Optical Surface Monitoring系统跟踪运动靶区的位置精度和剂量精度验证

目的:评估Optical Surface Monitoring System(OSMS)测量靶区运动的几何精度和使用OSMS跟踪运动靶区放疗时的剂量精度。方法:①分别使用OSMS与锥形束CT监测腹部模体的位置,通过移动治疗床使模体产生位移和旋转,对比二者对模体位置的测量结果;②用呼吸运动平台搭载MatriXX二维电离室阵列,测量不同跟踪阈值时模体内相对剂量,分别与计划剂量分布对比,分析不同跟踪阈值时计划验证的gamma通过率。结果:OSMS和锥形束CT系统对模体平移和旋转预设偏移的测量结果差异很小,使用OSMS门控技术时OSMS的阈值越小gamma通过率越高,相对剂量分布与计划剂量的一致性也越好。结论:OSMS系统对模体表面位置监测精度高,可以应用到实际放射治疗的辅助摆位。使用OSMS门控技术跟踪运动靶区可以提高靶区的剂量精度,而且对患者不附加任何电离辐射,适合应用于分次间的辅助摆位和分次内的靶区跟踪。     

放疗靶区呼吸运动对CT模拟定位图像重建的几何体积精度的体模实验研究

目的:研究放射治疗病人的不同呼吸运动状态对CT模拟定位扫描的图像重建精度的影响以及对放射治疗计划设计和评估的影响。方法:使用动态体模模拟放疗患者肺部靶区的呼吸运动,测试和计算不同运动周期和幅度下用于治疗计划设计的CT扫描的图像重建几何体积的变化。体模运动单元包含1cm和2cm的两个统一密度的球体和边长3cm的正方体,分别设定在沿CT定位床轴向以±1cm和±2cm的幅度作运动周期为3s,4s,5s,6s和10s的匀速振动。CT扫描条件为螺距1.5,层厚5mm,扫描速度1Slice/s。在CT模拟定位工作站上对扫描的原始数据进行三维图像重建,以自动阈值勾画方式计算模拟靶区体积大小,并与体模的实际几何体积比较确定误差。结果:(1)在体模运动方向有明显的几何体积误差,且可能形成明显的成像间隙。(2)重建的模拟运动靶区体积变化与其物理体积大小和运动状态相关。在选定的CT扫描参数和靶区体积的运动状态下,CT扫描图像重建的体积误差最大达66.7%,在振幅为2cm时相隔2cm的模体图像甚至可能发生部分重迭。(3)靶区图像的几何中心可能发生偏差,从而造成治疗计划设计的射野中心偏差。结论:在呼吸运动幅度和周期分别大于2cm和4s时有必要对定位患者采取呼吸限制方式进行CT模拟定位扫描或根据实际测量结果评估靶区体积误差可能带来的计划误差。     

用蒙特卡罗模拟评估小野条件下肺介质中AAA算法的精度

目的：用蒙特卡罗模拟评估放射治疗剂量计算使用的各向异性分析算法（Anisotropic Analytical Algorithm,AAA）在小野条件下肺介质中的计算精度。材料与方法：建立一包含肺介质的水模体,分别用AAA算法、笔形束卷积算法（Pencil Beam Convolution,PBC算法）（作为对比）和蒙特卡罗（Monte Carlo,MC）模拟计算2cm×2cm到8cm×8cm射野条件下该模体中的深度剂量和离轴比,并以MC模拟为标准比较深度剂量。用一维伽马分析对离轴比进行分析。结果：AAA算法在2cmx2cm射野肺介质区域高估了深度剂量,其它情况均低估了深度剂量,剂量偏差范围为-0．24％-2．66％．PBC算法在肺介质区域高估了深度剂量,剂量偏差的范围为1．18％～14．55％。AAA算法计算的离轴比和MC模拟,在射野剂量平坦区相对内收,在剂量跌落区向两侧发散,但AAA算法略高估了射野边缘的剂量,一维伽马分析（与MC相比）通过率为100％（3mm／3％）。PBC算法在射野剂量平坦区相对发散,而在剂量跌落区向两侧内收。一维伽马分析通过率范围为51％～88％。结论：在肺介质中,AAA剂量计算的结果与MC模拟的一致性较好,与PBC算法相比,剂量计算的精度较高。     

