IMP重离子治癌中的剂量计算方法

针对深部肿瘤重离子治疗临床试验的需求,首先在水介质中进行生物有效剂量的优化计算,然后根据CT图像中像素CT值与水等效长度转换系数之间的关系, 结合水中的深度剂量分布曲线对每个像素进行积分得到CT图像上的生物有效剂量分布。同时介绍了基于被动式束流配送系统适形照射时的剂量确定方式, 并提出二维适形放疗也应使用分层照射方式以适应治疗时的不同要求。 这些方法适合目前及今后在IMP进行的重离子治癌临床试验研究中治疗计划系统的需要。Basic algorithms of biological effective dose optimization and dose distribution on CT image for the heavy ion therapy project at the Institute of Modern Physics (IMP), Chinese Academy of Sciences (CAS) are reported in this paper. Firstly, biological effective dose optimization is conducted in water. According to the relationship between CT number and water equivalent path length, an integral algorithm is used to calculate the average dose within a pixel and then the dose distribution in tissue is derived. Secondly, the dose determination of layer stacking conformal irradiation is described and the layer stacking method is proposed to be applied in two dimensional conformal irradiation. These methods are sufficient to the requirements of the ongoing and future heavy ion clinical trials conducted at IMP.     

基于混合Batho修正的RBM剂量计算方法在仿真头模实验中的剂量学验证

基于混合Batho修正的规则束模型剂量计算方法可以根据放射治疗过程中加速器相关物理参数和从病人的医学影像数据中获取的人体组织非均匀信息计算出人体内的剂量场数据。在简要介绍该方法的基础上,通过将其计算结果与仿真头模的实测结果进行对比,初步证明了该方法可以作为一种较为可靠的快速剂量计算方法供临床治疗计划系统使用。The dose distribution can be calculated by the Regular Beam Model （RBM） dose engine based on the accelerator parameters and the inhomogeneity data of the patient for the radiotherapy planning. In this paper, the newly-developed mixed Batho correction based RBM is introduced, and then the calculation results are compared with the measured ones using the head phantom. The results demonstrate that the RBM dose engine can be used as a kind of reliable fast dose calculation tool in the clinical treatment planning system.     

Study of the online event filtering algorithm for BESⅢ

The results of a study of event tagging strategies for elementary physics processes in the τ-charm region are presented. The algorithm for online event filtering is optimized by adopting the information provided by different sub-detectors according to their strengths and capacities. The algorithm is tested with various generated physics and background events. The results indicate that the algorithm satisfies the requirements of BESⅢ physics analysis and its     

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.     

Fast local reconstruction by selective backprojection for low dose in dental computed tomography

The high radiation dose in computed tomography (CT) scans increases the lifetime risk of cancer, which becomes a major clinical concern. The backprojection-filtration (BPF) algorithm could reduce the radiation dose by reconstructing the images from truncated data in a short scan. In a dental CT, it could reduce the radiation dose for the teeth by using the projection acquired in a short scan, and could avoid irradiation to the other part by using truncated projection. However, the limit of integration for backprojection varies per PI-line, resulting in low calculation efficiency and poor parallel performance. Recently, a tent BPF has been proposed to improve the calculation efficiency by rearranging the projection. However, the memory-consuming data rebinning process is included. Accordingly, the selective BPF (S-BPF) algorithm is proposed in this paper. In this algorithm, the derivative of the projection is backprojected to the points whose x coordinate is less than that of the source focal spot to obtain the differentiated backprojection. The finite Hilbert inverse is then applied to each PI-line segment. S-BPF avoids the influence of the variable limit of integration by selective backprojection without additional time cost or memory cost. The simulation experiment and the real experiment demonstrated the higher reconstruction efficiency of S-BPF.     

Fast local reconstruction by selective backprojection for low dose in dental computed tomography

The high radiation dose in computed tomography （CT） scans increases the lifetime risk of cancer, which becomes a major clinical concern. The backprojection-filtration （BPF） algorithm could reduce the radiation dose by reconstructing the images from truncated data in a short scan. In a dental CT, it could reduce the radiation dose for the teeth by using the projection acquired in a short scan, and could avoid irradiation to the other part by using truncated projection. However, the limit of integration for backprojection varies per PI-line, resulting in low calculation efficiency and poor parallel performance. Recently, a tent BPF has been proposed to improve the calculation efficiency by rearranging the projection. However, the memory-consuming data rebinning process is included. Accordingly, the selective BPF （S-BPF） algorithm is proposed in this paper. In this algorithm, the derivative of the projection is backprojected to the points whose x coordinate is less than that of the source focal spot to obtain the differentiated backprojection. The finite Hilbert inverse is then applied to each PI-line segment. S-BPF avoids the influence of the variable limit of integration by selective backprojection without additional time cost or memory cost. The simulation experiment and the real experiment demonstrated the higher reconstruction efficiency of S-BPF.     

