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.     

基于蒙特卡罗EGS4算法的核素内照射吸收剂量计算方法探讨

目的 以蒙特卡罗EGS4算法(Monte Carlo EGS4,MC EGS4)为基础,用时序性SPECT/CT检查探讨核素内照射治疗吸收剂量的计算方法.方法 用体模标定153Sm放射性浓度与SPECT图像灰度值的关系;用RMI467型CT体模标定不同组织物理密度与CT图像灰度值的关系;优化MC EGS4计算程序.以此为基础,通过时序性SPECT/CT检查和累积尿液的放射性测定,计算4例肿瘤多发骨转移患者153Sm-乙二胺四亚甲基膦酸(EDTMP,按体重注射24.1 MBq/kg)内照射治疗后不同靶器官的三维吸收剂量分布和病灶、骨髓、脊髓、盆腔性腺组织的吸收剂量.结果 SPECT和CT图像的灰度值分别与153Sm放射性浓度和组织物理密度之间存在线性对应关系(P＜0.05).多发骨转移癌患者骨转移灶的153Sm-EDTMP吸收剂量分布明显不均,放射性累积中心点吸收剂量最高,边缘区域剂量降低.1例患者最高点内照射吸收剂量率为4.3×10-8 Gy·s-1,左髂骨转移灶最高吸收剂量约为5.6 Gy,病灶边缘吸收剂量为2.0 Gy.其他3例患者病灶最高点吸收剂量率分别为4.5×10-8,3.5×10-8,3.8×10-8 Gy·s-1.结论 基于MC EGS4算法,用时序性SPECT/CT可计算核素内照射治疗患者的病灶和其他靶器官吸收剂量及其三维分布.     

Evaluation of the RayStation electron Monte Carlo dose calculation algorithm

The aim of this work was to evaluate the accuracy of the RayStation treatment planning system electron Monte Carlo algorithm against measured data for a range of clinically relevant scenarios. This was done by comparing measured percentage depth dose data (PDD) in water, profiles at oblique incidence and with heterogeneities in the beam path, and output factor data and that generated using the RayStation treatment planning system Monte Carlo VMC++ based calculation algorithm. While electron treatments are widely employed in the radiotherapy setting accurate modelling is challenging (TPS) in the presence of patient being both heterogeneous and nonrectangular. Watertank-based measurements were made on a Varian TrueBeam linear accelerator covering electron beam energies 6 to 18 MeV. These included both normal and oblique incidence, heterogeneous geometries, and irregular shaped cut-outs. The measured geometries were replicated in RayStation and the Monte Carlo dose calculation engine used to generate dosimetric data for comparison against measurement in what were considered clinically relevant settings. Water-based PDDs and profile comparisons showed excellent agreement for all electron beam energies. Profiles measured with oblique beam incidence demonstrated acceptable agreement to the treatment planning system calculations although the correspondence worsened as the angle increased with the planning system overestimating the dose in the shoulder region. Profile measurements under inhomogeneities were generally good. The planning system had a tendency to overestimate dose under the heterogeneity and also demonstrated a broader penumbra than measurement. Of the 170 different output factors calculated in RayStation over the range of electron energies commissioned, 141 were within ± 3% of measured values and 164 within ± 5%. Four of the 6 comparisons beyond 5% were at 18 MeV and all had a cut-out edge within 3 cm of the beam central axis/measurement point. The RayStation implementation of a VMC++ electron Monte Carlo dose calculation algorithm shows good agreement with measured data for a range of scenarios studied and represented sufficient accuracy for clinical use.     

Influence of difference in cross-sectional dose profile in a CTDI phantom on X-ray CT dose estimation: a Monte Carlo study

The longitudinal dose profile in a computed tomography dose index (CTDI) phantom had been studied by many researchers. The cross-sectional dose profile in the CTDI phantom, however, has not been studied. It is also important to understand the cross-sectional dose profile in the CTDI phantom for dose estimation in X-ray CT. In this study, the cross-sectional dose profile in the CTDI phantom was calculated by use of a Monte Carlo (MC) simulation method. A helical or a 320-detector-row cone-beam X-ray CT scanner was simulated. The cross-sectional dose profile in the CTDI phantom from surface to surface through the center point was calculated by MC simulation. The shape of the calculation region was a cylinder of 1-mm-diameter. The length of the cylinder was 23, 100, or 300 mm to represent various CT ionization chamber lengths. Detailed analyses of the energy depositions demonstrated that the cross-sectional dose profile was different in measurement methods and phantom sizes. In this study, we also focused on the validation of the weighting factor used in weighted CTDI (CTDI _w ). As it stands now, the weighting factor used in CTDI _w is (1/3, 2/3) for the (central, peripheral) axes. Our results showed that an equal weighting factor, which is (1/2, 1/2) for the (central, peripheral) axes, is more suitable to estimate the average cross-sectional dose when X-ray CT dose estimation is performed.     

