Department of Radiation Oncology, University of Florida, Gainesville, Florida, USA.
Department of Industrial and Systems Engineering, University of Florida, Gainesville, Florida, USA.
Med Phys. 2022 Jun;49(6):4026-4042. doi: 10.1002/mp.15638. Epub 2022 Apr 12.
Most commercially available treatment planning systems (TPSs) approximate the continuous delivery of volumetric modulated arc therapy (VMAT) plans with a series of discretized static beams for treatment planning, which can make VMAT dose computation extremely inefficient. In this study, we developed a polar-coordinate-based pencil beam (PB) algorithm for efficient VMAT dose computation with high-resolution gantry angle sampling that can improve the computational efficiency and reduce the dose discrepancy due to the angular under-sampling effect.
6 MV pencil beams were simulated on a uniform cylindrical phantom under an EGSnrc Monte Carlo (MC) environment. The MC-generated PB kernels were collected in the polar coordinate system for each bixel on a fluence map and subsequently fitted via a series of Gaussians. The fluence was calculated using a detectors' eye view with off-axis and MLC transmission factors corrected. Doses of VMAT arc on the phantom were computed by summing the convolution results between the corresponding PB kernels and fluence for each bixel in the polar coordinate system. The convolution was performed using fast Fourier transform to expedite the computing speed. The calculated doses were converted to the Cartesian coordinate system and compared with the reference dose computed by a collapsed cone convolution (CCC) algorithm of the TPS. A heterogeneous phantom was created to study the heterogeneity corrections using the proposed algorithm. Ten VMAT arcs were included to evaluate the algorithm performance. Gamma analysis and computation complexity theory were used to measure the dosimetric accuracy and computational efficiency, respectively.
The dosimetric comparisons on the homogeneous phantom between the proposed PB algorithm and the CCC algorithm for 10 VMAT arcs demonstrate that the proposed algorithm can achieve a dosimetric accuracy comparable to that of the CCC algorithm with average gamma passing rates of 96% (2%/2mm) and 98% (3%/3mm). In addition, the proposed algorithm can provide better computational efficiency for VMAT dose computation using a PC equipped with a 4-core processor, compared to the CCC algorithm utilizing a dual 10-core server. Moreover, the computation complexity theory reveals that the proposed algorithm has a great advantage with regard to computational efficiency for VMAT dose computation on homogeneous medium, especially when a fine angular sampling rate is applied. This can support a reduction in dose errors from the angular under-sampling effect by using a finer angular sampling rate, while still preserving a practical computing speed. For dose calculation on the heterogeneous phantom, the proposed algorithm with heterogeneity corrections can still offer a reasonable dosimetric accuracy with comparable computational efficiency to that of the CCC algorithm.
We proposed a novel polar-coordinate-based pencil beam algorithm for VMAT dose computation that enables a better computational efficiency while maintaining clinically acceptable dosimetric accuracy and reducing dose error caused by the angular under-sampling effect. It also provides a flexible VMAT dose computation structure that allows adjustable sampling rates and direct dose computation in regions of interest, which makes the algorithm potentially useful for clinical applications such as independent dose verification for VMAT patient-specific QA.
大多数商业可用的治疗计划系统（TPS）通过一系列离散的静态射束来近似容积调强弧形治疗（VMAT）计划的连续输送，这使得 VMAT 剂量计算效率极低。在这项研究中，我们开发了一种基于极坐标的笔束（PB）算法，用于高效的 VMAT 剂量计算，具有高分辨率的旋转角度采样，可以提高计算效率并减少由于角度欠采样效应引起的剂量差异。
在 EGSnrc 蒙特卡罗（MC）环境下，在均匀圆柱体模型上模拟了 6 MV 笔束。在极坐标系统中为每个射野的每个像素收集 MC 生成的 PB 核，并通过一系列高斯拟合。使用探测器视线计算剂量，校正了离轴和 MLC 传输因子。通过将相应的 PB 核与极坐标系统中每个像素的射野的卷积结果相加，计算 VMAT 弧形的剂量。使用快速傅里叶变换（FFT）进行卷积，以加快计算速度。将计算出的剂量转换为笛卡尔坐标系，并与 TPS 的塌陷锥卷积（CCC）算法计算的参考剂量进行比较。创建了一个不均匀的体模来研究使用所提出的算法进行的不均匀性校正。包括十个 VMAT 弧形，以评估算法性能。使用伽马分析和计算复杂度理论分别测量剂量准确性和计算效率。
在 10 个 VMAT 弧形的均匀体模上，将提出的 PB 算法与 CCC 算法进行的剂量比较表明，该算法可以达到与 CCC 算法相当的剂量准确性，平均伽马通过率为 96%（2%/2mm）和 98%（3%/3mm）。此外，与利用双 10 核服务器的 CCC 算法相比，配备四核处理器的 PC 可用于 VMAT 剂量计算，从而为 VMAT 剂量计算提供更好的计算效率。此外，计算复杂度理论表明，对于均匀介质上的 VMAT 剂量计算，该算法在计算效率方面具有很大的优势，尤其是在应用较细的角度采样率时。这可以通过使用更精细的角度采样率来支持减少角度欠采样效应引起的剂量误差，同时保持实际的计算速度。对于不均匀体模的剂量计算，具有不均匀性校正的 PB 算法仍可以提供合理的剂量准确性，与 CCC 算法的计算效率相当。
我们提出了一种新的基于极坐标的笔束算法，用于 VMAT 剂量计算，在保持临床可接受的剂量准确性的同时，提高了计算效率，并减少了由于角度欠采样效应引起的剂量误差。它还提供了一种灵活的 VMAT 剂量计算结构，允许可调的采样率和感兴趣区域的直接剂量计算，这使得该算法在临床应用中具有潜在的用途，例如用于 VMAT 患者特定 QA 的独立剂量验证。