Motion estimation based on an improved block matching technique

An improved block-matching algorithm for fast motion estimation is proposed. The matching criterion is the sum of absolute difference. The basic idea is to obtain the best estimation of motion vectors by an optimization of the search process which can terminate the time-consuming computation of matching evaluation between the current block and the ineligible candidate block as early as possible and eliminate the search positions as many as possible in the search area. The performance of this algorithm is evaluated by theoretic analysis and compared with the full search algorithm (FSA). The simulation results demonstrate that the computation load of this algorithm is much less than that of FSA, and the motion vectors obtained by this algorithm are identical to those of FSA.     

Motion estimation based on an improved block matching technique

An improved block-matching algorithm for fast motion estimation is proposed. The matching criterion is the sum of absolute difference. The basic idea is to obtain the best estimation of motion vectors by an optimization of the search process which can terminate the time-consuming computation of matching evaluation between the current block and the ineligible candidate block as early as possible and eliminate the search positions as many as possible in the search area. The performance of this algorithm is evaluated by theoretic analysis and compared with the full search algorithm (FSA). The simulation results demonstrate that the computation load of this algorithm is much less than that of FSA, and the motion vectors obtained by this algorithm are identical to those of FSA.     

A simulation study of a dual-plate in-room PET system for dose verification in carbon ion therapy

During carbon ion therapy,lots of positron emitters such as11C,15O,10C are generated in irradiated tissues by nuclear reactions,and can be used to track the carbon beam in the tissue by a positron emission tomography(PET)scanner.In this study,an dual-plate in-room PET scanner has been designed and evaluated based on the GATE simulation platform to monitor patient dose in carbon ion therapy.The dual-plate PET is designed to avoid interference with the carbon beamline and with patient positioning.Its performance was compared with that of four-head and full-ring PET scanners.The dual-plate,four-head and full-ring PET scanners consisted of 30,60,60detector modules,respectively,with a 36 cm distance between directly opposite detector modules for dose deposition measurements.Each detector module consisted of a 24×24 array of 2 mm×2 mm×18 mm LYSO pixels coupled to a Hamamatsu H8500 PMT.To estimate the production yield of positron emitters,a 10 cm×15 cm×15 cm cuboid PMMA phantom was irradiated with 172,200,250 MeV/u12C beams.3D images of the activity distribution measured by the three types of scanner are produced by an iterative reconstruction algorithm.By comparing the longitudinal profile of positron emitters along the carbon beam path,it is indicated that use of the dual-plate PET scanner is feasible for monitoring the dose distribution in carbon ion therapy.     

Retrieval of C_n~2 profile from differential column image motion lidar using the regularization method

We develop a regularization-based algorithm for reconstructing the C_n~2 profile using the profile of Fried's transverse coherent length(r_0) of differential column image motion(DCIM) lidar. This algorithm consists of fitting the set of measured data to a spline function and a two-stage inversion method based on regularized least squares QR-factorization(LSQR) in combination with an adaptive selection method. The performance of this algorithm is analyzed by a simulated profile generated from the HV5∕7model and experimental DCIM lidar data. Both the simulation and experiment support the presented approach. It is shown that the algorithm can be applied to estimate a reliable C_n~2 profile from DCIM lidar.     

