均匀磁场下碳离子束剂量平均LET及纳剂量学量的分布

随着磁共振成像(MRI)技术的发展,图像引导放射治疗在放射肿瘤学中的作用和重要性正在迅速增加,本研究分析了外加均匀磁场对碳离子束的剂量平均LET以及纳剂量学量的影响。通过基于GEANT4内核的GATE蒙特卡罗(Monte Carlo, MC)模拟平台,模拟计算了不同磁场环境下,不同能量碳离子束剂量平均LET和纳剂量学量的分布。结果发现,平行磁场对碳离子束的剂量平均LET和纳剂量学量均无显著影响,垂直磁场对碳离子束的剂量平均LET及纳剂量学量的影响主要集中在布拉格峰区域,其影响主要是碳离子束在磁场中受到洛伦兹力作用而发生横向偏转,进而使得碳离子束布拉格峰位置发生横向侧移导致的。这些结果为进一步研究磁场对碳离子束治疗性能的影响打下了坚实的基础。     

基于TPS和蒙特卡罗计算的12C肿瘤放射治疗In-beam PET剂量监测研究

In-beam PET成像是碳离子放射治疗剂量监测的有效手段,可以对碳离子放疗过程的物理剂量分布和生物剂量分布进行实时监测。结合放射治疗计划系统(TPS)和蒙特卡罗(MC)模拟分别对静态均匀水模体和腹部肿瘤CT图像进行治疗计划设计、MC计算和PET成像,比较TPS肿瘤靶区剂量分布、MC模拟剂量分布和PET成像三者之间的一致性。TPS和MC模拟中相对生物学效应(RBE)的计算均采用线性二次模型(LQ)。研究结果显示,TPS和MC计算的静态均匀水模体、单野治疗腹部肿瘤的物理剂量、RBE加权剂量在SOBP区域的平均误差均在0.5%和2%以内。碳离子束流能量为120~400 MeV/u时,束流方向剂量深度分布与PET成像在SOBP区域的位置差异均在8 mm以内。In-beam PET可作为碳离子放射治疗中位置验证和剂量验证的有效手段。     

碳离子束射程快速验证方法的蒙特卡罗模拟研究

可通过楔形装置把离子束(质子和重离子)纵向上的的深度剂量分布转换成横向上的剂量分布，进行离子束射程的快速测量及验证。本工作通过基于GEANT4内核的GATE蒙特卡罗模拟平台，模拟计算了不同能量碳离子束在用于制作楔形装置的铜、铝、铁和有机玻璃等材料中的深度剂量分布，得到不同能量碳离子束在不同材料中Bragg峰位所处深度与能量之间的关系；模拟计算了不同能量碳离子束穿越单楔板、双楔板和大小组合楔板等楔形装置后横向上的剂量分布，得到了横向剂量峰值出现位置与碳离子束射程之间的关系。本文蒙特卡罗模拟研究为进一步开发重离子治疗当中的射程快速验证方法与设备奠定了坚实的基础。     

质子束治疗中非均匀组织的等效水厚度修正研究

非均匀组织等效水厚度修正是研究质子放射治疗的重要组成部分, 利用蒙特卡罗Fluka2011.2程序模拟了不同能量(50–250 MeV)的质子束入射到不同介质中的输运过程, 总结出了在不同介质中质子束初始能量与质子束Bragg峰深度关系, 并拟合出质子束在介质中的等效水厚度修正公式. 结果表明, 对不同能量的质子束入射到非均匀组织中, 通过拟合公式计算出Bragg峰深度值与Fluka模拟的质子束Bragg峰的位置相差在1 mm 之内. 如果建立起介质和水的Bragg峰比与电子密度比关系的数据库, 该公式有可能用于临床上的质子放射治疗的剂量计算中. 关键词： 蒙特卡罗 质子治疗 等效水厚度 Bragg 峰     

重离子加速器示范装置被动式束流配送系统次级中子特征的蒙特卡罗研究

在碳离子放射治疗中,碳离子束在剂量配送过程中会与束流输运线相互作用,形成以中子辐射为主的外辐射场.由于中子是高LET射线,具有较高的相对生物学效应,减少碳离子放疗中产生的次级中子有助于降低放疗后正常组织并发症几率及二次肿瘤风险.利用蒙特卡罗方法对保守情况(能量为400 MeV/u,多叶光栅完全闭合)下碳离子治疗被动式束...     

