装饰装修用石材所致室内γ外照射剂量率的估算方法研究

目的 建立更加准确估算装饰装修用石材所致室内γ外照射剂量率的方法。方法 结合室内空间模型和装饰装修情况,基于MonteCarlo方法计算铺设不同面积和厚度石材后室内离地1m中心处的外照射剂量率和剂量率转换系数,并开展实验测量验证;结合计算结果,进一步探讨装饰装修用石材的放射性含量限值。结果 在同一室内,随着石材铺设厚度或面积增加,石材中放射性核素所致室内γ剂量率随之增大。在相同铺设面积条件下,剂量率转换系数随铺设厚度近似线性增加;在相同铺设厚度条件下,剂量率转换系数随铺设面积增加而增加,但增加呈减缓趋势。在铺设相同石材情况下,房间越小,γ剂量率增加越大。模拟计算值与实测结果在±20%以内吻合。结论 装饰装修用石材所致室内γ外照射剂量率的增加不仅与石材中的放射性核素含量有关,而且还依赖于石材的铺设面积、厚度以及房间大小。本研究的计算方法可更加准确估算出装饰装修石材所致居民的附加外照射剂量,可为修订装饰装修材料中放射性核素含量限值标准提供理论依据。     

图像引导多源γ射束立体定向放射治疗临床测试

目的介绍图像引导的体部多源γ射束立体定向放射治疗系统性能测量。方法应用深圳市奥沃医学新技术发展有限公司生产的体部多源γ射束立体定向放射治疗系统及江苏瑞尔医疗科技有限公司的图像引导放射治疗定位系统,采用PTW-UNIDOS剂量仪、PTW 0.6 cc电离室、PTW 0.125 cc电离室、PTW 0.015 cc电离室、PTW-TW60008半导体探测器、EPSON 10000XL背透光扫描仪、EBT2免冲洗放疗胶片,测量γ射束立体定向放射治疗定位参考点偏差、焦点剂量率、治疗计划软件三维图像重建位置误差以及系统综合定位精度。结果定位参考点偏差Δ=0.433 mm,焦点吸收剂量率285.5 Gy/min,三维图像重建位置误差0.8 mm,综合定位精度最大值Δ=1.729 mm。结论该图像引导的体部多源γ射束立体定向放射治疗系统符合国家标准要求。     

用蒙特卡罗方法计算土壤中光子发射体的外照射剂量率转换因子

目的:用蒙特卡罗方法的MCNP、GEANT和MC三种程序分别计算均匀分布在无限大土壤中光子发射体的外照射剂量率转换因子Dcf,并比较其计算结果。方法:1MCNP程序是一种蒙特卡罗辐射输运程序,它能数字化的模拟中子、光子和电子输运。无限大土壤用高为1m、直径为40m的圆柱体来模拟,空气用高为1.5m、直径为40m的圆柱体来模拟,它们的Z轴相同。用此程序计算8种均匀分布在土壤中不同的光子发射体发射的能量范围在200～3000keV之间的光子外照射剂量率转换因子Dcf。使用两种虚拟探测器:点探测器,计算点取在Z轴离土壤1m处的空气中;半径为40cm的球探…     

β核素球囊预防血管再狭窄的临床剂量估算

目的 探讨核素球囊内照射血管内的吸收剂量分布规律。方法 ①依据吸收剂量点核函数模拟计算^90Y、^186Re、^32P灌注球囊时血管组织中的吸收剂量率分布；②用非线性最小二乘法对吸收剂量率随球囊外径及组织深度的变化进行曲线拟合，并由此导出便于临床使用的经验公式。结果 球囊中吸收剂量率峰值出现在血管腔内球囊中，血管表面位于吸收剂量率曲线的拐点处，血管壁及周围组织中的吸收剂量率以近双指数方式下降。吸收剂量、持续照射时间、初始放射性浓度、组织深度及球囊外半径间的关系可用一经验公式表达。结论 血管组织中的β核素吸收剂量分布呈快速下降。该经验公式具有实用价值。     