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。     

3DCT联合4DCBCT在中下叶肺癌SABR放疗靶区外放边界中的应用研究

目的:利用四维锥形束CT（4DCBCT）扫描获取放疗靶区摆位误差和呼吸运动误差,计算肿瘤立体定向消融放射治疗（SABR）中计划靶区体积（PTV）外放边界大小。方法:回顾性分析19例中下叶肺癌SABR治疗患者,治疗前4DCBCT扫描,共72次扫描图像。根据4DCBCT与定位CT的配准结果,评估放疗靶区分次间摆位和呼吸运动误差,确定PTV外放边界大小。结果:放疗靶区摆位误差在左右、上下、前后3个方向上分别为（0.11±0.29）、（0.02±0.58）、（0.05±0.26） cm,放疗靶区呼吸运动误差在3个方向上分别为（-0.06±0.34）、（0.09±0.68）、（0.06±0.23） cm,利用ICRU83#报告公式计算PTV外放边界,在3个方向上分别为1.13、2.15、0.90 cm。结论:4DCBCT可有效评估放疗靶区摆位和呼吸运动误差,并确定中下叶肺癌SABR治疗中PTV外放边界大小。利用本方法计算的外放边界比原来RTOG提出的外放标准更加精确,可个体化评估放疗靶区外放边界。     

GATE在核医学成像和放射治疗中的蒙特卡洛模拟

目的:探讨GATE在核医学成像SPECT和PET、光子和质子放射治疗中的蒙特卡洛模拟,并利用GATE平台研究碳纤维床板对光子放疗时剂量的影响。方法:首先模拟运行GATE V6.1提供的三个例子,分别对应于SPECT、PET和RT,其中RT又分为光子治疗和质子治疗。对SPECT和PET模拟中光子的散射情况进行统计分析,详细比较RT模拟中光子束和质子束在水模体中的能量沉积特性。然后在GATE平台上编程模拟了光子治疗束分别在有碳纤维床板和无床板时射入水模体中,比较并分析这两种情况下水模体中的剂量分布差异。结果:GATE V6.1的三个例子模拟中,SPECT中的未散射光子稳定在36%左右,PET中未散射的真符合计数稳定在44.5%左右,RT模拟中质子相比于光子在深度方向上有明显的剂量分布优势,而光子在横向方向的剂量分布稍好于质子。在碳纤维床板对光子放疗时剂量影响的模拟中,有碳纤维床板相对于无床板时,水模体的表层剂量有明显的提高。结论:GATE能够稳定准确的对核医学成像SPECT和PET及放射治疗过程进行蒙特卡洛模拟。它可以为放射治疗剂量验证、临床放射治疗计划以及核医学成像引导放射治疗的研究提供强大帮助。     

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。     

调强放射治疗“end to end”质量控制核查中应用针尖电离室测量小野剂量的初步研究

目的:在调强放射治疗“end to end”质量核查中,探讨应用针尖电离室对调强放射治疗小野照射进行绝对剂量测量的研究。方法:选择3省20家医院,将放有热释光剂量计TLD（距模体表面距离约7.5 cm）和胶片的国际原子能机构（IAEA）模体进行CT扫描,图像导入放射治疗计划系统（TPS）中,设计治疗计划,进行7野等中心调强照射,MLC照射野大小>2 cm×2 cm且<4 cm×4 cm。同时针尖电离室（0.015 cc）放在固体水模体距模体表面7.5 cm下进行点剂量绝对剂量验证:（1）将治疗计划中射野角度归零平移到固体水模体中进行剂量验证;（2）治疗计划射野角度不归零时为实际治疗照射方向,平移到固体水模体中进行绝对剂量验证。结果:在调强放射治疗多叶光栅小野照射的固体水模体中,用针尖电离室测量的绝对剂量与TPS计算得到的绝对剂量比较,7野照射方向归为零度时,比较偏差<5%;实际照射方向时,比较偏差<5%。验证后的计划,在IAEA模体上进行实际7野调强治疗,模体中的高剂量靶区胶片（Gafchromic EBT3 film）绝对剂量通过率均≥90%（Gamma分析:3%, 3 mm）,TLD偏差<7%。均符合IAEA提出的标准。结论:在调强放射治疗多叶光栅小野照射时,可以应用针尖电离室作为绝对剂量验证的一个方法。     