基于磁场调制的质子放疗新方法及其调制机制

提出一种基于磁场调制的质子放射治疗新方法, 探索肿瘤剂量、正常器官剂量与磁场调制方法之间的关联机制, 研究磁场调制质子放疗在器官环绕型肿瘤(肿瘤被正常器官环绕或包围)治疗中的应用。基于蒙特卡罗粒子输运程序Geant4,分别建立了理想器官环绕结构（由若干平行六面体结构组成）和含胰腺肿瘤人体腹部解剖结构两种几何构型。分别对两种几何构型内部施加磁场, 通过改变磁场强度和方向,调制质子束布拉格峰几何位置, 利用质子径迹的磁致偏转效应使质子束绕开正常器官对肿瘤进行照射。 对于理想器官环绕型构型, 磁场调制质子放疗可在保证95%剂量覆盖肿瘤情况下将受质子照射正常器官体积控制在接近于零的极低水平。对于胰腺肿瘤解剖构型, 磁场调制方法可使质子束在调制磁场作用下, 绕过脊髓和肾脏照射肿瘤, 并使肿瘤被95%剂量较好地覆盖。通过磁场调制, 可对质子束布拉格峰几何位置和质子径迹弯曲程度进行调制, 绕过正常器官对肿瘤进行照射,从而最大限度地减少正常器官的受照体积和剂量。     

MRI引导放射治疗中电子回转效应的蒙特卡罗研究

为了研究磁共振引导放射治疗对剂量分布的影响,采用蒙特卡罗方法研究了横向均匀磁场对6 MV光子束在4种不同人体组织材料与空气界面处因电子回转效应导致剂量分布的改变。模拟显示,对于电离能相近的几种材料,磁场对剂量分布扰动的差别较小,而且电离能较大的材料,这种扰动明显变小。结果表明,磁场的引入会影响光子束原有的剂量分布,且这种影响与材料的电离能有关。这意味着虽然磁共振引导放射治疗可以增强靶向精度,提高治疗效果,但磁场会导致光子束剂量分布的改变,且不同的组织这种改变也不相同,这将为相应的剂量算法研究带来了新的挑战。A Monte Carlo code was used to study the discrepancy resulted from the emergence of magnetic field in MRI guided radiotherapy. In this work, four different tissue phantoms with magnetic field and 6 MV photon were studied, and the dose distributions at the interface of phantom-air were evaluated. It is found that the differences of the dose perturbation are small between the materials with similar ionization energy. However, the dose perturbation decreased significantly for the material with high ionization energy. The results of this study demonstrate that magnetic field will change the dose distribution of photon beam and the dose perturbation associated with ionization energy of materials. It means that magnetic resonance imaging guided radiotherapy can enhance the target accuracy, but the magnetic field will change the dose distribution of photon beam, and the perturbation was not the same for the different materials of human tissue, it has brought new challenges for the research of dose algorithm.     

碳离子在不同材料叶片的多叶光栅上产生的次级粒子研究

在碳离子放射治疗中，碳离子束与治疗头设备和患者身体相互作用产生的次级粒子可以到达患者体内的许多区域，在产生的次级粒子中以中子和$\gamma $射线的产额为最大。在不影响束流配送功能的情况下，减少碳离子放疗中产生的次级中子和$\gamma $射线对于降低放疗后出现的正常组织并发症及二次肿瘤风险有着非常重要的意义。本文利用蒙特卡罗(Monte Carlo)方法模拟计算了被动式束流配送系统下，400 MeV/u碳离子束照射到由不同材料叶片构成的多叶光栅(MLC)形成典型的10 cm×10 cm方形射野时，在水模体中产生的次级中子和$\gamma $射线所沉积的剂量及空间分布等。模拟结果显示:碳离子束通过MLC形成射野后在水模体中产生的次级中子主要分布在水模体的入射端，次级$\gamma $射线较为均匀的分布在整个水模体内，且较多分布在具有展宽Bragg峰(SOBP)射野在水模体中贯穿时的坪区。对于MLC叶片材料的选择，则需根据实际情况对叶片厚度以及次级粒子当量剂量的要求来确定。本文通过模拟研究不同MLC叶片材料产生次级粒子的情况，为被动式束流配送系统中MLC叶片及其他元件的材料选择提供了科学依据。     

中性束束流品质对偏转系统窗口设计的影响

介绍了EAST第一条中性束注入束线内部偏转系统的结构。以参数为80 keV,70 A的氘束作为参考束,分析了经过中性化气体靶后,束流中高能粒子的能量成分;通过计算不同能量离子的偏转半径,说明了束截面在纵向的扩展情况;阐述了束散角的产生原理,分析了束散角对偏转系统窗口设计的影响。结果表明,偏转系统入射窗口和出射窗口尺寸一般和入射束的尺寸一致;偏转系统入射窗口和回转窗口的尺寸和相对位置满足一定条件,可使装置的热负荷相对较小,且各部分热负荷相对均匀。     

Active spot-scanning test with heavy ions at HIRFL-CSR

An active spot beam delivery system for heavy ion therapy has been developed based on the Cooling Storage Ring at HIRFL-CSR, where the pencil carbon-ion beams were scanned within a target volume transversely by a pair of orthogonal (horizontal and vertical) dipole magnets to paint the slices of the target volume and longitudinally by active energy variation of the synchrotron slice by slice. The unique techniques such as dose shaping via active energy variation and magnetic deflection constitute a promising three-dimensional conformal even intensity-modulated radiotherapy with heavy ions at HIRFL-CSR. In this paper, the verification of active energy variation and the calibration of steerable beam deflection are shown, as the basic functionality components of the active spot-scanning system. Additionally, based on the capability of creating homogeneous irradiation fields with steerable pencil beams, a radiobiological experiment like cell survival measurement has been performed aiming at comparison of the radiobiological effects under active and passive beam deliveries.     