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。     

重离子放射治疗双微小展宽峰组合照射方法

在基于被动式束流配送系统的分层适形重离子治疗当中,需要利用微型脊形过滤器（mini ridge filter,mini-RF）将单能重离子束Bragg峰展宽为峰区近似高斯分布的微小展宽峰（mini spread-out Bragg peak,mini-SOBP）,从而达到减少照射分层数及照射时间的目的。采用较宽的mini-SOBP可以有效减少照射分层数,但会增大展宽Bragg峰（SOBP）远端剂量跌落距离,增加对靶区后方正常组织或危及器官的辐照剂量。这一问题可以通过双mini-SOBP的组合照射方法来解决。使用2种mini-RF对单能重离子束Bragg峰略微展宽得到2种半高宽（FWHM）且剂量分布近似高斯分布的mini-SOBP,通过基于放射生物学模型的剂量优化,证实了在SOBP平顶区按生物有效剂量均匀和物理吸收剂量均匀的展宽情况下,双mini-SOBP组合照射方法均可以在减少照射分层数的同时较大幅度地减小SOBP远端剂量跌落距离。In layer-stacking conformal heavy-ion therapy based on passive beam delivery system,it is necessary to minimize the layer numbers and reduce irradiation time for layer-stacking conformal heavy-ion therapy.Gaussian shaped mini spread-out Bragg peaks (mini-SOBP) were generated by mini ridge filters (mini-RF) for monoenergetic heavy ion beams.It is effective to minimize the layer number by using mini-SOBPs with the bigger full width at the half maximum (FWHM),but in this way the distal dose fall-off distance of a spread-out Bragg peak (SOBP) will be enlarged,increasing the radiation damage to normal tissue or organ at risk behind the target volume.This issue could be solved by using mini-SOBPs based combinatorial irradiation method.In this study,Gaussian shaped mini-SOBPs with two different FWHMs were generated by two different mini-RFs for monoenergetic heavy ion beams.Based on radiobiological model calculations and dose optimizations,the mini-SOBPs based combinatorial irradiation method was confirmed to reduce the distal dose fall-off distances of SOBPs while minimizing the layer numbers for layer-stacking conformal heavy-ion therapy.     

A dose comparison of proton radiotherapy and photon radiotherapy for pediatric brain tumor

The purpose of this study was to investigate the effectiveness of photon radiotherapy and to compare the dose of treatment planning between proton radiotherapy and 3D conformal radiation therapy (3D-CRT) for pediatric brain tumor patients. This study was conducted in five pediatric brain tumor patients who underwent craniospinal irradiation treatment from October 2013 to April 2014 in the hospital. The study compared organs at risk (OARs) by assessing the dose distribution of normal tissue from the proton plan and 3D-CRT. Furthermore, this study assessed the treatment plans by looking at the homogeneity index (HI) and conformity index (CI). As a result, the study revealed OARs due to the small volume proton radiotherapy dose distribution in the normal tissue. Also, by comparing HI and CI between the 3D-CRT and proton radiotherapy plan, the study found that the dose of proton radiotherapy plan was homogenized. When conducting 3D-CRT and proton radiotherapy in a dose–volume histogram comparison, the dose of distribution turned out to be low. Consequently, proton radiotherapy is used for protecting the normal tissue, and is used in tumor tissue as a homogenized dose for effective treatment.     

初步的IMP重离子治疗计划系统

中国科学院近代物理研究所基于兰州重离子研究装置(HIRFL/HIRFL-CSR),在被动型束流配送系统下采用二维分层适形照射治疗技术开展重离子治癌临床试验研究。为了更好地利用重离子束在肿瘤放射治疗中的生物物理优势并保障重离子临床治疗试验的顺利实施,一个初级版本的重离子治疗计划系统已经设计完成。此计划系统是针对被动型束流配送系统下的二维分层适形照射治疗方式来进行设计的。介绍了此系统的设计框架、可提供的功能以及利用宽束算法进行剂量计算在此系统中的实现。通过人体仿真体模实验证实由该治疗计划系统给出的靶区计划剂量与实测剂量的偏差在5%之内。最后讨论了设计较为完备的重离子治疗计划系统仍需解决的问题。     

全局稳定性一致的共形卷积完全匹配层

在正交网格体系中建立物理模型共形描述的基础上,针对采用扩展元胞技术的共形时域有限差分(ECT-CFDTD)方法模拟计算波导器件遇到的开放端口截断问题,给出了积分形式的共形卷积完全匹配层方法,算法具有与ECT-CFDTD相同的数值稳定性。设置不同的完全匹配层的控制参数,对波导中有消逝波存在的情况进行长时间模拟计算,分析共形卷积完全匹配层对消逝波的长效截断能力,分析卷积完全匹配层的截断误差。计算结果显示:积分形式的共形卷积完全匹配层可有效截断波导器件的开放端口。     