β源支架剂量分布的蒙特卡罗算法

目的 比较数值积分和蒙特卡罗方法计算的放射性支架的剂量率分布。方法 以3种有代表性的剂量点核函数为计算模型，计算支架的剂量率分布。结果 分别计算了中心面的径向，支架表面及离支架表面0.5mm处的轴向剂量分布，径向最大差异为1．5％，轴向的差异也有1．5％之内。结论 3种函数用数值积分和蒙特卡罗方法计算的剂量分布是一致的，蒙特卡罗方法可用来计算放射性支架的剂量分布。     

蒙特卡罗模拟方法结合不同体模在剂量估算中的应用

随着核与辐射在人们日常生活中的应用越来越广泛,其所带来的危害也备受关注。剂量估算是辐射技术应用的重要一环,估算出人体所受的剂量对评价辐射造成的确定效应与随机效应起着重要作用。蒙特卡罗（MC）模拟与人体参考模型结合可对核事故、医疗照射和环境的辐射剂量进行估算,是一种快速且对硬件要求较少的剂量估算方法,目前正面临模型开发和计算耗时的瓶颈,笔者对此现状进行综述。     

Comparison between Acuros XB and Brainlab Monte Carlo algorithms for photon dose calculation

Purpose

The Acuros? XB dose calculation algorithm by Varian and the Monte Carlo algorithm XVMC by Brainlab were compared with each other and with the well-established AAA algorithm, which is also from Varian.

Methods

First, square fields to two different artificial phantoms were applied: (1) a “slab phantom” with a 3?cm water layer, followed by a 2?cm bone layer, a 7?cm lung layer, and another 18?cm water layer and (2) a “lung phantom” with water surrounding an eccentric lung block. For the slab phantom, depth–dose curves along central beam axis were compared. The lung phantom was used to compare profiles at depths of 6 and 14?cm. As clinical cases, the CTs of three different patients were used. The original AAA plans with all three algorithms using open fields were recalculated.

Results

There were only minor differences between Acuros and XVMC in all artificial phantom depth doses and profiles; however, this was different for AAA, which had deviations of up to 13% in depth dose and a few percent for profiles in the lung phantom. These deviations did not translate into the clinical cases, where the dose–volume histograms of all algorithms were close to each other for open fields.

Conclusion

Only within artificial phantoms with clearly separated layers of simulated tissue does AAA show differences at layer boundaries compared to XVMC or Acuros. In real patient CTs, these differences in the dose–volume histogram of the planning target volume were not observed.     

Microdosimetric calculation of penumbra for biological dose in wobbled carbon-ion beams with Monte Carlo Method

In carbon-ion radiotherapy, it is important to evaluate the biological dose because the relative biological effectiveness values vary greatly in a patient’s body. The microdosimetric kinetic model (MKM) is a method of estimating the biological effect of radiation by use of microdosimetry. The lateral biological dose distributions were estimated with a modified MKM, in which we considered the overkilling effect in the high linear-energy-transfer region. In this study, we used the Monte Carlo calculation of the Geant4 code to simulate a horizontal port at the Heavy Ion Medical Accelerator in Chiba of the National Institute of Radiological Sciences. The lateral biological dose distributions calculated by Geant4 were almost flat as the lateral absorbed dose in the flattened area. However, in the penumbra region, the lateral biological dose distributions were sharper than the lateral absorbed dose distributions. Furthermore, the differences between the lateral absorbed dose and biological dose distributions were dependent on the depth for each multi-leaf collimator opening size. We expect that the lateral biological dose distribution presented here will enable high-precision calculations for a treatment-planning system.     

A Monte Carlo brachytherapy study for dose distribution prediction in an inhomogeneous medium

The purpose of this study was to present a theoretical analysis of how the presence of bone in interstitial brachytherapy affects dose rate distributions. This study was carried out using a Monte Carlo simulation of the dose distribution in homogeneous medium for 3 commonly used brachytherapy seeds. The 3 seeds investigated in this study are iridium-192 (¹⁹²Ir) iodine-125 (¹²⁵I), and palladium-103 (¹⁰³Pd). The computer code was validated by comparing the specific dose rate (Λ), the radial dose function g(r), and anisotropy function F(r,θ) for all 3 seeds with the AAPM TG-43 dosimetry formalism and current literature. The ¹⁹²Ir seed resulted in a dose rate of 1.115 ± 0.001 cGy-hr⁻¹-U⁻¹, the ¹²⁵I seed resulted in a dose rate of 0.965 ± 0.006 cGy/h⁻¹/U⁻¹, and the ¹⁰³Pd seed resulted in a dose rate of 0.671 ± 0.002 cGy/h⁻¹/U⁻¹. The results for all 3 seeds are in good agreement with the AAPM TG-43 and current literature. The validated computer code was then applied to a simple inhomogeneous model to determine the effect bone has on dose distribution from an interstitial implant. The inhomogeneous model showed a decrease in dose rate of 2% for the ¹⁹²Ir, an increase in dose rate of 84% for ¹²⁵I, and an increase in dose rate of 83% for the ¹⁰³Pd at the surface of the bone nearest to the source.     