Spot-scanning beam delivery with laterally-and longitudinally-mixed spot size pencil beams in heavy ion radiotherapy

The three-dimensional(3D) spot-scanning method is one of the most commonly used irradiation methods in charged particle beam radiotherapy.Generally,spot-scanning beam delivery utilizes the same size pencil beam to irradiate the tumor targets.Here we propose a spot-scanning beam delivery method with laterally-and longitudinallymixed size pencil beams for heavy ion radiotherapy.This uses pencil beams with a bigger spot size in the lateral direction and wider mini spread-out Bragg peak(mini-SOBP) to irradiate the inner part of a target volume,and pencil beams with a smaller spot size in the lateral direction and narrower mini-SOBP to irradiate the peripheral part of the target volume.Instead of being controlled by the accelerator,the lateral size of the pencil beam was adjusted by inserting Ta scatterers in the beam delivery line.The longitudinal size of the pencil beam(i.e.the width of the mini-SOBP) was adjusted by tilting mini ridge filters along the beam direction.The new spot-scanning beam delivery using carbon ions was investigated theoretically and compared with traditional spot-scanning beam delivery.Our results show that the new spot-scanning beam delivery has smaller lateral penumbra,steeper distal dose fall-off and the dose homogeneity(1-standard deviation/mean) in the target volume is better than 95%.     

Experimental verification of therapeutic doses for the superficially-placed tumor radiotherapy with heavy ions at HIRFL

Up to now, clinical trials of heavy-ion radiotherapy for superficially placed tumors have been carried out for six times and over 60 selected patients have been treated with 80--100 MeV/u carbon ions supplied by the Heavy Ion Research Facility in Lanzhou (HIRFL) at the Institute of Modern Physics, Chinese Academy of Sciences since November, 2006. A passive irradiation system and a dose optimization method for radiotherapy with carbon-ion beams have been developed. Experimental verification of longitudinally therapeutic dose distributions was conducted under the condition of simulating patient treatment in the therapy terminal at HIRFL. The measured depth-dose distributions basically coincide with the expected ones. These results indicate that the irradiation system and the dose optimization method are effective in the ongoing carbon-ion radiotherapy for shallow-seated tumors at HIRFL.     

Gafchromic® EBT films for ion dosimetry

For dose delivery to patients, scanning ion beams are going to be increasingly used in the upcoming ion beam therapy facilities. Especially carbon ion beams are able to produce steep dose gradients. However, the currently used method for patient dose verification, employing ionization chamber arrays, provides a spatial resolution of 1 cm only. As continuous media, EBT films, widely used in photon therapy, are interesting candidates to be used for this purpose. The EBT film is the ancestor of the currently available EBT2 film. In our contribution two dimensional dosimetry and film response quenching in ion beams were investigated. For a real ¹²C patient plan a good qualitative agreement with the planned dose distribution including a high signal-to-noise ratio and a good resolution in the measured photon-equivalent dose was found. The depth-dose response of EBT films for a ¹²C ion beam shows response quenching, which rises towards the Bragg peak. It was quantified by the relative efficiency determined at different depths. Furthermore, the relative efficiency was measured in monoenergetic proton and carbon ion beams. All the measured efficiencies show no significant dependency on the dose up to the highest measured doses of 6 Gy. However, differences between proton and carbon ions as well as between carbon ion beams of different energies were observed. The measurements reveal, that the use of EBT films for absolute dose verification measurements requires to take the relative efficiency into account, dependent on the ion type and energy.     

Heavy-ion cancer therapy

The HIMAC Heavy-Ion Medical Accelerator in Chiba was completed in 1993, and clinical trials of particle radiotherapy for cancer were started using carbon beams accelerated by HIMAC in 1994. Since then, about 2200 patients have been treated in the carbon radiotherapy up to the end of February 2005. A heavy-ion beam generates a Bragg peak in a medium, and it provides biological effects of high RBE and low OER in the Bragg peak region. In heavy-ion radiotherapy, therefore, the beam delivers a high radiation dose to a target volume highly locally. These are excellent advantages for radiotherapy over the case of X-rays. The clinical trials have proved that carbon radiotherapy brings various good results.     

碳离子束混合LET辐照的EBT3辐射变色胶片剂量修正方法

针对EBT3辐射变色胶片对碳离子束混合LET辐照的剂量欠响应比较了两种剂量修正方法。利用260 MeV/u的碳离子束通过被动降能得到多种剂量平均LET的碳离子束,利用这些碳离子束进行了胶片剂量响应刻度辐照,选择最佳的拟合公式得到了胶片剂量刻度曲线。使用RE(Relative Efficiency)量化了EBT3胶片随LET的剂量欠响应,并使用RE剂量修正法修正了混合LET辐照胶片的剂量。此外,根据剂量刻度曲线公式中拟合参数随不同LET所占剂量比例的变化规律,提出了拟合参数剂量修正法并修正了混合LET辐照胶片的剂量。最后比较了这两种方法的结果,表明拟合参数方法得到的剂量偏差在5%以内,优于RE方法10%以内的剂量偏差。