Department of Radiation Oncology, Mayo Clinic, Rochester, Minnesota, USA.
Med Phys. 2023 Jan;50(1):600-618. doi: 10.1002/mp.15938. Epub 2022 Aug 31.
Although intensity-modulated radiation therapy and volumetric arc therapy have revolutionized photon external beam therapies, the technological advances associated with electron beam therapy have fallen behind. Modern linear accelerators contain technologies that would allow for more advanced forms of electron treatments, such as beam collimation, using the conventional photon multi-leaf collimator (MLC); however, no commercial solutions exist that calculate dose from such beam delivery modes. Additionally, for clinical adoption to occur, dose calculation times would need to be on par with that of modern dose calculation algorithms.
This work developed a graphics processing unit (GPU)-accelerated Monte Carlo (MC) engine incorporating the Varian TrueBeam linac head geometry for a rapid calculation of electron beams collimated using the conventional photon MLC.
A compute unified device architecture framework was created for the following: (1) transport of electrons and photons through the linac head geometry, considering multiple scattering, Bremsstrahlung, Møller, Compton, and pair production interactions; (2) electron and photon propagation through the CT geometry, considering all interactions plus the photoelectric effect; and (3) secondary particle cascades through the linac head and within the CT geometry. The linac head collimating geometry was modeled according to the specifications provided by the vendor, who also provided phase-space files. The MC was benchmarked against EGSnrc/DOSXYZnrc/GEANT by simulating individual interactions with simple geometries, pencil, and square beam dose calculations in various phantoms. MC-calculated dose distributions for MLC and jaw-collimated electron fields were compared to measurements in a water phantom and with radiochromic film.
Pencil and square beam dose distributions are in good agreement with DOSXYZnrc. Angular and spatial distributions for multiple scattering and secondary particle production in thin slab geometries are in good agreement with EGSnrc and GEANT. Dose profiles for MLC and jaw-collimated 6-20-MeV electron beams showed an average absolute difference of 1.1 and 1.9 mm for the FWHM and 80%-20% penumbra from measured profiles. Percent depth doses showed differences of <5% for as compared to measurement. The computation time on an NVIDIA Tesla V100 card was 2.5 min to achieve a dose uncertainty of <1%, which is ∼300 times faster than published results in a similar geometry using a single-CPU core.
The GPU-based MC can quickly calculate dose for electron fields collimated using the conventional photon MLC. The fast calculation times will allow for a rapid calculation of electron fields for mixed photon and electron particle therapy.
尽管调强放射治疗和容积弧形治疗已经彻底改变了光子外束治疗，但与电子束治疗相关的技术进步却落后了。现代直线加速器包含了可实现更先进形式的电子治疗的技术，例如使用传统光子多叶准直器（MLC）进行光束准直；然而，目前还没有商业解决方案可以计算出这种束流传输模式下的剂量。此外，为了实现临床应用，剂量计算时间需要与现代剂量计算算法相匹配。
本研究开发了一个基于图形处理单元（GPU）的蒙特卡罗（MC）引擎，该引擎结合了瓦里安 TrueBeam 直线加速器头的几何形状，用于快速计算使用传统光子 MLC 准直的电子束。
创建了一个计算统一设备架构框架，用于以下方面：（1）通过直线加速器头几何形状传输电子和光子，考虑多次散射、韧致辐射、Møller、康普顿和对产生相互作用；（2）电子和光子通过 CT 几何形状传播，考虑所有相互作用以及光电效应；（3）在直线加速器头和 CT 几何形状内进行二次粒子级联。根据供应商提供的规格对直线加速器头准直几何形状进行建模，供应商还提供了相空间文件。通过模拟简单几何形状、铅笔和正方形束剂量计算，使用 EGSnrc/DOSXYZnrc/GEANT 对 MC 进行了基准测试，以在各种体模中进行单个相互作用。在水模体和放射色胶片上比较了 MLC 和 jaw-collimated 电子场的 MC 计算剂量分布与测量值。
铅笔和正方形束剂量分布与 DOSXYZnrc 吻合良好。在薄平板几何形状中，多次散射和次级粒子产生的角度和空间分布与 EGSnrc 和 GEANT 吻合良好。对于 MLC 和 jaw-collimated 6-20-MeV 电子束的剂量分布，FWHM 和 80%-20%半影的平均绝对差值分别为 1.1 和 1.9mm，与测量的分布相比。与测量值相比，百分深度剂量的差异<5%。在 NVIDIA Tesla V100 卡上的计算时间为 2.5 分钟，可达到<1%的剂量不确定性，这比在类似几何形状下使用单个 CPU 内核发布的结果快约 300 倍。
基于 GPU 的 MC 可以快速计算使用传统光子 MLC 准直的电子束剂量。快速的计算时间将允许快速计算混合光子和电子粒子治疗的电子场。
