灵活调控笔形束参数的点扫描照射新方法研究

Heavy-ion cancer therapy is the forefront and hotspot in the field of radiation therapy research. In the beam delivery technique of three-dimensional spot scanning, the sharp Bragg peak of a pencil-like pristine heavy-ion beam generally needs to be extended slightly with a mini ridge filter so as to form a primary mini spread-out Bragg peak (mini-SOBP), where a Gaussian-shaped dose distribution in the mini-SOBP is presented. The width (full-width at the half maximum) of the primary mini-SOBP and the beam spot size are basically unchanged during the process of spot scanning at the scan points in the target volume. To improve the conformity degree, these pramaters should be maintained at a smaller scale. In this way, it is necessary to reduce the scan step for the spot scanning so as to allocate enormous scan points in the target volume. To resolve this problem, we propose this research project aiming at establishing a novel method of spot scanning irradiation with flexibly adjustable pencil beam parameters. In the method, pencil beams with smaller beam spots are used to irradiate the peripheral scan points and with bigger ones to the internal scan points. Meanwhile, the pencil beam parameters are elaborately modulated by means of tilting different angles for the mini ridge filter and then variable pencil beams with different widths of the primary mini-SOBP and beam-spot sizes are acquired flexibly. There are some markedly advantages such as shortening the irradiation time and decreasing the dose deviations at scan points as well as the dose inhomogeneity in the target volume by using the new method. In this research project, we wish to establish the new method for spot scanning beam delivery through a systematic investigation and verify its validity and superiority via experimental measurements. Clearly, this research project is of importance in making full use of the benefits of heavy ions in radiotherapy.

重离子治癌是放疗研究领域的前沿与热点。在重离子三维点扫描照射技术中，单能重离子笔形束尖锐的Bragg峰需通过微型脊形过滤器略微展宽成峰区剂量分布为高斯型的初级展宽峰。在照射靶区内各个扫描点的过程中，初级展宽峰的半高宽及束斑尺寸基本不变。为提高适形程度，这些参数需保持在较小的尺度上，这导致点扫描时需要减小扫描步长，在靶区设置庞大的扫描点数量。本项目旨在建立一种新的点扫描照射方法，即利用较小束斑的笔形束照射靶区的边缘区域，较大束斑的笔形束照射靶区的内部区域，从而减少靶区内扫描点数量，同时通过倾斜微型脊形过滤器不同角度来调控笔形束参数，获得纵向上初级展宽峰半高宽及横向上束斑大小灵活可变的笔形束。该方法具有缩短照射时间、减小扫描点的剂量偏差和靶区内剂量不均匀性等优势。本项目拟通过系统研究建立该新型点扫描照射方法，通过实验测量验证该方法的有效性及优势，这对进一步发挥重离子在放疗中的优势具有重要意义。

在传统的重离子三维点扫描照射技术中，单能重离子笔形束尖锐的Bragg峰需要通过微型脊形过滤器略微展宽成峰区剂量为高斯型的初级微小展宽Bragg峰（mini-SOBP）。在照射靶区内各个扫描点的过程中，初级mini-SOBP的半高宽及横向束斑尺寸基本不变。为提高照射适形程度，即减小剂量半影，需要将束斑横向及纵向尺寸控制在较小的尺度，这导致照射整个靶区需要很多的扫描点数，进而导致靶区中心区域扫描点剂量权重的减小，增加照射控制的难度。基于上述背景，本项目提出将可变尺寸的束斑运用到重离子三维点扫描照射技术中，即实现靶区外围利用小束斑照射，靶区内部利用大束斑照射的新方法。纵向上，本项目采用倾斜微型脊形过滤器的方法获得不同展宽程度的mini-SOBP。通过MC模拟研究发现，倾斜微型脊形过滤器30、45o、60度可分别将mini-SOBP的半高宽（FWHM）增大16%、42%、100%。因此，倾斜微型脊形过滤器的方法可以有效调节重离子笔形束的mini-SOBP纵向尺寸。此外还发现，倾斜微型脊形过滤器方法还可以扩展重离子笔形束的横向尺寸，但是扩展效果并不是很明显。因此单纯采用倾斜微型脊形过滤器的方法不能满足三维重离子笔形束尺寸调节的实际需求。针对这一问题，本项目提出在主动式束流配送系统束流线上插入钽（Ta）散射体。通过MC模拟发现，对360MeV/u的碳离子加入0.4mm Ta散射体后，束斑横向尺寸可以增大60%。通过MC模拟，本项目建立起来了一整套用于剂量优化及计算的基础数据。在此基础上，本项目在水模体中建立了一个直径为6cm的虚拟球形靶体，对该靶体边缘区域采用小束斑照射，靶体内部采用大束斑照射，以靶区物理剂量均匀分布为目标进行优化获得剂量分布。计算结果表明，利用该照射方法可以有效减少靶区剂量半影、减少数量、增加靶区中心区域的扫描点剂量权重，并使整个靶区扫描点权重分布向高一个数量级方向偏移。另外，本项目还研究了主动式束流配送系统的设置，发现将真空窗与等中心距离小于125cm及微型脊形过滤器结构周期为2mm的束流配送条件下，100-400 Mev/u之间治疗能量范围内的碳离子束在等中心处的束斑尺寸及剂量平坦度能满足主动式点扫描照射治疗的要求。这些研究内容将对进一步提升我国重离子治疗技术水平提供有力支持。