Heng Veng Jean, Serban Monica, Renaud Marc-André, Freeman Carolyn, Seuntjens Jan
Department of Physics and Medical Physics Unit, McGill University, Montreal, Canada.
Princess Margaret Cancer Centre and Department of Radiation Oncology, University of Toronto, Toronto, Canada.
Med Phys. 2023 Oct;50(10):6502-6513. doi: 10.1002/mp.16709. Epub 2023 Sep 8.
Mixed electron-photon beam radiation therapy (MBRT) is an emerging technique in which external electron and photon beams are simultaneously optimized into a single treatment plan. MBRT exploits the steep dose falloff and high surface dose of electrons while maintaining target conformity by leveraging the sharp penumbra of photons.
This study investigates the dosimetric benefits of MBRT for soft tissue sarcoma (STS) patients.
A retrospective cohort of 22 STS of the lower extremity treated with conventional photon-based Volumetric Modulated Arc Therapy (VMAT) were replanned with MBRT. Both VMAT and MBRT treatments were planned on the Varian TrueBeam linac using the Millenium multi-leaf collimator. No electron applicator, cutout or additional collimating devices were used for electron beams of MBRT plans. MBRT plans were optimized to use a combination of 6 MV photons and five electron energies (6, 9, 12, 16, 20 MeV) by a robust column generation algorithm. Electron beams in this study were planned at standard 100 cm source-axis distance (SAD). The dose to the clinical target volume (CTV), bone, normal tissue strip and other organs-at-risk (OARs) were compared using a Wilcoxon signed-rank test.
As part of the original VMAT treatment, tissue-equivalent bolus was required in 10 of the 22 patients. MBRT plans did not require bolus by virtue of the higher electron entrance dose. CTV coverage by the prescription dose was found to be clinically equivalent between plans of either modality: (MBRT) = 97.9 ± 0.2% versus (VMAT) = 98.1 ± 0.6% (p=0.34). Evaluating the absolute paired difference between doses to OARs in MBRT and VMAT plans, we observed lower to normal tissue in MBRT plans by 14.9 ± 3.2% ( ). Similarly, to bone was found to be decreased by 8.2 ± 4.0% ( ) of the bone volume.
For STS with subcutaneous involvement, MBRT offers statistically significant sparing of OARs without sacrificing target coverage when compared to VMAT. MBRT plans are deliverable on conventional linacs without the use of electron applicators, shortened source-to-surface distance (SSD) or bolus. This study shows that MBRT is a logistically feasible technique with clear dosimetric benefits.
混合电子 - 光子束放射治疗（MBRT）是一种新兴技术，其中外部电子束和光子束同时优化到单个治疗计划中。MBRT利用电子束陡峭的剂量下降和高表面剂量，同时通过利用光子束的锐利半值层来保持靶区适形性。
本研究调查MBRT对软组织肉瘤（STS）患者的剂量学优势。
对22例接受传统基于光子的容积调强弧形治疗（VMAT）的下肢STS患者进行回顾性队列研究，用MBRT重新计划。VMAT和MBRT治疗均在瓦里安TrueBeam直线加速器上使用 Millennium多叶准直器进行计划。MBRT计划的电子束未使用电子限光筒、挡块或额外的准直装置。通过稳健的列生成算法，将MBRT计划优化为使用6 MV光子和五种电子能量（6、9、12、16、20 MeV）的组合。本研究中的电子束在标准源轴距（SAD）100 cm处计划。使用Wilcoxon符号秩检验比较临床靶区（CTV）、骨骼、正常组织条带和其他危及器官（OARs）的剂量。
作为原始VMAT治疗的一部分，22例患者中有10例需要组织等效填充物。由于电子束的入射剂量较高，MBRT计划不需要填充物。两种治疗方式的计划中，处方剂量对CTV的覆盖在临床上相当：（MBRT）= 97.9 ± 0.2%，而（VMAT）= 98.1 ± 0.6%（p = 0.34）。评估MBRT和VMAT计划中OARs剂量的绝对配对差异，我们观察到MBRT计划中对正常组织的剂量降低了14.9 ± 3.2%（ ）。同样，对骨骼的剂量降低了骨骼体积的8.2 ± 4.0%（ ）。
对于有皮下受累的STS，与VMAT相比，MBRT在不牺牲靶区覆盖的情况下，对OARs有统计学上显著的剂量节省。MBRT计划可在传统直线加速器上实施，无需使用电子限光筒、缩短源皮距（SSD）或填充物。本研究表明，MBRT是一种在逻辑上可行的技术，具有明显的剂量学优势。