γ射线高度计在模拟航天器回收舱着陆的静态实验中剂量分布的测量及其评价

γ射线高度计用于航天器回收舱回收着陆过程中的近地高度测量。其内装有活度为２．９６×１０１０Ｂｑ的１３７Ｃｓγ放射源。为了解仪表舱附近，特别是回收舱内有效载荷所在处的γ剂量分布，利用ＦＪ－３１７Ｃ＿γ剂量率仪和ＡＨＰИ－０１－０２γ剂量率仪对单机试验现场的γ剂量场进行了测量。结果表明，当高度计下降到距地面０．１６ｍ时，地面上距γ源５ｍ处的吸收剂量率低于辐射防护限值。随着回收舱高度的增加，地面对γ射线的反射作用将下降。因此，回收舱内有效载荷处的γ剂量率水平将满足安全飞行的要求。     

利用MC方法计算小鼠体内辐射剂量的体素模型的建立

目的建立一个与蒙特卡罗方法结合进行辐射剂量计算的小鼠体素模型。方法采用一只28g雄性裸鼠的序列切片图像,对此序列图片进行配准、组织识别、分割、颜色赋值后,最后在蒙特卡罗程序MCNP中赋予一定的物理属性。结果建立了一个可以满足剂量分布计算所需精度,并且模拟计算时间适中的小鼠体素模型,此模型器官数量为14个,体素大小为0.2mm×0.2mm×0.2mm。结论该研究所建立的小鼠体素模型可用来进行电离辐射剂量学的模拟计算,以开展放射医学、核医学及空间辐射医学等相关领域的研究。     

基于蒙特卡罗方法的脑转移瘤术中放疗施照器设计及剂量学评估

目的 设计适用脑转移瘤术中放疗的施照器并评估其剂量学特征。方法 施照器设计首先通过模拟计算电子束经过一系列不同厚度散射箔后的散射角和剂量率,确定散射箔厚度;其次,建立散射箔位于不同高度的位置评估模型,通过计算模型表面平均能量方差,确定散射箔位置;最后,建立调节层几何结构特征与施照器表面剂量之间的关系,确定调制器的内表面特征。使用蒙特卡罗（MC）EGSnrc/BEAMnrc和EGS4/DOSXYZ程序完成Mobetron加速器、位置评估模型、调节层、施照器建模和剂量学分析。结果 半球囊状施照器的限光筒直径为2.5 cm、筒壁厚0.5 cm,材料为0.2 cm厚水等效材料加0.3 cm厚不锈钢;散射箔厚度0.14 cm,材料为金属钨,位置高度为0.5 cm;调制器为月牙形,材料为水等效材料。该施照器能够使Mobetron 12 MeV的电子束产生半球面剂量分布,剂量率为160 cGy/min,治疗深度为0.85 cm。结论 采用MC模拟设计的适用于高能电子束的半球囊状施照器,能产生半球面剂量分布。     

宇宙辐射电离成分随高度和纬度的变化

用RSS-111型高压电离室徊FD-71型闪烁辐射仪, 除进行了海平面上的跨纬度测量以及不同高度不同纬度的大水面测量外, 还特别设计了垂直性飞机航测及不同高度上的跨纬度水平性飞机航测。利用测量得到的数据, 总结出用解析式表达的低空宇宙辐射电离成分空气吸收剂量率随海拔高度以及在不同高度随地磁纬度变化的经验公式。经验公式计算值与本题实验值相比符合非常好, 全部数据在10%以内一致。与美国发表的同类测量数据进行比较检验, 符合得也令人满意, 98%的数据在10%以内一致。     

基于可视中国数字人体素模型的比吸收分数计算

目的开展器官比吸收分数的计算研究,以定量计算人体的内照射剂量。方法在高分辨可视中国人三维解剖结构数据集基础上,对原始图片进行分割、标识、赋值和低抽样,构建了基于体素数据的内辐射模型。采用基于蒙特卡洛方法的MCNPX软件,模拟了光子在人体器官组织间的传输过程,得到了各能量下源器官对目标器官的比吸收分数,并与中国数学模型的计算结果进行了比较。结果两组数据在趋势上具有相似性,说明了计算方法的正确性;体素模型的计算结果较数学模型大,主要源自模型间器官质量和器官距离的不同;数学模型中部分结果具有明显差异性,说明其计算结果可能不准确。结论基于中国数字人体素模型计算的比吸收分数,提供了更为准确和可靠的数据。     

