重离子点扫描中基于笔形束低剂量包络的非均一束斑变间距排布方法研究

Heavy ion spot scanning method is the frontier issue and hot topic in current particle radiotherapy research field. Dose delivery with scanned beam is currently depended on the position of uniform pencil beam spot arranged regularly and the Gaussian-like lateral dose distribution of beam spot along the penetration depth direction. New research suggests that a low-dose envelope exists over an extended distance from the central axis of a scanning spot, which contributed up to 15% to the total scanning dose. So the spot size is beyond the scope of the 2D Gaussian model，which lead to the inaccuracy of the dose distribution caused by beam spot superposition. Also the artificial arrangement of spot position which demand small spot spacing lead to time-consuming and low efficiency of spot scanning..To resolve this problem, we propose this research project aiming at establishing a novel basic method of spot scanning technique with precise pencil beam model and the optimization of scanning point arrangement. In this method, by firstly investigating the low-dose lateral profile at long off-axis distance, we focus on modeling individual scanning spot precisely, which contribute to the dose calculation accuration of spot scanning irradiation. Then we develop an optimization method of spot number and position using polygonal space filling theory based on a set of pencil beam spot sizes which we study previously. There are some significant advantages such as improving the dose calculation accuracy and increase scanning efficiency by decreasing the spot numbers..In this research project, we wish through a systematic investigation to develop the new basic method for spot scanning method and verify its validity and superiority. Clearly, this study is of importance in making full use of the benefits of heavy ions in radiotherapy.

重离子点扫描技术研究是当前粒子放射治疗研究的前沿和热点。目前点扫描剂量配送基于人工规则排布的均一束斑和笔形束二维横向高斯分布模型及实测深度剂量分布进行。但笔形束存在离轴较远的横向低剂量包络，且剂量贡献超过15%，而二维高斯模型无法描述此低剂量包络，影响了点扫描笔形束迭加剂量分布的准确性；同时，以二维高斯分布为基础排布均一束斑时要求间距紧凑，导致扫描点数目多，照射效率低。因此，精确笔形束模型和优化扫描点排布方案是改善点扫描剂量准确性和照射效率的有效切入点。本项目旨在创新点扫描基础方法，在精确测量笔形束剂量包络并建模的基础上，自动优化非均一笔形束数目和扫描点排布，建立重离子非均一笔形束变间距点扫描初步方法，以达到减少扫描点数目，提高剂量配送准确性，提高效率的目的。从束流基础到技术应用的研究，将丰富重离子点扫描方法，对进一步发挥重离子放疗优势具有重要意义。

首先实现了碳离子笔形束横向散射10^-4Gy数量级的低剂量包络分布的测量，验证了低剂量包络的影响范围，同时运用蒙特卡洛方法进行模拟验证，阐明了笔形束剂量包络来源、性质及随能量、深度两个参数变化的趋势；其次基于笔形束实测数据及大量模拟剂量分布，发展了以二重高斯配合Logistic修正公式来补充描述离轴低剂量包络的精确笔形束模型；接下来基于笔形束剖面模型进行均匀照射野剂量分布计算，考察了碳离子不同笔形束模型对照射野剂量分布的影响，结果验证了二重高斯+Logistic公式模型在理论计算精度方面的优势相比常规的一重、二重高斯模型更明显。接下来实现了扫描点最短路径的优化排布算法，并整合进现有的碳离子治疗计划系统中。最后，通过靶区点扫描方法的模拟照射验证，考察了点扫描模式下，碳离子不同笔形束模型对球形靶区点扫描权重的影响，证实了二重高斯+Logistic公式模型下的点扫描治疗计划剂量计算结果与模拟结果误差最小，约为0.06%。double Gaussian-logistic模型可视为TPS剂量计算的理想模型。此研究为新型模型的应用奠定了理论基础。