扫描治疗头的蒙特卡罗模型研究

目的:评估质子治疗中扫描治疗头对束流品质的影响。方法:通过扫描治疗头的蒙特卡罗模型研究深度剂量曲线的变化,计算射程移位器对束斑截面的影响以及分析扫描磁场对单质子束的偏转情况。结果:随着能量的增加,质子在水中的射程增加,同时散射也越严重,最终布拉格峰变宽,尾端变胖。相比于直接入射水模,通过治疗头后质子在水中的射程缩短了约0.6 cm,但布拉格峰形基本保持不变;将4 cm厚度聚乙烯射程移位器放置于距离水模表面0、10、20、30、40和50 cm分别进行独立计算,发现与水模距离越远,质子的散射越大,因此治疗过程中射程移位器应尽量靠近患者;当扫描磁铁加载磁场后,束斑将偏离束流中心。设置纵向扫描磁场Bx=0.1 T,横向扫描磁场By=0.3 T,180 MeV质子束在Y方向偏离了2.693 cm,横向扫描磁场使质子在-X方向上偏离了8.427 cm。当束流有偏转的时候,要求射程移位器横截面足够大以满足宽扫描场的需要。结论:扫描治疗头的蒙特卡罗模型将有助于理解质子治疗这一新兴的放疗方法以及熟悉扫描治疗的束流特性,在调试和质量保证中提供参考。     

基于TOPAS计算的磁场下质子辐射剂量分析

目的:分析磁场作用下质子束在模体中的剂量分布,为核磁共振引导的质子放疗提供数据参考。方法:采用蒙特卡罗软件TOPAS计算治疗用质子束在核磁共振横向磁场影响下水体模中的剂量分布。同时采用水-空气-水模型研究磁场下的电子回转效应对不同介质交界处质子剂量分布的影响。结果:在均匀水体模中,当磁场强度在0.5 T以内,质子能量在150 MeV以下时,质子布拉格峰位置沿深度方向偏移在1 mm以内,但与束流入射方向平行的XZ面的高剂量区横向侧移在4 mm左右;当磁场强度达到1.5 T时,150 MeV的质子布拉格峰位置偏移在1 mm以内,但横向侧移达10 mm以上。研究结果还发现在磁场作用下,质子在水与空气交界处的剂量无明显变化。结论:利用蒙特卡罗方法可以准确分析磁场下的质子辐射剂量。横向磁场的存在对质子在深度方向的剂量影响较小,但对横向剂量侧移影响较大,且与能量、磁场强度成正比,而电子回转效应对质子在水与空气交界处的剂量影响近似可以忽略。     

扫描式质子放疗系统自动化束斑测量与分析方法的设计和应用

目的:针对具有录像功能的闪烁体探测器Lynx-PT和笔形束扫描（PBS）质子放疗系统，设计一种质子束斑测量与分析方法，为PBS质子放疗系统的临床调试、束流建模和周期性质控测量提供可靠、高效的解决方案。方法:配置PBS质子放疗系统和Lynx-PT的相关参数，设计数据预处理和束斑分析流程，将两种数据处理流程嵌入自研软件（SpotCheck），实现从束斑采集到数据预处理再到束斑特性分析的全流程自动化。结果:SpotCheck输出的束斑尺寸结果与现有商业化软件（myQA, 比利时IBA公司）以及厂商的现场验收测量结果均保持一致，能成功识别所有束斑录像文件的数据质量问题，并将质子束流占用时间从4 d缩短至0.5 d。结论:本文方法的束斑尺寸计算结果准确、束斑采集速度快、数据处理流程自动化程度高，极大提升了PBS质子放疗系统临床调试和束流建模的效率。     