一种高能X射线任意形状野剂量计算模型的建立方法

目的探讨建立一种可用于模拟计算任意形状和强度的高能X射线外照射射束分布的快速剂量计算模型.方法基于蒙特卡罗方法建立的源模型包含一个由方形网格元素组成的光子注量矩阵,通过对源模型进行重构,以反映物理射束经过加速器射束成形部件后所引起的形状改变.结果比较规则野和不规则野的计算和测量结果,规则野误差小于2%,不规则野在10%等剂量线误差稍大.结论这一模型能够较为精确的模拟计算任意形状和强度的外照射放射治疗射束分布.     

Experimental measurements and Monte Carlo calculations of dosimetric parameters of the IRA1-(103)Pd brachytherapy source.

This work presents a brachytherapy source having (103)Pd adsorbed onto a cylindrical silver rod that has been developed by Agricultural, Medical and Industrial Research School for permanent implant applications. Dosimetric characteristics (dose-rate constant, radial dose function, anisotropy function and anisotropy factor) of this source were experimentally and theoretically determined in terms of the updated AAPM Task Group 43 (TG-43U1) recommendations. Measurements were performed using TLD-GR200A circular chip dosimeters using standard methods employing thermoluminescent dosimeters in a Perspex phantom. Precision machined bores in the phantom located dosimeters and source in a reproducible fixed geometry providing for transverse-axis and angular dose profiles over a range of distances from 0.5 to 5cm. The Monte Carlo N-Particle (MCNP) code, version 4C was used to evaluate the dose-rate distributions around this model (103)Pd source in water and Perspex phantoms. The Monte Carlo calculated dose-rate constant of the IRA1-(103)Pd source in water was found equal to Lambda=0.669cGy/h/U with approximate uncertainties of +/-0.1%. The anisotropy function, F(r, theta), and the radial dose function, g(L)(r), of the IRA1-(103)Pd source were also measured in Perspex phantom and calculated in both Perspex and liquid water phantom.     

Monte Carlo dose calculations for a 6-MV photon beam in a thorax phantom

Purpose In this study we evaluated the accuracy of the Monte Carlo (MC) and effective path length (EPL) methods for dose calculations in the inhomogeneous thorax phantom. Materials and methods The Philips SL 75/5 linear accelerator head was modeled using the MCNP4C Monte Carlo code. An anatomic inhomogeneous thorax phantom was irradiated with a 6-MV photon beam, and the doses along points of the central axis of the beam were measured by a small ionization chamber. The central axis relative dose was calculated by the MCNP4C code and the EPL method in a conventional treatment planning system. The results of calculations and measurements were compared. Results For all measured points on the thorax phantom the results of the MC method were in agreement with the actual measurement (local difference was less than 2%). For the EPL method, the amount of error was dependent on the field size and the point location in the phantom. The maximum error was +19.5 and +26.8 for field sizes of 10 × 10 and 5 × 5 cm² for lateral irradiation. Conclusion Our study showed large, unacceptable errors for EPL calculations in the lung for both field sizes. The accuracy of the MC method was better than the recommended value of 3%. Thus, application of this method is strongly recommended for lung dose calculations, especially for small field sizes.     

3D dose distribution calculation in a voxelized human phantom by means of Monte Carlo method

The aim of this work is to provide the reconstruction of a real human voxelized phantom by means of a MatLab^® program and the simulation of the irradiation of such phantom with the photon beam generated in a Theratron 780^® (MDS Nordion) ⁶⁰Co radiotherapy unit, by using the Monte Carlo transport code MCNP (Monte Carlo N-Particle), version 5. The project results in 3D dose mapping calculations inside the voxelized antropomorphic head phantom.The program provides the voxelization by first processing the CT slices; the process follows a two-dimensional pixel and material identification algorithm on each slice and three-dimensional interpolation in order to describe the phantom geometry via small cubic cells, resulting in an MCNP input deck format output. Dose rates are calculated by using the MCNP5 tool FMESH, superimposed mesh tally, which gives the track length estimation of the particle flux in units of particles/cm². Furthermore, the particle flux is converted into dose by using the conversion coefficients extracted from the NIST Physical Reference Data.The voxelization using a three-dimensional interpolation technique in combination with the use of the FMESH tool of the MCNP Monte Carlo code offers an optimal simulation which results in 3D dose mapping calculations inside anthropomorphic phantoms. This tool is very useful in radiation treatment assessments, in which voxelized phantoms are widely utilized.     

