总数从使用Eclipse计划系统的61个减少到使用所提出算法的35个；对于头颈患者，七个射野的子段总数从107个减少到28个。还将头颈病例的结果与每个射野使用相等数量的四个子段的情况进行了比较。比较结果表明，使用特定射野的子段数量可实现显著改善的剂量分布。