质子束流蒙特卡罗模型的建立及对脊形滤波器的探究

目的:探究脊形滤波器结构对质子束流展宽的影响。方法:利用蒙特卡罗程序FLUKA建立质子束流模型，并进行验证。模拟质子束流通过三棱柱型（A型）和金字塔型（B型）两种脊形滤波器，比较使用和不使用脊形滤波器的模拟值:束流前端最大剂量50%到束流末端最大剂量50%的宽度（E50-D50）、束流前端80%到束流末端80%的宽度（E80-D80）及束流末端80%到束流末端20%的宽度（D80-D20）。结果:根据模型计算出的121.1 MeV质子对应的模拟值绘制的积分深度剂量曲线与实际测量的积分深度剂量曲线，E50、E70和D80位置偏差不超过0.06 mm；A型相比B型将E50-D50平均多展宽了0.80 mm，将E80-D80 平均多展宽了0.27 mm，将D80-D20 平均多展宽了0.08 mm。结论:建立的质子束流蒙特卡罗模型合理，三棱柱型（A型）脊形滤波器展宽质子束流的效果更好。     

一种新型质子治疗剂量递送系统的设计研究及模拟验证

目的:针对激光等离子体加速的质子束流特性,设计用于剂量递送的新型紧凑治疗头系统,并通过模拟计算验证该方法的有效性与适用性。方法:基于实验上已实现的激光质子束流参数,利用散射体设计软件NEU（Nozzles with Everything Upstream）进行流线型散射体设计。通过散角选择和能散调制进一步优化剂量递送效率,并利用蒙特卡罗模拟计算软件TOPAS（TOol for PArticle Simulation）及底层的Geant4（GEometry ANd Tracking）计算引擎分析并验证激光质子通过此剂量递送方法后水模体中的剂量分布。结果:在直径6 cm、高5 cm的圆柱形靶区内,深度剂量分布平坦度在±1%以内,横向剂量分布在±3%以内。结论:此剂量递送方法及系统适用于现阶段激光质子束流特性,水模体靶区内剂量递送均匀、高效且稳定。     

基于GAMOS的蒙特卡罗方法模拟放疗电子线照射的剂量学研究

目的:探讨基于GAMOS的蒙特卡罗（MC）方法模拟电子线放疗的剂量精确性。方法:运用GAMOS MC程序,建立Varian Rapidarc加速器3档能量（6、9和12 MeV）及3种限光筒［（6×6）、（10×10）和（15×15） cm2］的束流模型,模拟束流在水模体中的剂量分布,并与测量得到的百分深度剂量和等平面剂量分布比较,评估GAMOS软件模拟电子线照射的精确性和运算效率。结果:模拟的粒子数越多,模拟与测量结果的误差越小;当模拟粒子的数量达到5×108时,各个能量的电子线射程（Rp）和50%剂量深度（R50）的模拟结果与测量结果一致;除建成区外,百分深度剂量模拟和测量的结果误差在2%以内;等平面剂量分布模拟和测量的结果误差也在2%以内,模拟的照射野大小与测量结果一致。运算效率中,能量越大,限光筒尺寸越大,并行同步模拟所用的时间越多,模拟时间的变化越大。结论:基于GAMOS的MC方法可准确地模拟放疗电子线照射剂量的分布,粒子数的增加可提高模拟的精确性,并行同步计算可提高模拟的效率。     

碳离子放疗过程中病人体内器官次级中子剂量的蒙特卡洛计算

目的:使用蒙特卡洛方法模拟在碳离子放疗过程中产生的次级中子对人体主要器官的吸收剂量和当量剂量。方法:基于中国科学院近代物理研究所(IMP)的重离子深层肿瘤治疗的束流配送系统治疗头,使用MCNPX对该系统的初级准直器、脊型过滤器、射程移位器以及多叶准直器进行建模,模拟计算400 Me V/u碳离子均匀照射野经过IMP被动式束流配送系统,入射到RPI-Adult男性体模后,统计全身主要器官内的次级中子能谱和吸收剂量与当量剂量。结果:当脑垂体瘤接受治疗并给予50 Gy处方剂量时,不同器官内的中子能谱显示所产生的次级中子能量范围比较大,最大能量高达几百Me V;而且脑部部分器官的中子当量剂量相对比较高,大脑、头盖骨和眼晶体当量剂量为53.18、32.43、33.20 m Sv;离脑部较远的器官,如胸部、肺以及前列腺剂量很低,都小于0.4 m Sv。结论:利用蒙特卡洛方法和计算机仿真人体模型模拟了碳离子放疗过程,并统计了全身大部分器官内次级中子能谱和受到的剂量。本研究计算的结果和结论,再结合相关资料,可以为临床上研究碳离子放疗的远期效应提供参考。     

