Department of Physics, Carleton University, Ottawa, ON, K1S 5B6, Canada.
Phys Med Biol. 2018 Oct 26;63(21):21NT01. doi: 10.1088/1361-6560/aae65e.
Although VMAT delivery features continuous gantry rotation and leaf motion, dose calculation is often performed under the dual assumption of discrete apertures changing instantaneously from one discrete angle to the next. In this work, the validity of these two approximations is determined, as well as their impact on the quality of optimized plans. Further, an accurate method of fluence calculation is derived which does not use the discrete aperture approximation, but instead calculates the fluence as the multi-leaf collimator leaves sweep from one position to another. This continuous aperture fluence calculation is integrated in the VMAT optimization process using the open-source treatment planning system matRad. The three-step approach of VMAT optimization is used: fluence map optimization followed by leaf sequencing and direct aperture optimization, with variable leaf speed, gantry rotation speed, and MU rate. The benefit of the continuous aperture VMAT method over the discrete aperture method is determined by comparing the plan quality of discrete aperture and continuous aperture optimized plans, when the former is recalculated using the continuous aperture fluence calculation. Discrete aperture VMAT plans calculated at 4° spacing result in significant dose errors (10%-35%, depending on the anatomical site) as compared to the reference dose (continuous aperture fluence calculation at 0.5° spacing). These errors are greatly reduced (to 0.8%-2%) when the continuous aperture fluence calculation method was used at the same 4° spacing, implying that the dose error is primarily due to the discrete aperture approximation. Whereas all dose objectives were met by the discrete aperture VMAT optimized plan, many of them failed when the dose was recalculated with the continuous aperture fluence calculation. All objectives were met once again when the plan was optimized with the new continuous aperture VMAT optimization. Further, using only half of the beam angles, the continuous aperture VMAT optimization can achieve the same degree of accuracy with only 40% of the computing time as compared with the standard discrete aperture VMAT.
尽管 VMAT 传输具有连续的机架旋转和叶片运动，但剂量计算通常是在离散孔径从一个离散角度瞬间变为下一个离散角度的双重假设下进行的。在这项工作中，确定了这两种近似的有效性，以及它们对优化计划质量的影响。此外，还推导出了一种精确的通量计算方法，该方法不使用离散孔径近似，而是计算多叶准直器叶片从一个位置扫到另一个位置时的通量。这种连续孔径通量计算方法在使用开源治疗计划系统 matRad 的 VMAT 优化过程中得到了集成。使用三步法进行 VMAT 优化：通量图优化，然后是叶片排序和直接孔径优化，同时改变叶片速度、机架旋转速度和 MU 率。通过比较离散孔径和连续孔径优化计划的计划质量来确定连续孔径 VMAT 方法相对于离散孔径方法的优势，当使用连续孔径通量计算方法重新计算前者时。与参考剂量（0.5°间隔的连续孔径通量计算）相比，以 4°间隔计算的离散孔径 VMAT 计划会导致显著的剂量误差（取决于解剖部位，为 10%-35%）。当以相同的 4°间隔使用连续孔径通量计算方法时，这些误差大大降低（至 0.8%-2%），这表明剂量误差主要是由于离散孔径近似所致。虽然离散孔径 VMAT 优化计划满足了所有剂量目标，但当使用连续孔径通量计算重新计算剂量时，其中许多目标都失败了。一旦使用新的连续孔径 VMAT 优化再次优化该计划，所有目标都再次得到满足。此外，仅使用一半的射束角度，连续孔径 VMAT 优化可以以仅为标准离散孔径 VMAT 的 40%的计算时间实现相同的精度。