运动模体中靶区长度与锥形束CT扫描速度的相关性

目的:研究锥形束CT扫描速度与动态模体中靶区长度的相关性。方法 :选用QUASAR(Modus,Germany)呼吸运动模体,模体内含运动插件,插件中心内嵌入一边长3 cm的立方体,用其来模拟运动靶区。设置模体振幅为0.5、1、2 cm,每一振幅分别设20、15、10次/min三种频率,在每振幅下分别行300°、180°、90°/min运动速度的CBCT扫描。计算CBCT图像靶区长度及靶区长度覆盖率与理论结果比较。结果:振幅为5 mm时,300、180、90°/min扫描时所得不同频率下靶区长度分别为(30.17,30.33,30.5)mm;(31.17,31.83,32)mm;(32.5,33.67,33.67)mm;振幅为10 mm时,300°、180°、90°/min扫描时所得靶区长度分别为(32.67,33.67,35.67)mm;(36,37.5,37.65)mm;(40.17,40.5,41.17)mm;振幅为15 mm时,300°、180°、90°/min扫描时所得靶区长度分别为(39.33,41,41.83)mm;(43,46,46.5)mm;(47.83,48.83,49.17)mm。结论 :CBCT扫描速度、模体振幅以及模体运动频率对靶区长度均有影响。扫描速度越慢,图像所得靶区长度越接近靶区长度真实值,但是各种速度下均小于理论靶区长度。振幅越小时所得靶区长度越接近于靶区理论值,靶区覆盖率越高。临床实践中使用CBCT对动态肿瘤监控时应使用患者平静呼吸的慢速扫描。     

Octavius 1600SRS三维剂量验证系统临床可行性测试

目的:通过模体实验对PTW Octavius 1600SRS三维剂量验证系统进行临床应用前性能测试,评估其对立体定向放射治疗（SBRT）计划进行剂量验证的可行性。方法:选用PTW Octavius 1600SRS体模的CT图像,模拟勾画7个球形靶区,中心靶区（PTV0）直径大小为3 cm,其余各靶区直径大小分别为1.0 cm（2个）、1.5 cm（2个）和2.0 cm（2个）,各靶区中心点距PTV0中心点距离为3~6 cm。设置PTV0的中心点为计划中心,在治疗计划系统中制定SBRT计划（Plan0）,处方剂量为8 Gy×3 F。实验分别对Plan0引入临床常见偏差,包括叶片MLC位置偏差（1、2、3 mm）、计划中心点（ISO）位置偏差（1、2、3 mm）和机架位置偏差（0.5°、1.0°、2.0°）,并生成相应的偏差计划。使用1600SRS验证系统分别对原计划和偏差计划进行测量,比较两者γ通过率和靶区覆盖率的差别,以评估系统对放疗剂量偏差的敏感性。另外,对6例临床SBRT计划进行治疗前剂量验证,并与EPID验证结果进行比较,以评估其临床计划验证性能。结果:1600SRS验证系统对MLC偏差检测非常敏感,当MLC出现1 mm偏差时,其γ通过率与各靶区覆盖率均出现显著下降,且随着MLC偏差变大,其下降越明显。当MLC出现3 mm偏差时,（3 mm/3%）和（2 mm/3%）的γ通过率分别从99.6%和98.0%下降至92.8%和81.7%,7个靶区体积的覆盖率（V98%）平均下降（58.8±6.8）%。1600SRS对机架旋转偏差和ISO平移误差检测亦敏感,在机架旋转出现2°偏差或ISO出现3 mm偏差时,其（2 mm/2%）的γ通过率分别从95.1%下降至89.5%或86.2%。另外,6例临床SBRT放疗计划的（2 mm/3%）γ通过率平均为（95.5±1.5）%。结论:Octavius 1600SRS能敏感地检测出SBRT计划中MLC到位偏差、机架旋转角度偏差与ISO偏差,能较好地应用于SBRT计划的治疗前剂量验证。     

