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 as 11C, 15O, 11C 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, 60 detector modules, respectively, with a 36 cm distance measurements. Each detector module consisted of a between directly opposite detector modules for dose deposition 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/u 12C 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.     

Investigation of scatter from out of the field of view and multiple scatter in PET using Monte Carlo simulations

In fully three-dimensional (3D) positron emission tomography (PET) imaging, the scatter fraction (SF) is about 40%-60%, which may degrade the imaging quality severely. Scatter correction is important for high quality image reconstruction. Model-based scatter correction has been proved to be accurate and available in clinical PET. However, it does not correct the scatter from out of the field of view (OFOV) and multiple scatters. In this study, we demonstrate the radial and axial distribution of scatters from OFOV when the source is located in different radial positions. In order to apply the above conclusions to different PET systems, we characterize the scatters from OFOV as a function of the ratio of the scanner diameter to the length of the axial field of view (AFOV) by modeling several typical whole-body and micro PET systems. The proportions of true events (S_0-0), single scatter of one photon (S_1-0) , single scatter of both photons (S_1-1) , double scatter of one photon (S_2-0) and multiple scatter (S_m) are also calculated and compared. Here the 3D-PET Monte Carlo simulations are performed with the Geant4 Application for Tomography Emission (GATE). In summary, the scatters from OFOV tend to be recorded on the lines of response (LOR) far away from the source. They have a much more serious impact on whole-body PET than micro PET depending on the ratio of scanner diameter to the length of AFOV. In whole-body PET, twice scatters including single scatter of both photons (S_1-1) and double scatter of one photon (S_2-0) add up to about 12% so that twice scatter correction must be taken into account to acquire a high quality reconstruction image.     

Neutron dose measurement in carbon ion radiation therapy at HIRFL (IMP)

For radiation protection purposes, the neutron dose in carbon ion radiation therapy at the HIRFL (Heavy Ion Research Facility in Lanzhou) was investigated. The neutron dose from primary 12C ions with a specific energy of 100 MeV/u delivered from SSC was roughly measured with a standard Anderson-Broun rem-meter using a polyethylene target at various distances. The result shows that a maximum neutron dose contribution of 19 mSv in a typically surface tumor treatment was obtained, which is less than 1% of the planed heavy ion dose and is in reasonable agreement with other reports. Also the γ-ray dose was measured in this experiment using a thermo luminescent detector.     

Neutron dose measurement in carbon ion radiation therapy at HIRFL （IMP）

For radiation protection purposes, the neutron dose in carbon ion radiation therapy at the HIRFL （Heavy Ion Research Facility in Lanzhou） was investigated. The neutron dose from primary ^12C ions with a specific energy of 100 MeV/u delivered from SSC was roughly measured with a standard Anderson-Broun rem-meter using a polyethylene target at various distances. The result shows that a maximum neutron dose contribution of 19 mSv in a typically surface tumor treatment was obtained, which is less than 1% of the planed heavy ion dose and is in reasonable agreement with other reports. Also the T-ray dose was measured in this experiment using a thermo luminescent detector.     

An efficient numerical tool for dose deposition prediction applied to synchrotron medical imaging and radiation therapy

Medical imaging and radiation therapy are widely used synchrotron‐based techniques which have one thing in common: a significant dose delivery to typically biological samples. Among the ways to provide the experimenters with image guidance techniques indicating optimization strategies, Monte Carlo simulation has become the gold standard for accurately predicting radiation dose levels under specific irradiation conditions. A highly important hampering factor of this method is, however, its slow statistical convergence. A track length estimator (TLE) module has been coded and implemented for the first time in the open‐source Monte Carlo code GATE/Geant4. Results obtained with the module and the procedures used to validate them are presented. A database of energy‐absorption coefficients was also generated, which is used by the TLE calculations and is now also included in GATE/Geant4. The validation was carried out by comparing the TLE‐simulated doses with experimental data in a synchrotron radiation computed tomography experiment. The TLE technique shows good agreement versus both experimental measurements and the results of a classical Monte Carlo simulation. Compared with the latter, it is possible to reach a pre‐defined statistical uncertainty in about two to three orders of magnitude less time for complex geometries without loss of accuracy.     

