发散型准直器限束孔应用于被动散射质子放疗可行性研究

被动散射质子放疗（Passive Scattering Proton Therapy，PSPT）是质子治疗的主要技术之一，束流通过准直器限束孔（Aperture）时因边缘散射效应导致患者体内剂量分布偏离理想状态。使用蒙特卡洛软件TOPAS（TOol for PArticle Simulation）对质子束流经过发散型与传统型准直器限束孔后进入水模体中的过程建模，分析两种准直器的边缘散射效应对剂量及中子能谱分布的影响，分别测试70，110，160，200，230，250 MeV能量下的质子束流，发现传统Aperture在70 MeV的质子束下边缘散射效应最明显，在距水箱表面0.5 cm深度处横向剂量曲线平坦度、均匀度分别达到4.63%，108.05%，随着深度增加边缘散射效应逐渐减弱，在布拉格峰位置处接近消失。使用发散型准直器限束孔后，在70 MeV下平坦度、均匀度分别降至1.28%，101.31%，对于100，160，200 MeV质子束均有不同程度改善。对于能量接近250 MeV的质子束，发散型准直器限束孔设置下横向剂量曲线并无优势。边缘散射效应导致的剂量不均随水深增加而减弱，对于各个能量质子，使用发散型准直器限束孔后次级中子减少。研究结果表明，发散型准直器限束孔应用于PSPT效果显著，为进一步应用于临床提供数据支撑。Passive Scattering Proton Therapy (PSPT) is one of the main technologies for proton radiation therapy. The dose distribution in the patient deviates from the ideal state due to the edge scattering effect when the beam passes through the aperture. In this paper, TOPAS, a Monte Carlo software, was used to simulate the passive scattering treatment head. The influence of the edge scattering effect of the two aperture sets on the dose distribution was compared. The proton beam at 70, 110, 160, 200, 230 and 250 MeV was tested respectively. We found that the scattering effect of the conventional aperture is most obvious at 70 MeV, and the flatness and hetergeneity of the lateral dose curve at the inlet of 0.5 cm of the tank reach 4.63%, 108.05%, respectively. The dose shift caused by the edge scattering effect decreases with increasing water depth and disappears at the Bragg peak. After using the divergent aperture, the flatness and uniformity at 70 MeV are reduced to 1.28% and 101.31%, respectively, and the 100, 160, and 200 MeV proton beams are improved in different extents. For a proton beam with an energy close to 250 MeV, there is no advantage in the lateral dose curve of the divergent aperture. For all energy protons, the secondary neutrons are reduced with divergent aperture. The results show that the divergent aperture is effective for PSPT and this study provides data support for further application in clinical practice.

Feasibility Study on Whether Divergent Aperture can be Used on Proton Therapy Radiotherapy

Passive Scattering Proton Therapy (PSPT) is one of the main technologies for proton radiation therapy. The dose distribution in the patient deviates from the ideal state due to the edge scattering effect when the beam passes through the aperture. In this paper, TOPAS, a Monte Carlo software, was used to simulate the passive scattering treatment head. The influence of the edge scattering effect of the two aperture sets on the dose distribution was compared. The proton beam at 70, 110, 160, 200, 230 and 250 MeV was tested respectively. We found that the scattering effect of the conventional aperture is most obvious at 70 MeV, and the flatness and hetergeneity of the lateral dose curve at the inlet of 0.5 cm of the tank reach 4.63%, 108.05%, respectively. The dose shift caused by the edge scattering effect decreases with increasing water depth and disappears at the Bragg peak. After using the divergent aperture, the flatness and uniformity at 70 MeV are reduced to 1.28% and 101.31%, respectively, and the 100, 160, and 200 MeV proton beams are improved in different extents. For a proton beam with an energy close to 250 MeV, there is no advantage in the lateral dose curve of the divergent aperture. For all energy protons, the secondary neutrons are reduced with divergent aperture. The results show that the divergent aperture is effective for PSPT and this study provides data support for further application in clinical practice.