Benchmarking analytical calculations of proton doses in heterogeneous matter

A proton dose computational algorithm, performing an analytical superposition of infinitely narrow proton beamlets (ASPB) is introduced. The algorithm uses the standard pencil beam technique of laterally distributing the central axis broad beam doses according to the Moliere scattering theory extended to slablike varying density media. The purpose of this study was to determine the accuracy of our computational tool by comparing it with experimental and Monte Carlo (MC) simulation data as benchmarks. In the tests, parallel wide beams of protons were scattered in water phantoms containing embedded air and bone materials with simple geometrical forms and spatial dimensions of a few centimeters. For homogeneous water and bone phantoms, the proton doses we calculated with the ASPB algorithm were found very comparable to experimental and MC data. For layered bone slab inhomogeneity in water, the comparison between our analytical calculation and the MC simulation showed reasonable agreement, even when the inhomogeneity was placed at the Bragg peak depth. There also was reasonable agreement for the parallelepiped bone block inhomogeneity placed at various depths, except for cases in which the bone was located in the region of the Bragg peak, when discrepancies were as large as more than 10%. When the inhomogeneity was in the form of abutting air-bone slabs, discrepancies of as much as 8% occurred in the lateral dose profiles on the air cavity side of the phantom. Additionally, the analytical depth-dose calculations disagreed with the MC calculations within 3% of the Bragg peak dose, at the entry and midway depths in the phantom. The distal depth-dose 20%-80% fall-off widths and ranges calculated with our algorithm and the MC simulation were generally within 0.1 cm of agreement. The analytical lateral-dose profile calculations showed smaller (by less than 0.1 cm) 20%-80% penumbra widths and shorter fall-off tails than did those calculated by the MC simulations. Overall, this work validates the usefulness of our ASPB algorithm as a reasonably fast and accurate tool for quality assurance in planning wide beam proton therapy treatment of clinical sites either composed of homogeneous materials or containing laterally extended inhomogeneities that are comparable in density and located away from the Bragg peak depths.     

Comparison of whole-body phantom designs to estimate organ equivalent neutron doses for secondary cancer risk assessment in proton therapy

Secondary neutron fluence created during proton therapy can be a significant source of radiation exposure in organs distant from the treatment site, especially in pediatric patients. Various published studies have used computational phantoms to estimate neutron equivalent doses in proton therapy. In these simulations, whole-body patient representations were applied considering either generic whole-body phantoms or generic age- and gender-dependent phantoms. No studies to date have reported using patient-specific geometry information. The purpose of this study was to estimate the effects of patient–phantom matching when using computational pediatric phantoms. To achieve this goal, three sets of phantoms, including different ages and genders, were compared to the patients' whole-body CT. These sets consisted of pediatric age specific reference, age-adjusted reference and anatomically sculpted phantoms. The neutron equivalent dose for a subset of out-of-field organs was calculated using the GEANT4 Monte Carlo toolkit, where proton fields were used to irradiate the cranium and the spine of all phantoms and the CT-segmented patient models. The maximum neutron equivalent dose per treatment absorbed dose was calculated and found to be on the order of 0 to 5 mSv Gy(-1). The relative dose difference between each phantom and their respective CT-segmented patient model for most organs showed a dependence on how close the phantom and patient heights were matched. The weight matching was found to have much smaller impact on the dose accuracy except for very heavy patients. Analysis of relative dose difference with respect to height difference suggested that phantom sculpting has a positive effect in terms of dose accuracy as long as the patient is close to the 50th percentile height and weight. Otherwise, the benefit of sculpting was masked by inherent uncertainties, i.e. variations in organ shapes, sizes and locations.Other sources of uncertainty included errors associated with beam positioning, neutron weighting factor definition and organ segmentation. This work demonstrated the importance of hybrid phantom height matching for more accurate organ dose calculation in proton therapy and the potential limitations of reference phantoms released by regulatory bodies for radiation therapy applications.     

A polygon-surface reference Korean male phantom (PSRK-Man) and its direct implementation in Geant4 Monte Carlo simulation

Even though the hybrid phantom embodies both the anatomic reality of voxel phantoms and the deformability of stylized phantoms, it must be voxelized to be used in a Monte Carlo code for dose calculation or some imaging simulation, which incurs the inherent limitations of voxel phantoms. In the present study, a voxel phantom named VKH-Man (Visible Korean Human-Man), was converted to a polygon-surface phantom (PSRK-Man, Polygon-Surface Reference Korean-Man), which was then adjusted to the Reference Korean data. Subsequently, the PSRK-Man polygon phantom was directly, without any voxelization process, implemented in the Geant4 Monte Carlo code for dose calculations. The calculated dose values and computation time were then compared with those of HDRK-Man (High Definition Reference Korean-Man), a corresponding voxel phantom adjusted to the same Reference Korean data from the same VKH-Man voxel phantom. Our results showed that the calculated dose values of the PSRK-Man surface phantom agreed well with those of the HDRK-Man voxel phantom. The calculation speed for the PSRK-Man polygon phantom though was 70-150 times slower than that of the HDRK-Man voxel phantom; that speed, however, could be acceptable in some applications, in that direct use of the surface phantom PSRK-Man in Geant4 does not require a separate voxelization process. Computing speed can be enhanced, in future, either by optimizing the Monte Carlo transport kernel for the polygon surfaces or by using modern computing technologies such as grid computing and general-purpose computing on graphics processing units programming.     