A Monte Carlo brachytherapy study for dose distribution prediction in an inhomogeneous medium

The purpose of this study was to present a theoretical analysis of how the presence of bone in interstitial brachytherapy affects dose rate distributions. This study was carried out using a Monte Carlo simulation of the dose distribution in homogeneous medium for 3 commonly used brachytherapy seeds. The 3 seeds investigated in this study are iridium-192 (¹⁹²Ir) iodine-125 (¹²⁵I), and palladium-103 (¹⁰³Pd). The computer code was validated by comparing the specific dose rate (Λ), the radial dose function g(r), and anisotropy function F(r,θ) for all 3 seeds with the AAPM TG-43 dosimetry formalism and current literature. The ¹⁹²Ir seed resulted in a dose rate of 1.115 ± 0.001 cGy-hr⁻¹-U⁻¹, the ¹²⁵I seed resulted in a dose rate of 0.965 ± 0.006 cGy/h⁻¹/U⁻¹, and the ¹⁰³Pd seed resulted in a dose rate of 0.671 ± 0.002 cGy/h⁻¹/U⁻¹. The results for all 3 seeds are in good agreement with the AAPM TG-43 and current literature. The validated computer code was then applied to a simple inhomogeneous model to determine the effect bone has on dose distribution from an interstitial implant. The inhomogeneous model showed a decrease in dose rate of 2% for the ¹⁹²Ir, an increase in dose rate of 84% for ¹²⁵I, and an increase in dose rate of 83% for the ¹⁰³Pd at the surface of the bone nearest to the source.     

Estimating statistical uncertainty of Monte Carlo efficiency-gain in the context of a correlated sampling Monte Carlo code for brachytherapy treatment planning with non-normal dose distribution

Correlated sampling Monte Carlo methods can shorten computing times in brachytherapy treatment planning. Monte Carlo efficiency is typically estimated via efficiency gain, defined as the reduction in computing time by correlated sampling relative to conventional Monte Carlo methods when equal statistical uncertainties have been achieved. The determination of the efficiency gain uncertainty arising from random effects, however, is not a straightforward task specially when the error distribution is non-normal. The purpose of this study is to evaluate the applicability of the F distribution and standardized uncertainty propagation methods (widely used in metrology to estimate uncertainty of physical measurements) for predicting confidence intervals about efficiency gain estimates derived from single Monte Carlo runs using fixed-collision correlated sampling in a simplified brachytherapy geometry. A bootstrap based algorithm was used to simulate the probability distribution of the efficiency gain estimates and the shortest 95% confidence interval was estimated from this distribution. It was found that the corresponding relative uncertainty was as large as 37% for this particular problem. The uncertainty propagation framework predicted confidence intervals reasonably well; however its main disadvantage was that uncertainties of input quantities had to be calculated in a separate run via a Monte Carlo method. The F distribution noticeably underestimated the confidence interval. These discrepancies were influenced by several photons with large statistical weights which made extremely large contributions to the scored absorbed dose difference. The mechanism of acquiring high statistical weights in the fixed-collision correlated sampling method was explained and a mitigation strategy was proposed.     

蒙特卡罗程序MCNP、EGSnrc、DPM剂量计算比较研究

目的 验证3个蒙特卡罗程序MCNP、EGSnrc、DPM计算结果的一致性问题。方法基于简单均匀及非均匀模型和复杂临床头部实例模型，对3个蒙特卡罗程序MCNP、EGSnrc、DPM的模型建模、计算结果、计算时间进行了比较研究。结果尽管3个程序在粒子输运原理、模拟参数设置、几何描述模型以及材料截面数据等方面存在不同，但是计算结果仍符合很好。结论3个蒙特卡罗程序计算复杂模型具有相当的可靠性；简单快速蒙特卡罗程序DPM具有明显的优势。     

Effects of heterogeneities in dose distributions under nonreference conditions: Monte Carlo simulation vs dose calculation algorithms