重离子治疗肿瘤点扫描控制系统

为了实现重离子治疗肿瘤点扫描,对扫描磁铁进行相应的控制。治疗计划进行剂量与肿瘤适形数据的生成,并传输至点扫描控制器与剂量控制器。同时也向点扫描控制器中存入事例数据,以纳入整个加速器控制时序,等待同步时间事例系统的事例触发。点扫描控制器根据剂量控制器的脉冲控制换点治疗操作以及进行换点治疗的扫描磁铁插值算法平滑处理。剂量控制器对气体电离室前端电子学已经刻度的剂量脉冲进行计数。当达到治疗剂量时,控制法拉第筒来阻挡束流,实现点扫描安全治疗。实验证明在现有的电源与磁铁等设备条件以及束流品质下,点扫描控制系统能实现2mm点间距扫描。     

Monte Carlo simulation of glandular dose in a dedicated breast CT system

A dedicated breast CT system (DBCT) is a new method for breast cancer detection proposed in recent years. In this paper, the glandular dose in the DBCT is simulated using the Monte Carlo method. The phantom shape is half ellipsoid, and a series of phantoms with different sizes, shapes and compositions were constructed. In order to optimize the spectra, monoenergy X-ray beams of 5-80 keV were used in simulation. The dose distribution of a breast phantom was studied: a higher energy beam generated more uniform distribution, and the outer parts got more dose than the inner parts. For polyenergtic spectra, four spectra of Al filters with different thicknesses were simulated, and the polyenergtic glandular dose was calculated as a spectral weighted combination of the monoenergetic dose.     

用于HIRFL重离子治癌的PET成像实验研究与蒙特卡罗模拟

为了HIRFL重离子治癌的需要,使用两个位置灵敏闪烁体探测器组成了一个简单的PET成像系统.对PET成像进行了实验研究,实验测量得到了物体成像以及γ射线能谱,对于511keV全能峰处的能量分辨率为186%,峰总比为52.4%.采用GEANT3程序对该系统进行了模拟计算,并与实验进行比较,最后对该系统进行了优化设计     

Preliminary study of depth of interaction measurement for a PET detector

In this work we studied the feasibility of detecting the depth of interaction (DOI) with two layers of crystal arrays of LYSO and BGO scintillators coupled to a position-sensitive photomultiplier tube (PS-PMT) R8900-C12. A front-end electronics was designed, with which we got different pulse shapes for different crystals to obtain depth information. With the double integration method, we got the DOI histogram of a divided integration ratio of two crystals as the standard to determine the layer-of-interaction. The DOI accuracy, measured by scanning a ²²Na slit source along the side of the module, was 98% for the LYSO layer and 95% for the BGO layer. The energy resolution at 511keV was 13.1% for LYSO and 17.1% for BGO. We obtained good crystal separation in 2D position histograms of both layers. These results could be useful in the manufacture of PET scanners with high spatial resolutions.     

重离子治疗肿瘤点扫描控制系统

为了实现重离子治疗肿瘤点扫描,对扫描磁铁进行相应的控制。治疗计划进行剂量与肿瘤适形数据的生成,并传输至点扫描控制器与剂量控制器。同时也向点扫描控制器中存入事例数据,以纳入整个加速器控制时序,等待同步时间事例系统的事例触发。点扫描控制器根据剂量控制器的脉冲控制换点治疗操作以及进行换点治疗的扫描磁铁插值算法平滑处理。剂量控制器对气体电离室前端电子学已经刻度的剂量脉冲进行计数。当达到治疗剂量时,控制法拉第筒来阻挡束流,实现点扫描安全治疗。实验证明在现有的电源与磁铁等设备条件以及束流品质下,点扫描控制系统能实现2 mm点间距扫描。     

重离子治癌终端的束流强度及剂量监测系统

为实现重离子肿瘤治疗临床实验中对照射束流状态的实时监测,研制了束流强度及剂量监测系统,包括积分电离室、后续电流频率转换电路及LabVIEW数据获取处理三部分。利用闪烁体探测器和标准剂量计测试了系统的线性响应和得到剂量的准确性,结果表明：系统在束流临床照射流强范围内的线性响应好于90%,其实时反馈的临床照射剂量偏差小于5%。同时束流强度及剂量监测系统与安全控制系统相结合,保证束流照射状态满足临床照射的安全要求。     

Benchmarking and validation of a Geant4–SHADOW Monte Carlo simulation for dose calculations in microbeam radiation therapy

Microbeam radiation therapy (MRT) is a synchrotron‐based radiotherapy modality that uses high‐intensity beams of spatially fractionated radiation to treat tumours. The rapid evolution of MRT towards clinical trials demands accurate treatment planning systems (TPS), as well as independent tools for the verification of TPS calculated dose distributions in order to ensure patient safety and treatment efficacy. Monte Carlo computer simulation represents the most accurate method of dose calculation in patient geometries and is best suited for the purpose of TPS verification. A Monte Carlo model of the ID17 biomedical beamline at the European Synchrotron Radiation Facility has been developed, including recent modifications, using the Geant4 Monte Carlo toolkit interfaced with the SHADOW X‐ray optics and ray‐tracing libraries. The code was benchmarked by simulating dose profiles in water‐equivalent phantoms subject to irradiation by broad‐beam (without spatial fractionation) and microbeam (with spatial fractionation) fields, and comparing against those calculated with a previous model of the beamline developed using the PENELOPE code. Validation against additional experimental dose profiles in water‐equivalent phantoms subject to broad‐beam irradiation was also performed. Good agreement between codes was observed, with the exception of out‐of‐field doses and toward the field edge for larger field sizes. Microbeam results showed good agreement between both codes and experimental results within uncertainties. Results of the experimental validation showed agreement for different beamline configurations. The asymmetry in the out‐of‐field dose profiles due to polarization effects was also investigated, yielding important information for the treatment planning process in MRT. This work represents an important step in the development of a Monte Carlo‐based independent verification tool for treatment planning in MRT.     