Shielding design for a laser-accelerated proton therapy system

In this paper, we present the shielding analysis to determine the necessary neutron and photon shielding for a laser-accelerated proton therapy system. Laser-accelerated protons coming out of a solid high-density target have broad energy and angular spectra leading to dose distributions that cannot be directly used for therapeutic applications. A special particle selection and collimation device is needed to generate desired proton beams for energy- and intensity-modulated proton therapy. A great number of unwanted protons and even more electrons as a side-product of laser acceleration have to be stopped by collimation devices and shielding walls, posing a challenge in radiation shielding. Parameters of primary particles resulting from the laser-target interaction have been investigated by particle-in-cell simulations, which predicted energy spectra with 300 MeV maximum energy for protons and 270 MeV for electrons at a laser intensity of 2 x 10(21) W cm(-2). Monte Carlo simulations using FLUKA have been performed to design the collimators and shielding walls inside the treatment gantry, which consist of stainless steel, tungsten, polyethylene and lead. A composite primary collimator was designed to effectively reduce high-energy neutron production since their highly penetrating nature makes shielding very difficult. The necessary shielding for the treatment gantry was carefully studied to meet the criteria of head leakage <0.1% of therapeutic absorbed dose. A layer of polyethylene enclosing the whole particle selection and collimation device was used to shield neutrons and an outer layer of lead was used to reduce photon dose from neutron capture and electron bremsstrahlung. It is shown that the two-layer shielding design with 10-12 cm thick polyethylene and 4 cm thick lead can effectively absorb the unwanted particles to meet the shielding requirements.     

The Monte Carlo SRNA-VOX code for 3D proton dose distribution in voxelized geometry using CT data

This paper describes the application of the SRNA Monte Carlo package for proton transport simulations in complex geometry and different material compositions. The SRNA package was developed for 3D dose distribution calculation in proton therapy and dosimetry and it was based on the theory of multiple scattering. The decay of proton induced compound nuclei was simulated by the Russian MSDM model and our own using ICRU 63 data. The developed package consists of two codes: the SRNA-2KG, which simulates proton transport in combinatorial geometry and the SRNA-VOX, which uses the voxelized geometry using the CT data and conversion of the Hounsfield's data to tissue elemental composition. Transition probabilities for both codes are prepared by the SRNADAT code. The simulation of the proton beam characterization by multi-layer Faraday cup, spatial distribution of positron emitters obtained by the SRNA-2KG code and intercomparison of computational codes in radiation dosimetry, indicate immediate application of the Monte Carlo techniques in clinical practice. In this paper, we briefly present the physical model implemented in the SRNA package, the ISTAR proton dose planning software, as well as the results of the numerical experiments with proton beams to obtain 3D dose distribution in the eye and breast tumour.     

Application of an inverse kernel concept to Monte Carlo based IMRT

Inverse treatment planning by means of pencil beam algorithms can lead to errors in the calculation of dose in areas without secondary electron equilibrium. Monte Carlo (MC) simulations give accurate results in such areas but result in increased computation times. We present a new, so-called inverse kernel concept that offers MC precision in inverse treatment planning with acceptable computation times and memory consumption. Inverse kernels are matrices that describe the dose contribution from all bixels of a beam to a distinct voxel of the patient phantom. The concept is similar to other generalized pencil-beam concepts, except that inverse kernel elements are precalculated using a single MC simulation and stored as binary trees. In this procedure a modified MC code (XVMC) is applied to trace the photon history for each dose deposition. Iterative optimization is then applied in a second step. The inverse process is separated into (i) a slower MC simulation and (ii) a faster iterative optimization, followed by (iii) the segmentation procedure, and (iv) a final MC dose calculation step including a segment weight reoptimization. Inverse kernel optimization, or IKO, with segmentation and reoptimization steps is demonstrated by means of a lung cancer case. To demonstrate the superiority of an inverse MC system over pencil-beam or collapsed-cone based systems, the final result of the IKO is compared to plans where all segments have been calculated by pencil beam or collapsed cone, respectively. Dose-volume histograms and dose-difference histograms show remarkable differences, which can be attributed to systematic errors in both algorithms. IKO is a precise, nonhybrid, inverse MC treatment planning system which suits current clinical needs, as several optimization steps can follow one single MC-simulation step for a distinct beam setup.     

