Nguyen Dan, Lyu Qihui, Ruan Dan, O'Connor Daniel, Low Daniel A, Sheng Ke
Department of Radiation Oncology, University of California Los Angeles, Los Angeles, California 90024.
Med Phys. 2016 Jul;43(7):4263. doi: 10.1118/1.4953832.
Volumetric modulated arc therapy (VMAT) is a widely employed radiation therapy technique, showing comparable dosimetry to static beam intensity modulated radiation therapy (IMRT) with reduced monitor units and treatment time. However, the current VMAT optimization has various greedy heuristics employed for an empirical solution, which jeopardizes plan consistency and quality. The authors introduce a novel direct aperture optimization method for VMAT to overcome these limitations.
The comprehensive VMAT (comVMAT) planning was formulated as an optimization problem with an L2-norm fidelity term to penalize the difference between the optimized dose and the prescribed dose, as well as an anisotropic total variation term to promote piecewise continuity in the fluence maps, preparing it for direct aperture optimization. A level set function was used to describe the aperture shapes and the difference between aperture shapes at adjacent angles was penalized to control MLC motion range. A proximal-class optimization solver was adopted to solve the large scale optimization problem, and an alternating optimization strategy was implemented to solve the fluence intensity and aperture shapes simultaneously. Single arc comVMAT plans, utilizing 180 beams with 2° angular resolution, were generated for a glioblastoma multiforme case, a lung (LNG) case, and two head and neck cases-one with three PTVs (H&N3PTV) and one with foue PTVs (H&N4PTV)-to test the efficacy. The plans were optimized using an alternating optimization strategy. The plans were compared against the clinical VMAT (clnVMAT) plans utilizing two overlapping coplanar arcs for treatment.
The optimization of the comVMAT plans had converged within 600 iterations of the block minimization algorithm. comVMAT plans were able to consistently reduce the dose to all organs-at-risk (OARs) as compared to the clnVMAT plans. On average, comVMAT plans reduced the max and mean OAR dose by 6.59% and 7.45%, respectively, of the prescription dose. Reductions in max dose and mean dose were as high as 14.5 Gy in the LNG case and 15.3 Gy in the H&N3PTV case. PTV coverages measured by D95, D98, and D99 were within 0.25% of the prescription dose. By comprehensively optimizing all beams, the comVMAT optimizer gained the freedom to allow some selected beams to deliver higher intensities, yielding a dose distribution that resembles a static beam IMRT plan with beam orientation optimization.
The novel nongreedy VMAT approach simultaneously optimizes all beams in an arc and then directly generates deliverable apertures. The single arc VMAT approach thus fully utilizes the digital Linac's capability in dose rate and gantry rotation speed modulation. In practice, the new single VMAT algorithm generates plans superior to existing VMAT algorithms utilizing two arcs.
容积调强弧形放疗（VMAT）是一种广泛应用的放射治疗技术，与静态射束调强放疗（IMRT）相比，其剂量测定相当，但监测单位和治疗时间减少。然而，当前的VMAT优化采用了各种贪婪启发式算法来获得经验性解决方案，这危及了计划的一致性和质量。作者引入了一种新颖的VMAT直接孔径优化方法来克服这些限制。
将综合VMAT（comVMAT）计划制定为一个优化问题，带有一个L2范数保真项以惩罚优化剂量与规定剂量之间的差异，以及一个各向异性全变差项以促进注量图中的分段连续性，为直接孔径优化做好准备。使用水平集函数来描述孔径形状，并对相邻角度的孔径形状差异进行惩罚以控制多叶准直器（MLC）的运动范围。采用近端类优化求解器来解决大规模优化问题，并实施交替优化策略以同时求解注量强度和孔径形状。针对多形性胶质母细胞瘤病例、肺部（LNG）病例以及两个头颈部病例（一个有三个计划靶体积（PTV）（H&N3PTV），另一个有四个PTV（H&N4PTV））生成了单弧comVMAT计划，利用180束射束，角分辨率为2°，以测试其有效性。使用交替优化策略对计划进行优化。将这些计划与使用两个重叠共面弧进行治疗的临床VMAT（clnVMAT）计划进行比较。
comVMAT计划的优化在块最小化算法的600次迭代内收敛。与clnVMAT计划相比，comVMAT计划能够持续降低所有危及器官（OAR）的剂量。平均而言，comVMAT计划将最大和平均OAR剂量分别降低了处方剂量的6.59%和7.45%。在LNG病例中，最大剂量和平均剂量的降低分别高达14.5 Gy和15.3 Gy，在H&N3PTV病例中分别为15.3 Gy。通过D95、D98和D99测量的PTV覆盖率在处方剂量的0.25%以内。通过全面优化所有射束，comVMAT优化器有自由允许一些选定的射束提供更高的强度，从而产生类似于经过射束方向优化的静态射束IMRT计划的剂量分布。
这种新颖的非贪婪VMAT方法同时优化弧形中的所有射束，然后直接生成可交付的孔径。因此，单弧VMAT方法充分利用了数字直线加速器在剂量率和机架旋转速度调制方面的能力。在实际应用中，新的单VMAT算法生成的计划优于现有的使用两个弧的VMAT算法。
