~(125)Ⅰ植入治疗源的剂量学参数的Monte Carlo模拟

放射性粒子源125I已被广泛用于前列腺和眼睛的植入治疗中。本文采用EGS5蒙特卡罗代码计算了美国医用物理学协会（AAPM）TG-43U1报告中推荐的型号为6711125I近距治疗源（活性区长取0.28cm）的剂量学参数,如剂量率常数、径向剂量函数和各向异性函数。剂量率常数为0.959cGy/h/U,与TG-43U1推荐值和Dolan等已发表的值相差在2.0%以内 径向剂量函数数值与二者均符合较好 随着角度和距离的增加,各向异性函数值数值与二者的复合程度趋佳。并给出了实用性较强的径向剂量函数的拟合公式。     

Monte Carlo dose parameters of the BrachySeed model LS-1 I brachytherapy source

Using a modified EGS4 code and associated user code DOSCGC, the two-dimensional dose rate distribution in water and air-kerma strength are calculated for a BrachySeed (model LS-1) ¹²⁵I brachytherapy source, based on geometry and material data provided by the manufacturer. The AAPM TG-43 dose parameters derived from these results include the dose rate constant, the radial dose function, the anisotropy function, and the anisotropy factor and constant. The value of the dose rate constant so obtained is 0.932±0.003 cGy h⁻¹ U⁻¹. The source strength calculation excludes the contribution from titanium characteristic X-rays (4.5 and 4.9 keV) in the source in order to comply with a new primary calibration standard implemented by the National Institute of Standards and Technology in 1999. A sampling procedure for simulating silver characteristic X-ray production in the mixture material of the source core is developed in the EGS4 code. The calculated results reveal the good dose isotropy of the LS-1 source. The Monte Carlo dose parameters obtained are compared with measurements and calculations of other investigators.     

A discretized approach to determining TG-43 brachytherapy dosimetry parameters: case study using Monte Carlo calculations for the MED3633 Pd source

It is of interest to discern the energy-dependence of American Association of Physicists in Medicine (AAPM) TG-43 brachytherapy dosimetry parameters. Using Monte Carlo calculation geometry and techniques (MCNP), dependence of these parameters was calculated as a function of photon energy, in general, and for the MED3633 ¹⁰³Pd source using a discretized approach. Results were weighted and summed to determine the total contribution for comparison with the ¹⁰³Pd source literature. Comprehensive 2-D results are discussed, and the level of agreement with other assessments are presented.     

瓦里安高剂量率铱源剂量学参数的蒙特卡罗模拟研究

目的 采用美国医学物理师学会（AAPM）和欧洲放射治疗和肿瘤学会（ESTRO）推荐的蒙特卡罗方法对瓦里安GammaMed Plus HDR ¹⁹²Ir源的剂量学参数进行模拟研究。方法 基于EGSnrc蒙特卡罗软件,建立该型号¹⁹²Ir源精确的计算模型。采用公式推导、双线性插值及单位转换等方法,分别得到了单位活度空气比释动能强度、剂量率常数、径向剂量函数以及各向异性函数,并将结果与文献报道数据进行分析比较。结果 研究得到的单位活度空气比释动能强度为9.781×10^-8 U/Bq,剂量率参数为1.113 cGy·h^-1·U^-1,与文献报道的相差在0.4%以内。本研究的径向剂量函数、各向异性函数与文献数据能较好吻合。结论 基于EGSnrc蒙特卡罗软件能对¹⁹²Ir源剂量学特性进行定量研究,这将为进一步研究后装剂量分布,精确评价临床放疗剂量提供理论依据。     

基于实验与蒙特卡罗模拟法探究125I粒子间剂量衰减

目的 研究多颗¹²⁵I放射性粒子间剂量衰减规律.方法 利用Geant 4软件包进行蒙特卡罗模拟单颗粒子和多颗粒子周围剂量分布,将模拟结果与TG43-U1报告中推荐剂量计算方法所得结果进行对比,并利用实验测得数据验证蒙特卡罗模拟结果.结果 单颗粒子周围剂量分布的蒙特卡罗模拟结果与TG43-U1计算和实验结果差值分别在±3%和±5%以内.多颗粒子的蒙特卡罗模拟结果对比TG43-U1线性叠加结果,粒子间剂量衰减为3.8%~13.2%,平均剂量衰减为7.2%,实验所得结果与蒙特卡罗模拟结果差值在6%以内.结论 空间中存在多颗放射性¹²⁵I粒子时,由于粒子间的相互影响导致7%左右的剂量衰减,其最大值可超过13%,在人体组织中剂量衰减值会更大.因此,利用TG43-U1方法进行线性叠加计算临床中的剂量分布不够精确.     