数值方法分析电子线笔形束模型的能量展宽函数

目的:探讨用离轴比曲线分析电子束照射野笔形束模型能量展宽函数的方法.方法:用PTW mp3三维水箱测量Synergy加速器所有电子束能量、限光筒、空气间隙在不同深度的射野离轴比曲线.用数值分析方法对射野离轴比曲线进行分析,得到电子线照射野笔形束模型能量展宽函数σp(z)随电子束标称能量、限光筒大小和限光筒底端面到体模表面空气间隙变化的规律.将计算得到的σp(z)输入到PLATO治疗计划系统,计算吸收剂量,并与相同条件下用0.6 cc电离室剂量仪测量的结果进行比较.结果:能量展宽σp(z)随深度增加而变大,接近电子最大射程末端,很快减小,呈液滴状分布.能量展宽和电子的标称能量以及限光简大小有关,这主要是电子在体模中的单次和多次散射作用引起的.能量展宽随限光筒低端面到体模表面的空气间隙线性变化.标准条件下吸收剂量的计算值和测量值很接近,最大误差小于±5%.结论:电子束照射野笔形束模型充分考虑电子在体模内的作用特点和过程,是比较好的计算模型.用射野离轴比数据分析电子束照射野笔形束模型的特征参数.结果准确可靠.     

MM50加速器电子线降能器的研究

目的：研制医用电子回旋加速器MM50电子线降能器，以增加MM50加速器电子线能量档i方法：分析高能电子束与物质发生作用损失能量的机理，通过蒙特卡罗模拟计算对降能器进行理论研究，并选择适当的高纯材料和合理的厚度，设计并研制出多种降能器；研究MM50加速器治疗头靶转换器结构并进行适当改造，安装不同的降能器进行调试；实验测量电子线百分深度剂量曲线求得半峰值剂量深度R50（cm），再使用IAEA和NACP推荐的算法获得如。结果：达到了将电子线能量降低2MeV，而没有X射线沾污影响的效果，从而使MM50加速器电子线能量档从现在3档（10MeV、25MeV、50MeV）变为6档（8MeV、10MeV、23MeV、25MeV、47MeV、50MeV）。结论：测量值与参考值吻合很好，并且各种方法所得的结果相一致。该方法可用于其他高能电子加速器、质子和其他重离子等加速器的能量转换或选择系统。     

质子治疗的物理与生物学基础

近几十年来质子治疗在临床上取得了巨大成就,这是因为质子束在物理学和生物学上具有独特的优势。在肿瘤治疗学上质子比常规射线(^60Co、X射线、电子)有两个主要优势：(1)可根据肿瘤在体内的深度,使质子束精确地定位在肿瘤病灶处,以使肿瘤受到最大的照射剂量而不伤害健康组织,从而达到适形治疗。(2)可根据肿瘤的形状改变质子在微观尺度能量沉积的形状,实现辐射生物学效应的改变。基于此,对于形状较复杂的大实体瘤,质子治疗比常规治疗有更高的精度。质子的这些在治疗学上特异的可能性是由其剂量学和辐射生物学特性决定的。剂量学的性质与能量在宏观尺度的沉积特征有关,作为带电粒子,质子在介质中有确定的射程和相对小的散射歧离,此外在射程前端剂量相对较小,而到射程末端剂量达到最大,形成一个尖锐的Bragg峰,基于这屿特点使得肿瘤受到高剂量的照射而周围的健康组织受到很小的伤害;相对生物学效应与能量在微观尺度的沉积特征有关,与重离子相比虽然质子属于低LET射线,但就其能量在微观尺度沉积的性质与常规射线相比质子足致密电离辐射,因此目前已有实验证实质子治疗比常规射线治疗增加了相对生物学效应,然而目前对能量的微观沉积与生物学效应关系的原理仍需要进一步从理论上和实验上研究证明。文中分析了质子与介质的作用过程、以及传能线密度(LET)、相对生物学效应(RBE)、氧增比(OER)等放射治疗学的一般概念,讨论了质子用于肿瘤治疗的物理学与生物学性质。     