A 3D point-kernel multiple scatter model for parallel-beam SPECT based on a gamma-ray buildup factor

A three-dimensional (3D) point-kernel multiple scatter model for point spread function (PSF) determination in parallel-beam single-photon emission computed tomography (SPECT), based on a dose gamma-ray buildup factor, is proposed. This model embraces nonuniform attenuation in a voxelized object of imaging (patient body) and multiple scattering that is treated as in the point-kernel integration gamma-ray shielding problems. First-order Compton scattering is done by means of the Klein-Nishina formula, but the multiple scattering is accounted for by making use of a dose buildup factor. An asset of the present model is the possibility of generating a complete two-dimensional (2D) PSF that can be used for 3D SPECT reconstruction by means of iterative algorithms. The proposed model is convenient in those situations where more exact techniques are not economical. For the proposed model's testing purpose calculations (for the point source in a nonuniform scattering object for parallel beam collimator geometry), the multiple-order scatter PSF generated by means of the proposed model matched well with those using Monte Carlo (MC) simulations. Discrepancies are observed only at the exponential tails mostly due to the high statistic uncertainty of MC simulations in this area, but not because of the inappropriateness of the model.     

Parametrization and application of scatter kernels for modelling scanned proton beam collimator scatter dose

Collimators are routinely used in proton radiotherapy to laterally confine the field and improve the penumbra. Collimator scatter contributes up to 15% of the local dose and is therefore important to include in treatment planning dose calculation. We present a method for reconstruction of the collimator scatter phase space based on the parametrization of pre-calculated scatter kernels. Collimator scatter distributions, generated by the Monte Carlo (MC) package GEANT4.8.2, were scored differential in direction and energy. The distributions were then parametrized so as to enable a fast reconstruction by sampling. MC calculated dose distributions in water based on the parametrized phase space were compared to full MC simulations that included the collimator in the simulation geometry, as well as to experimental data. The experiments were performed at the scanned proton beam line at the The Svedberg Laboratory (TSL) in Uppsala, Sweden. Dose calculations using the parametrization of this work and the full MC for isolated typical cases of collimator scatter were compared by means of the gamma index. The result showed that in total 96.7% (99.3%) of the voxels fulfilled the gamma 2.0%/2.0 mm (3.0%/3.0 mm) criterion. The dose distribution for a collimated field was calculated based on the phase space created by the collimator scatter model incorporated into the generation of the phase space of a scanned proton beam. Comparing these dose distributions to full MC simulations, including particle transport in the MLC, yielded that in total for 18 different collimated fields, 99.1% of the voxels satisfied the gamma 1.0%/1.0 mm criterion and no voxel exceeded the gamma 2.6%/2.6 mm criterion. The dose contribution of collimator scatter along the central axis as predicted by the model showed good agreement with experimental data.     

Combined modulated electron and photon beams planned by a Monte-Carlo-based optimization procedure for accelerated partial breast irradiation

The purpose of this study was to present a Monte-Carlo (MC)-based optimization procedure to improve conventional treatment plans for accelerated partial breast irradiation (APBI) using modulated electron beams alone or combined with modulated photon beams, to be delivered by a single collimation device, i.e. a photon multi-leaf collimator (xMLC) already installed in a standard hospital. Five left-sided breast cases were retrospectively planned using modulated photon and/or electron beams with an in-house treatment planning system (TPS), called CARMEN, and based on MC simulations. For comparison, the same cases were also planned by a PINNACLE TPS using conventional inverse intensity modulated radiation therapy (IMRT). Normal tissue complication probability for pericarditis, pneumonitis and breast fibrosis was calculated. CARMEN plans showed similar acceptable planning target volume (PTV) coverage as conventional IMRT plans with 90% of PTV volume covered by the prescribed dose (D(p)). Heart and ipsilateral lung receiving 5% D(p) and 15% D(p), respectively, was 3.2-3.6 times lower for CARMEN plans. Ipsilateral breast receiving 50% D(p) and 100% D(p) was an average of 1.4-1.7 times lower for CARMEN plans. Skin and whole body low-dose volume was also reduced. Modulated photon and/or electron beams planned by the CARMEN TPS improve APBI treatments by increasing normal tissue sparing maintaining the same PTV coverage achieved by other techniques. The use of the xMLC, already installed in the linac, to collimate photon and electron beams favors the clinical implementation of APBI with the highest efficiency.