Influence of thyroid volume reduction on absorbed dose in 131I therapy studied by using Geant4 Monte Carlo simulation

A simulation study has been performed to quantify the effect of volume reduction on the thyroid absorbed dose per decay and to investigate the variation of energy deposition per decay due to β- and γ-activity of 131I with volume/mass of thyroid, for water, ICRP- and ICRU-soft tissue taken as thyroid material. A Monte Carlo model of the thyroid, in the Geant4 radiation transport simulation toolkit was constructed to compute the β- and γ-absorbed dose in the simulated thyroid phantom for various values of its volume. The effect of the size and shape of the thyroid on energy deposition per decay has also been studied by using spherical, ellipsoidal and cylindrical models for the thyroid and varying its volume in 1-25 cm3 range. The relative differences of Geant4 results for different models with each other and MCNP results lie well below 1.870%. The maximum relative difference among the Geant4 estimated results for water with ICRP and ICRU soft tissues is not more than 0.225%. S-values for ellipsoidal, spherical and cylindrical thyroid models were estimated and the relative difference with published results lies within 3.095%. The absorbed fraction values for beta particles show a good agreement with published values within 2.105% deviation. The Geant4 based simulation results of absorbed fractions for gammas again show a good agreement with the corresponding MCNP and EGS4 results （±6.667%） but have 29.032% higher values than that of MIRD calculated values. Consistent with previous studies, the reduction of the thyroid volume is found to have a substantial effect on the absorbed dose. Geant4 simulations confirm dose dependence on the volume/mass of thyroid in agreement with MCNP and EGS4 computed values but are substantially different from MIRD8 data. Therefore, inclusion of size/mass dependence is indicated for 131I radiotherapy of the thyroid.     

多模式离子推力器栅极系统三维粒子模拟仿真

栅极系统是离子推力器推力产生的主要部件,推力器的性能和寿命都与栅极系统密切相关.对于具有多种工作模态的离子推力器,基于电流电压入口的仿真可以有效评估推力器的工作状况.采用三维粒子模拟方法对两栅极系统等离子体输运过程进行了仿真,获得了不同模式下的推力器性能参数,对比NSTAR的在轨测试参数,验证了模型的正确性;分析了工作模式变化对栅极区域电场分布和束流状态的影响以及离子推力器多模式设计需求.分析结果表明:远离栅极系统的外凸型屏栅鞘层和内凹型零等势面、低鞍点电势值和平缓的下游电势分布,有利于提高栅极系统离子通过率,抑制电子返流,减小Pits-and-Grooves腐蚀,是离子推力器工作模式的设计方向;提高束流电压会导致发散角损失增大,但可扩展栅极工作电流范围,在束流强度较大的模式下,使束流具有较好的聚焦状态,有利于减小Barrel腐蚀.研究结果为多模式离子推力器工作模式设计提供了参考.     

Influence of thyroid volume reduction on absorbed dose in ~(131)I therapy studied by using Geant4 Monte Carlo simulation

A simulation study has been performed to quantify the effect of volume reduction on the thyroid absorbed dose per decay and to investigate the variation of energy deposition per decay due to β- and γ-activity of ~(131)I with volume/mass of thyroid,for water,ICRP- and ICRU-soft tissue taken as thyroid material.A Monte Carlo model of the thyroid,in the Geant4 radiation transport simulation toolkit was constructed to compute the β- and γ-absorbed dose in the simulated thyroid phantom for various values of its volume.The effect of the size and shape of the thyroid on energy deposition per decay has also been studied by using spherical,ellipsoidal and cylindrical models for the thyroid and varying its volume in 1-25 cm~3 range.The relative differences of Geant4 results for different models with each other and MCNP results he well below 1.870%.The maximum relative difference among the Geant4estimated results for water with ICRP and ICRU soft tissues is not more than 0.225%.S-values for ellipsoidal,spherical and cylindrical thyroid models were estimated and the relative difference with published results lies within3.095%.The absorbed fraction values for beta particles show a good agreement with published values within 2.105%deviation.The Geant4 based simulation results of absorbed fractions for gammas again show a good agreement with the corresponding MCNP and EGS4 results(±6.667%) but have 29.032%higher values than that of MIRD calculated values.Consistent with previous studies,the reduction of the thyroid volume is found to have a substantial effect on the absorbed dose.Geant4 simulations confirm dose dependence on the volume/mass of thyroid in agreement with MCNP and EGS4 computed values but are substantially diiferent from MIRD8 data.Therefore,inclusion of size/mass dependence is indicated for ~(131)I radiotherapy of the thyroid.     