Borehole prompt gamma neutron activation and comparison with Monte Carlo simulation using MCNP code: Borehole PGNAA experiment comparison with MCNP

A borehole experiment using prompt gamma neutron activation analysis has been performed in a large sample box having a volume of 1 m³. Brine solutions having a salt concentration in the range of 0–10 wt% of sodium chloride has been used. Chlorine prompt gamma spectral response as a function of the salt concentrations have been obtained. A simulation of the above experiments has also been carried out using the MCNP4A Monte Carlo code. Comparison of the experimental spectral response versus the simulated MCNP4A data has produced excellent agreement. In view of the good benchmark data it is proposed that due to the inherent problems associated with the ordinary calibration procedures for the borehole logging tools, one could employ a combined calibration/simulation scheme to circumvent these difficulties and achieve more effective results.     

A quantitative three-dimensional dose attenuation analysis around Fletcher-Suit-Delclos due to stainless steel tube for high-dose-rate brachytherapy by Monte Carlo calculations

PurposeThe commercially available brachytherapy treatment-planning systems today, usually neglects the attenuation effect from stainless steel (SS) tube when Fletcher-Suit-Delclos (FSD) is used in treatment of cervical and endometrial cancers. This could lead to potential inaccuracies in computing dwell times and dose distribution. A more accurate analysis quantifying the level of attenuation for high-dose-rate (HDR) iridium 192 radionuclide (¹⁹²Ir) source is presented through Monte Carlo simulation verified by measurement.Methods and MaterialsIn this investigation a general Monte Carlo N-Particles (MCNP) transport code was used to construct a typical geometry of FSD through simulation and compare the doses delivered to point A in Manchester System with and without the SS tubing. A quantitative assessment of inaccuracies in delivered dose vs. the computed dose is presented. In addition, this investigation expanded to examine the attenuation-corrected radial and anisotropy dose functions in a form parallel to the updated AAPM Task Group No. 43 Report (AAPM TG-43) formalism. This will delineate quantitatively the inaccuracies in dose distributions in three-dimensional space. The changes in dose deposition and distribution caused by increased attenuation coefficient resulted from presence of SS are quantified using MCNP Monte Carlo simulations in coupled photon/electron transport. The source geometry was that of the Vari Source wire model VS2000. The FSD was that of the Varian medical system. In this model, the bending angles of tandem and colpostats are 15° and 120°, respectively. We assigned 10 dwell positions to the tandem and 4 dwell positions to right and left colpostats or ovoids to represent a typical treatment case. Typical dose delivered to point A was determined according to Manchester dosimetry system.Results and ConclusionsBased on our computations, the reduction of dose to point A was shown to be at least 3%. So this effect presented by SS–FSD systems on patient dose is of concern.     

Expression of Cytoskeletal Elements in Proliferating Cells Following Radiation Exposure

Summary

Previous work has demonstrated that radiation exposure modulates the expression of a series of genes, including those that encode cytoskeletal elements. The experiments reported here were designed to examine (1) the comparative effects of neutrons administered at high versus low dose-rates, (2) the comparative effects of neutrons on cycling versus resting cells and (3) the comparative effects of neutrons versus γ-rays on β- and γ-actin mRNA accumulation in Syrian hamster embryo (SHE) cells 1 and 3 h post-irradiation. JANUS fission-spectrum neutrons from Argonne National Laboratory's JANUS reactor administered at high (12 cGy/min) dose-rates had little effect on resting cells, but at very low dose-rates (0·1 cGy/min) had a repressive effect on γ-actin mRNA accumulation. Increased accumulation of β-actin mRNA was detected following the exposure of cells to neutrons administered at high dose-rates, but repression of β-actin mRNA was observed when neutrons were administered at low dose-rates. Cycling cells (unexposed and neutron irradiated) in all cases expressed higher levels of all actin-specific mRNAs than resting cells; β-actin mRNA (but not γ-actin mRNA) was induced to a greater extent in cycling cells than in resting cells during the first hour following neutron exposure. In resting cells, however, low dose-rate neutrons were more effective than low dose-rate γ-rays at repressing both γ- and β-actin mRNA accumulation. These results demonstrate the differential effects of radiation quality (neutrons versus γ-rays) and cell-cycle state on the modulation of actin isotype-specific gene expression.     