A beam source model for scanned proton beams

A beam source model, i.e. a model for the initial phase space of the beam, for scanned proton beams has been developed. The beam source model is based on parameterized particle sources with characteristics found by fitting towards measured data per individual beam line. A specific aim for this beam source model is to make it applicable to the majority of the various proton beam systems currently available or under development, with the overall purpose to drive dose calculations in proton beam treatment planning. The proton beam phase space is characterized by an energy spectrum, radial and angular distributions and deflections for the non-modulated elementary pencil beam. The beam propagation through the scanning magnets is modelled by applying experimentally determined focal points for each scanning dimension. The radial and angular distribution parameters are deduced from measured two-dimensional fluence distributions of the elementary beam in air. The energy spectrum is extracted from a depth dose distribution for a fixed broad beam scan pattern measured in water. The impact of a multi-slab range shifter for energy modulation is calculated with an own Monte Carlo code taking multiple scattering, energy loss and straggling, non-elastic and elastic nuclear interactions in the slab assembly into account. Measurements for characterization and verification have been performed with the scanning proton beam system at The Svedberg Laboratory in Uppsala. Both in-air fluence patterns and dose points located in a water phantom were used. For verification, dose-in-water was calculated with the Monte Carlo code GEANT 3.21 instead of using a clinical dose engine with approximations of its own. For a set of four individual pencil beams, both with the full energy and range shifted, 96.5% (99.8%) of the tested dose points satisfied the 1%/1 mm (2%/2 mm) gamma criterion.     

Adjustment of the lateral and longitudinal size of scanned proton beam spots using a pre-absorber to optimize penumbrae and delivery efficiency

In scanned-beam proton therapy, the beam spot properties, such as the lateral and longitudinal size and the minimum achievable range, are influenced by beam optics, scattering media and drift spaces in the treatment unit. Currently available spot scanning systems offer few options for adjusting these properties. We investigated a method for adjusting the lateral and longitudinal spot size that utilizes downstream plastic pre-absorbers located near a water phantom. The spot size adjustment was characterized using Monte Carlo simulations of a modified commercial scanned-beam treatment head. Our results revealed that the pre-absorbers can be used to reduce the lateral full width at half maximum (FWHM) of dose spots in water by up to 14 mm, and to increase the longitudinal extent from about 1 mm to 5 mm at residual ranges of 4 cm and less. A large factor in manipulating the lateral spot sizes is the drift space between the pre-absorber and the water phantom. Increasing the drift space from 0 cm to 15 cm leads to an increase in the lateral FWHM from 2.15 cm to 2.87 cm, at a water-equivalent depth of 1 cm. These findings suggest that this spot adjustment method may improve the quality of spot-scanned proton treatments.     

Two-dimensional pencil beam scaling: an improved proton dose algorithm for heterogeneous media

New dose delivery techniques with proton beams, such as beam spot scanning or raster scanning, require fast and accurate dose algorithms which can be applied for treatment plan optimization in clinically acceptable timescales. The clinically required accuracy is particularly difficult to achieve for the irradiation of complex, heterogeneous regions of the patient's anatomy. Currently applied fast pencil beam dose calculations based on the standard inhomogeneity correction of pathlength scaling often cannot provide the accuracy required for clinically acceptable dose distributions. This could be achieved with sophisticated Monte Carlo simulations which are still unacceptably time consuming for use as dose engines in optimization calculations. We therefore present a new algorithm for proton dose calculations which aims to resolve the inherent problem between calculation speed and required clinical accuracy. First, a detailed derivation of the new concept, which is based on an additional scaling of the lateral proton fluence is provided. Then, the newly devised two-dimensional (2D) scaling method is tested for various geometries of different phantom materials. These include standard biological tissues such as bone, muscle and fat as well as air. A detailed comparison of the new 2D pencil beam scaling with the current standard pencil beam approach and Monte Carlo simulations, performed with GEANT, is presented. It was found that the new concept proposed allows calculation of absorbed dose with an accuracy almost equal to that achievable with Monte Carlo simulations while requiring only modestly increased calculation times in comparison to the standard pencil beam approach. It is believed that this new proton dose algorithm has the potential to significantly improve the treatment planning outcome for many clinical cases encountered in highly conformal proton therapy.     