Prompt gamma detection for range verification in proton therapy

It is an on-going challenge to verify the proton range in situ during proton therapy. Since the protons stop in target tissue, measurement of gamma-rays emitted either promptly from nuclear de-excitation or in pair from positron annihilation is the feasible method to monitor the proton range in-vivo. Using the technique of gamma collimation, we empirically demonstrated that the proton range and prompt gamma distribution are well correlated in the therapy energy range, and that measuring prompt gammas is a viable method for the clinical application. However, this collimation technique appears not to be applicable to passively scattered proton beams. The device chosen for gamma imaging in 2D is an electron tracking Compton camera, which images single-emission photons employing a gas chamber to induce Compton scattering. Images of prompt gammas were attained at the proton beam energy of 140 MeV. Measurements showed that gamma image in the energy range of 800-2000 keV provides a better match with the proton range compared to the image by lower energy gammas.     

γ射线和PET用晶体单元相互作用特性的研究

γ射线与晶体单元的相互作用特性对于PET探测器的研制有着重要的参考意义. 通过实验测得了不同能量的γ射线在BGO, CsI, NaI 3种闪烁晶体中的探测效率、 能量分辨率等相互作用特性参数, 并与蒙特卡洛模拟结果进行了比较. 结果给出了最适合PET探测器的晶体单元. It’s important for development of PET detector that the interaction effect of γ rays and scintillator units are studied. The detection efficiencies and energy resolutions of γ rays for BGO, CsI and NaI crystal detector units are measured. Geant4 Monte Carlo simulations are compared with the experimental results. The results of the best detection units for PET detector are presented.     

单链表在离子发动机光学系统粒子模拟中的应用

目前离子发动机光学系统数值模拟大多采用PIC方法,由于该方法需要跟踪单个粒子的运动,因此需要存储每个粒子的位置信息与运动信息。传统的粒子模拟程序中,保存粒子信息大多采用数组的方式,但是采用这种方式存在弊端,例如对粒子总数变化的自适应不好。基于此开发了一种使用链式存储结构的粒子模拟程序,该程序使用带头节点的单向链表存储粒子信息。使用基于链式存储结构的PIC方法对离子发动机光学系统进行了粒子模拟,验证了链式粒子信息存储方法在粒子模拟中的可用性。模拟表明：（1）在粒子模拟中采用链式存储结构存储粒子信息,无需预先指定最大粒子总数,程序可自适应粒子总数的变化,因此无需进行试算,节省了计算时间;（2）在粒子模拟中采用链式存储结构,由于不存在内存资源的浪费,因而可显著提高程序的存储效率与计算效率。     

100 GW直线变压器驱动源的物理设计与模拟

 基于已有四开关组直线变压器驱动源(LTD)模块设计了对1 Ω负载输出电流100 kA、脉冲上升时间小于60 ns的八开关组LTD模块,该模块采用轮辐式结构,由8个储能电容、气体开关、峰化电容器组并联向中心负载放电。在此基础上,给出了峰值功率100 GW、共10级的LTD装置的物理设计,该装置为同轴感应电压叠加型脉冲发生器。通过电路模拟和PIC模拟,对物理设计进行了检验,结果表明：10级八开关组100 kA LTD模块串联可使10 Ω负载获得超过100 GW的功率输出,脉冲上升时间小于60 ns,所选取的结构和参数能保证电子流的磁绝缘,高压脉冲能有效传输到负载。     

单链表在离子发动机光学系统粒子模拟中的应用

 目前离子发动机光学系统数值模拟大多采用PIC方法,由于该方法需要跟踪单个粒子的运动,因此需要存储每个粒子的位置信息与运动信息。传统的粒子模拟程序中,保存粒子信息大多采用数组的方式,但是采用这种方式存在弊端,例如对粒子总数变化的自适应不好。基于此开发了一种使用链式存储结构的粒子模拟程序,该程序使用带头节点的单向链表存储粒子信息。使用基于链式存储结构的PIC方法对离子发动机光学系统进行了粒子模拟,验证了链式粒子信息存储方法在粒子模拟中的可用性。模拟表明：（1）在粒子模拟中采用链式存储结构存储粒子信息,无需预先指定最大粒子总数,程序可自适应粒子总数的变化,因此无需进行试算,节省了计算时间;（2）在粒子模拟中采用链式存储结构,由于不存在内存资源的浪费,因而可显著提高程序的存储效率与计算效率。