The purpose of this study is to evaluate the performance of dose calculation algorithms used in radiotherapy treatment planning systems (TPSs) in comparison with Monte Carlo (MC) simulations in nonelectronic equilibrium conditions. MC simulations with PENELOPE package were performed for comparison of doses calculated by pencil beam convolution (PBC), analytical anisotropy algorithm (AAA), and Acuros XB TPS algorithms. Relative depth dose curves were calculated in heterogeneous water phantoms with layers of bone (1.8?g/cm³) and lung (0.3?g/cm³) equivalent materials for radiation fields between 1?×?1?cm² and 10?×?10?cm². Analysis of relative depth dose curves at the water-bone interface shows that PBC and AAA algorithms present the largest differences to MC calculations (u_MC?=?0.5%), with maximum differences of up to 4.3% of maximum dose. For the lung-equivalent material and 1?×?1?cm² field, differences can be up to 24.3% for PBC, 11.5% for AAA, and 7.5% for Acuros. Results show that Acurus presents the best agreement with MC simulation data with equivalent accuracy for modeling radiotherapy dose deposition especially in regions where electronic equilibrium does not hold. For typical (nonsmall) fields used in radiotherapy, AAA and PBC can exhibit reasonable agreement with MC results even in regions of heterogeneities.     

蒙特卡罗方法在质子重离子加速器治疗场所屏蔽计算中的应用

目的 利用蒙特卡罗方法建立质子重离子加速器治疗场所的屏蔽计算模型,为治疗场所的屏蔽设计提供可靠的计算方法。方法 采用基于蒙特卡罗方法的FLUKA程序建立质子重离子治疗场所的屏蔽计算模型,模拟质子重离子加速器治疗场所辐射场的分布,通过对质子重离子加速器治疗场所的检测,验证计算模型。结果 FLUKA程序模拟计算结果与现场检测结果具有较好的符合性。结论 FLUKA程序建立的质子重离子加速器治疗场所屏蔽计算模型能够模拟质子重离子产生的辐射场。基于FLUKA程序建立的屏蔽计算模型,质子重离子治疗场所屏蔽设计应根据加速器最高可达的束流强度及能量进行计算。在质子和重离子加速器运行时的治疗室辐射场中,中子对剂量当量的贡献是主要的,因此,屏蔽设计中应重点考虑中子的屏蔽。     

Monte Carlo MCNP-4B-based absorbed dose distribution estimates for patient-specific dosimetry.

This study was intended to verify the capability of the Monte Carlo MCNP-4B code to evaluate spatial dose distribution based on information gathered from CT or SPECT. METHODS: A new three-dimensional (3D) dose calculation approach for internal emitter use in radioimmunotherapy (RIT) was developed using the Monte Carlo MCNP-4B code as the photon and electron transport engine. It was shown that the MCNP-4B computer code can be used with voxel-based anatomic and physiologic data to provide 3D dose distributions. RESULTS: This study showed that the MCNP-4B code can be used to develop a treatment planning system that will provide such information in a time manner, if dose reporting is suitably optimized. If each organ is divided into small regions where the average energy deposition is calculated with a typical volume of 0.4 cm(3), regional dose distributions can be provided with reasonable central processing unit times (on the order of 12-24 h on a 200-MHz personal computer or modest workstation). Further efforts to provide semiautomated region identification (segmentation) and improvement of marrow dose calculations are needed to supply a complete system for RIT. It is envisioned that all such efforts will continue to develop and that internal dose calculations may soon be brought to a similar level of accuracy, detail, and robustness as is commonly expected in external dose treatment planning. CONCLUSION: For this study we developed a code with a user-friendly interface that works on several nuclear medicine imaging platforms and provides timely patient-specific dose information to the physician and medical physicist. Future therapy with internal emitters should use a 3D dose calculation approach, which represents a significant advance over dose information provided by the standard geometric phantoms used for more than 20 y (which permit reporting of only average organ doses for certain standardized individuals)     

The examination of source distribution in a large sample by Monte Carlo simulation

In this work, realistic Monte Carlo simulations were carried out for several distributions of activity in a waste drum to observe the dependence of the efficiency on the source distribution and to test whether the efficiency can be correlated with the shape of the spectra.     

~(252)Cf裂变中子源在组织等效模体中的中子和γ辐射剂量分布的计算

目的　计算2 5 2 Cf裂变中子源的中子和γ辐射在组织等效模体内的剂量分布 ,为使用2 5 2 Cf裂变中子源进行中子放疗提供有用的剂量学参数。方法　建立2 5 2 Cf源和组织等效模体的三维几何计算模型 ,利用蒙特卡罗方法进行中子和γ辐射联合输运计算。结果　计算了两种医用2 5 2 Cf裂变中子源在水、血液、肌肉、皮肤、骨骼和肺组织等效材料构成的模体中距源不同距离点处的中子和γ辐射吸收剂量。结论　蒙特卡罗计算结果与文献数据以及使用双电离室实验测量的结果符合得较好。对2 5 2 Cf裂变中子源在 5种组织材料构成的模体中中子和γ辐射的剂量分布进行了比较 ,使用水作为组织等效材料对2 5 2 Cf裂变中子源在以肌肉、血液和皮肤构成的局部组织内的剂量分布进行模拟计算 ,可取得比较可靠的结果。