Validation of the TOPAS Monte Carlo toolkit for HDR brachytherapy simulations

PurposeThe goal of this work is to validate the user-friendly Geant4-based Monte Carlo toolkit TOol for PArticle Simulation (TOPAS) for brachytherapy applications.Methods and MaterialsBrachytherapy simulations performed with TOPAS were systematically compared with published TG-186 reference data. The photon emission energy spectrum, the air-kerma strength, and the dose-rate constant of the model-based dose calculation algorithm (MBDCA)-WG generic Ir-192 source were extracted. For dose calculations, a track-length estimator was implemented. The four Joint AAPM/ESTRO/ABG MBDCA-WG test cases were evaluated through histograms of the local and global dose difference volumes. A prostate, a palliative lung, and a breast case were simulated. For each case, the dose ratio map, the histogram of the global dose difference volume, and cumulative dose-volume histograms were calculated.ResultsThe air-kerma strength was (9.772 ± 0.001) × 10^?8 U Bq^?1 (within 0.3% of the reference value). The dose-rate constant was 1.1107 ± 0.0005 cGy h^?1 U^?1 (within 0.01% of the reference value). For all cases, at least 96.9% of voxels had a local dose difference within [?1%, 1%] and at least 99.9% of voxels had a global dose difference within [?0.1%, 0.1%]. The implemented track-length estimator scorer was more efficient than the default analog dose scorer by a factor of 237. For all clinical cases, at least 97.5% of voxels had a global dose difference within [?1%, 1%]. Dose-volume histograms were consistent with the reference data.ConclusionsTOPAS was validated for high-dose-rate brachytherapy simulations following the TG-186 recommended approach for MBDCAs. Built on top of Geant4, TOPAS provides broad access to a state-of-the-art Monte Carlo code for brachytherapy simulations.     

Analysis of the dose rate produced by control rods discharged from a BWR into the irradiated fuel pool

BWR control rods become activated by neutron reactions into the reactor. Therefore, when they are withdrawn from the reactor, they must be stored into the storage pool for irradiated fuel at a certain depth under water. Dose rates on the pool surface and the area surrounding the pool should be lower than limits for workers. The MCNP code based on the Monte Carlo method has been applied to model this situation and to calculate dose rates at points of interest.     

A study of neutron spectra from medical linear accelerators.

Medical accelerators with photon energies over 10 MeV generate an undesired fast neutron contamination in the therapeutic beam. In this work, the Monte Carlo code MCNP was used to simulate the transport of these photoneutrons across the head of various medical accelerators of high energy. The average and most probable neutron energies were obtained from these spectra, before and after crossing the accelerator shielding. The degradation of these spectra, when they cross concrete barriers with thickness which vary between 25 and 100 cm, was also studied.     

Effect of dose-rate on induction of neoplastic transformation in vitro by low doses of 232 MeV protons

Purpose:?To determine the effect of dose-rate on induction of neoplastic transformation in vitro by low doses of 232?MeV protons.

Materials and methods:?The experimental system used was the human hybrid cell assay. The dose-rates examined were 50?cGy/min and 20?cGy/h. The dose-rate 20?cGy/h was chosen as this is in the range of the maximum dose-rate that can be experienced in an unshielded space environment following a solar flare. At low dose-rate (LDR), doses from 0.5–100?cGy were studied. At high dose rate (HDR), the dose range was 0.5–200?cGy.

Results:?The data indicated no significant differences between the two dose-rates at doses up to 100?cGy.

Conclusion:?For the endpoint of neoplastic transformation in vitro, high dose-rate data may be sufficient to estimate low dose-rate effects (20?cGy/h) in the dose range up to 100?cGy from 232?MeV protons. The data are of relevance to risk estimation for space travel.