Implementation of pencil kernel and depth penetration algorithms for treatment planning of proton beams

The implementation of two algorithms for calculating dose distributions for radiation therapy treatment planning of intermediate energy proton beams is described. A pencil kernel algorithm and a depth penetration algorithm have been incorporated into a commercial three dimensional treatment planning system (Helax-TMS, Helax AB, Sweden) to allow conformal planning techniques using irregularly shaped fields, proton range modulation, range modification and dose calculation for non-coplanar beams. The pencil kernel algorithm is developed from the Fermi Eyges formalism and Molière multiple-scattering theory with range straggling corrections applied. The depth penetration algorithm is based on the energy loss in the continuous slowing down approximation with simple correction factors applied to the beam penumbra region and has been implemented for fast, interactive treatment planning. Modelling of the effects of air gaps and range modifying device thickness and position are implicit to both algorithms. Measured and calculated dose values are compared for a therapeutic proton beam in both homogeneous and heterogeneous phantoms of varying complexity. Both algorithms model the beam penumbra as a function of depth in a homogeneous phantom with acceptable accuracy. Results show that the pencil kernel algorithm is required for modelling the dose perturbation effects from scattering in heterogeneous media.     

Monte Carlo calculations and measurements of absorbed dose per monitor unit for the treatment of uveal melanoma with proton therapy

The treatment of uveal melanoma with proton radiotherapy has provided excellent clinical outcomes. However, contemporary treatment planning systems use simplistic dose algorithms that limit the accuracy of relative dose distributions. Further, absolute predictions of absorbed dose per monitor unit are not yet available in these systems. The purpose of this study was to determine if Monte Carlo methods could predict dose per monitor unit (D/MU) value at the center of a proton spread-out Bragg peak (SOBP) to within 1% on measured values for a variety of treatment fields relevant to ocular proton therapy. The MCNPX Monte Carlo transport code, in combination with realistic models for the ocular beam delivery apparatus and a water phantom, was used to calculate dose distributions and D/MU values, which were verified by the measurements. Measured proton beam data included central-axis depth dose profiles, relative cross-field profiles and absolute D/MU measurements under several combinations of beam penetration ranges and range-modulation widths. The Monte Carlo method predicted D/MU values that agreed with measurement to within 1% and dose profiles that agreed with measurement to within 3% of peak dose or within 0.5 mm distance-to-agreement. Lastly, a demonstration of the clinical utility of this technique included calculations of dose distributions and D/MU values in a realistic model of the human eye. It is possible to predict D/MU values accurately for clinical relevant range-modulated proton beams for ocular therapy using the Monte Carlo method. It is thus feasible to use the Monte Carlo method as a routine absolute dose algorithm for ocular proton therapy.     

Clinical implementation of full Monte Carlo dose calculation in proton beam therapy

The goal of this work was to facilitate the clinical use of Monte Carlo proton dose calculation to support routine treatment planning and delivery. The Monte Carlo code Geant4 was used to simulate the treatment head setup, including a time-dependent simulation of modulator wheels (for broad beam modulation) and magnetic field settings (for beam scanning). Any patient-field-specific setup can be modeled according to the treatment control system of the facility. The code was benchmarked against phantom measurements. Using a simulation of the ionization chamber reading in the treatment head allows the Monte Carlo dose to be specified in absolute units (Gy per ionization chamber reading). Next, the capability of reading CT data information was implemented into the Monte Carlo code to model patient anatomy. To allow time-efficient dose calculation, the standard Geant4 tracking algorithm was modified. Finally, a software link of the Monte Carlo dose engine to the patient database and the commercial planning system was established to allow data exchange, thus completing the implementation of the proton Monte Carlo dose calculation engine ('DoC++'). Monte Carlo re-calculated plans are a valuable tool to revisit decisions in the planning process. Identification of clinically significant differences between Monte Carlo and pencil-beam-based dose calculations may also drive improvements of current pencil-beam methods. As an example, four patients (29 fields in total) with tumors in the head and neck regions were analyzed. Differences between the pencil-beam algorithm and Monte Carlo were identified in particular near the end of range, both due to dose degradation and overall differences in range prediction due to bony anatomy in the beam path. Further, the Monte Carlo reports dose-to-tissue as compared to dose-to-water by the planning system. Our implementation is tailored to a specific Monte Carlo code and the treatment planning system XiO (Computerized Medical Systems Inc.). However, this work describes the general challenges and considerations when implementing proton Monte Carlo dose calculation in a clinical environment. The presented solutions can be easily adopted for other planning systems or other Monte Carlo codes.     