谱域OCT的印刷电路板三防漆厚度测量

三防漆是一种广泛应用在印刷电路板（PCB）的保护性涂层,可以有效保护PCB使其免受恶劣环境的损害。三防漆的厚度是评价三防漆涂层质量的关键指标,因此需要对三防漆涂层进行厚度检测。提出了将谱域光学相干断层扫描成像技术（spectral domain optical coherence tomography,SD-OCT）与图像分割算法相结合,实现对三防漆涂层厚度快速无损测量。为了提高测量精度,选用了宽带SLD光源（带宽：180 nm）来设计SD-OCT系统,系统轴向分辨率达到1.72 μm。同时设计了基于边缘跟踪的涂层分割算法来实现三防漆涂层的快速准确分割。为了评估所设计方法的准确性,利用传统的金相切片方法进行了三防漆的厚度测量,将测量结果与该方法测量结果进行比较,分析了两种方法检测到的涂层上下边界吻合程度以及厚度差异。此外,还将所提出的涂层分割算法与我们组之前研究的基于图像梯度的边缘检测算法进行对比,分析了两种方法在测量结果的准确性和运行效率方面的差异,以此来评估该方法的优劣势。结果表明,所设计的三防漆厚度测量方法与传统的金相切片方法的测量结果具有很好的一致性,可以准确地实现三防漆的厚度测量;基于SD-OCT系统的三维成像能力可以直观地看到三防漆厚度地形图,克服了传统的金相切片方法无法进行区域性的三防漆厚度测量的缺陷;相比之前提出的基于图像梯度的边缘检测算法,此方法测量结果更加准确,效率显著提升,具备实时测量的潜力。     

放疗绝对剂量的数学算法模型

本文提出一种通过物理模型计算放疗过程中每一个组织深度处绝对剂量的算法,它可代替蒙特卡罗仿真的部分工作且耗费时间更少.这个算法是基于对照射野内X射线产生电子的能量注量的积分运算,并考虑了射线的能谱及二次散射线,得到了后向散射对表面剂量的贡献比例,同时得到前向散射、后向散射及原射线剂量贡献的关系.比较了二次光子和二次电子的三维能谱,得出该能谱是粒子注量关于粒子能量和粒子运动方向的函数.为了得到每一深度处的光子注量,计算了有连续能谱的X射线的期望质量衰减系数.上述算法计算得到的绝对剂量与蒙特卡罗方式仿真的结果趋势一致,两者的差异在于算法未考虑高于二次的散射线.最后将算法应用到非均匀模体剂量计算,能准确反映其中剂量分布特点且具有较小的误差.     

精确放射治疗中改进的楔形挡板剂量修正算法

在精确放射治疗中,精确计算楔形挡板的楔形因子是精确修正加入楔形挡板后剂量值的关键。提出了基于衰减系数法思想的改进楔形野剂量修正算法,首先计算了楔形挡板材料的线性衰减系数,再根据楔形挡板的实际剖面尺寸,建立了线性拟合公式计算射线束穿过楔形挡板的实际厚度,精确计算楔形因子,并以AAPM55号报告提供的45°楔形挡板进行了验证。该方法在保证计算精度的同时减少了大量离轴测量数据,减轻了物理师的负担,可以作为一种有效的楔形挡板修正算法应用于放射治疗计划系统中。     

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.     

Discussion on the usefulness of dose dynamic multi-leaf collimator-based plan to overcome dose limit of spinal cord in high-dose radiotherapy

In this study, the conventional plan was compared with the plan that was based on a dose dynamic multi-leaf collimator (MLC), and a dose dynamic MLC was used to evaluate its usefulness. Then, this study examined if it was possible to perform a high-dose radiation therapy by adjusting the dose limit of the spinal cord when the dose dynamic MLC-based plan was used. First of all, linear accelerator was used to compare the conventional plan with the dose dynamic MLC-based plan. Then, the study was conducted in two methods in order to examine the proper range of the shield for the spinal cord when the dose dynamic MLC was used to adjust the dose of the spinal cord. In the first method, X-omat film was used to perform film dosimetry. In the second method, radiation treatment planning (RTP) system was used to find out the proper range among 0, 3, 6, and 9 mm. When film scan was performed in the each range, respectively, from the spinal cord and under the same conditions, it was confirmed to be appropriate to use the range of 3 mm. When the RTP system was used to perform planning in the shield range of each range, respectively, from the spinal cord, dose-volume histogram (DVH) was a slight difference could be found in the region from 25% to 35%. On the contrary, no radiation exposure was found in the region of 35% or higher for all of the each range. Consequently, if MLC is selected in consideration of the planning target volume (PTV), the most proper value can be obtained by selecting the range of 3 mm. Next, the DVH was compared to examine the relationship in PTV when the each range was used for planning. All of the ranges showed the same pattern up to the point of 90%. However, the results were different in the region of higher than 90% because the dose was low for the spinal cord, and a relatively useful dose was used for PTV when the range was 3 mm.