Fluence correction factors in plastic phantoms for clinical proton beams

In recent codes of practice for reference dosimetry in clinical proton beams using ionization chambers, it is recommended to perform the measurement in a water phantom. However, in situations where the positioning accuracy is very critical, it could be more convenient to perform the measurement in a plastic phantom. In proton beams, a similar approach as in electron beams could be applied by introducing fluence correction factors in order to account for the differences in particle fluence distributions at equivalent depths in plastic and water. In this work, fluence correction factors as a function of depth were determined for proton beams with different energies using the Monte Carlo code PTRAN for PMMA and polystyrene with reference to water. The influence of non-elastic nuclear interaction cross sections was investigated. It was found that differences in proton fluence distributions are almost entirely due to differences in non-elastic nuclear interaction cross sections between the plastic materials and water. For proton beams with energies lower than 100 MeV, for which the contributions from non-elastic interactions become small compared to the total dose, the fluence corrections are smaller than 1%. For beams with energies above 200 MeV, depending on the cross sections dataset for non-elastic nuclear interactions, fluence corrections of 2-5% were found at the largest depths. The results could, with an acceptable accuracy, be represented as a correction per cm penetration of the beam, yielding values between 0.06% and 0.15% per cm for PMMA and 0.06% to 0.20% per cm for polystyrene. Experimental information on these correction factors was obtained from depth dose measurements in PMMA and water. The experiments were performed in 75 MeV and 191 MeV non-modulated and range-modulated proton beams. From the experiments, values ranging from 0.03% to 0.15% per cm were obtained. A decisive answer about which dataset for non-elastic nuclear interactions would result in a better representation of the measurements could not be given. We conclude that below 100 MeV, dosimetry could be performed in plastic phantoms without a dramatic loss of accuracy. On the other hand, in clinical high-energy proton beams, where accurate positioning in water is in general not an issue, substantial correction factors would be required for converting dose measurements in a plastic phantom to absorbed dose to water. It is therefore not advisable to perform absorbed dose measurements nor to measure depth dose distributions in a plastic phantom in high-energy proton beams.     

Photon scatter in portal images: accuracy of a fluence based pencil beam superposition algorithm

The accuracy of a pencil beam algorithm to predict scattered photon fluence into portal imaging systems was studied. A data base of pencil beam kernels describing scattered photon fluence behind homogeneous water slabs (1-50 cm thick) at various air gap distances (0-100 cm) was generated using the EGS Monte Carlo code. Scatter kernels were partitioned according to particle history: singly-scattered, multiply-scattered, and bremsstrahlung and positron annihilation photons. Mean energy and mean angle with respect to the incident photon pencil beam were also scored. This data allows fluence, mean energy, and mean angular data for each history type to be predicted using the pencil beam algorithm. Pencil beam algorithm predictions for 6 and 24 MV incident photon beams were compared against full Monte Carlo simulations for several inhomogeneous phantoms, including approximations to a lateral neck, and a mediastinum treatment. The accuracy of predicted scattered photon fluence, mean energy, and mean angle was investigated as a function of air gap, field size, photon history, incident beam resolution, and phantom geometry. Maximum errors in mean energies were 0.65 and 0.25 MeV for the higher and lower energy spectra, respectively, and 15 degrees for mean angles. The ability of the pencil beam algorithm to predict scatter fluence decreases with decreasing air gap, with the largest error for each phantom occurring at the exit surface. The maximum predictive error was found to be 6.9% with respect to the total fluence on the central axis. By maintaining even a small air gap (approximately 10 cm), the error in predicted scatter fluence may be kept under 3% for the phantoms and beam energies studied here. It is concluded that this pencil beam algorithm is sufficiently accurate (using International Commission on Radiation Units and Measurements Report No. 24 guidelines for absorbed dose) over the majority of clinically relevant air gaps, for further investigation in a portal dose prediction algorithm.