Monte Carlo dosimetry of the eye plaque design used at the St. Erik Eye Hospital for 125I brachytherapy

PurposeAt St. Erik Eye Hospital in Stockholm, Sweden, ocular tumors of apical height above 6 mm are treated with brachytherapy, using iodine-125 seeds attached to a gold alloy plaque while the treatment planning is performed assuming homogeneous water surroundings. The aim of this work was to investigate the dose-modifying effects of the plaque and the seed fixating silicone rubber glue.Methods and MaterialsThe impact of the gold plaque and silicone rubber glue was studied with the Monte Carlo N-particle transport code, version 5.ResultsFor the 2 cm most proximal to the plaque surface along the plaque's central axis, the eyeball received 104.6–93.0% of the dose in all-water conditions.ConclusionsThe 0.3 mm thick layer of silicone rubber glue, used for seed fixation, attenuates photons little enough to allow characteristic X-rays from the gold alloy plaque to reach the eyeball. Close to the plaque, the dose rates were higher with the plaque and glue present, than in homogeneous water conditions. This is in contrast to what has been reported for more commonly used eye plaques, demonstrating the importance of investigating the dosimetry of individual treatment systems.     

A Monte Carlo evaluation of the dosimetric characteristics of the Best Model 2301 125I brachytherapy source.

Recently a new design of a 125I brachytherapy source was introduced for interstitial seed implants, particularly for prostate seed implants. This new source is the Best Model 2301 brachytherapy source. Due to the differences in source design and manufacturing process from one new source to the next, their dosimetric parameters should be determined according to the AAPM TG-43 guidelines. As per AAPM recommendation (Med. Phys. 25 (12) (1998) 2269), it is required to perform the seed dosimetry using at least one experimental study and one Monte Carlo simulation, preferably done by two separate investigators. Other investigators have experimentally determined the dosimetric parameters of this new source. In this project, the Monte Carlo simulated dosimetric parameters of the Best Model 2301 125I source have been provided. The results of this evaluation indicate the value of dose rate constant of 1.01 +/- 3% cGyh-1U-1 in liquid water, which is in good agreement with 1.02 +/- 8% cGyh-1U-1 reported by Nath and Yue, 2002. The anisotropy constant was found to be 0.98 in liquid water.     

Experimental and Monte Carlo dosimetric characterization of a 1 cm 103Pd brachytherapy source

PurposeTo determine the in-air azimuthal anisotropy and in-water dose distribution for the 1 cm length of a new elongated ¹⁰³Pd brachytherapy source through both experimental measurements and Monte Carlo (MC) simulations. Measured and MC-calculated dose distributions were used to determine the American Association of Physicists in Medicine Task Group No. 43 (TG-43) dosimetry parameters for this source.Methods and MaterialsThe in-air azimuthal anisotropy of the source was measured with a NaI scintillation detector and was simulated with the MCNP5 radiation transport code. Measured and MC results were normalized to their respective mean values and then compared. The source dose distribution was determined from measurements with LiF:Mg,Ti thermoluminescent dosimeter (TLD) microcubes and MC simulations. TG-43 dosimetry parameters for the source, including the dose-rate constant, Λ, two-dimensional anisotropy function, F(r, θ), and line-source radial dose function, g_L(r), were determined from the TLD measurements and MC simulations.ResultsNaI scintillation detector measurements and MC simulations of the in-air azimuthal anisotropy of the source showed that ≥95% of the normalized values for each source were within 1.2% of the mean value. TLD measurements and MC simulations of Λ, F(r, θ), and g_L(r) agreed to within the associated uncertainties.ConclusionsThis new ¹⁰³Pd source exhibits a high level of azimuthal symmetry as indicated by the measured and MC-calculated results for the in-air azimuthal anisotropy. TG-43 dosimetry parameters for the source were determined through TLD measurements and MC simulations.     

Dosimetric investigation of 103Pd permanent breast seed implant brachytherapy based on Monte Carlo calculations

PurposePermanent breast seed implant using ¹⁰³Pd is emerging as an effective adjuvant radiation technique for early stage breast cancer. However, clinical dose evaluations follow the water-based TG-43 approach with its considerable approximations. Toward clinical adoption of advanced TG-186 model-based dose evaluations, this study presents a comprehensive investigation for permanent breast seed implant considering both target and normal tissue doses.Methods and MaterialsDose calculations are performed with the free open-source Monte Carlo (MC) code, egs_brachy, using two types of virtual patient models: TG43sim (simulated TG-43 conditions) and MCref (heterogeneous tissue modeling from patient CT, seeds at implant angle) for 35 patients. The sensitivity of dose metrics to seed orientation and tissue segmentation are assessed.ResultsIn the target volume, D₉₀ is 14.1 ± 5.8% lower with MCref than with TG43sim, on average. Conversely, normal tissue doses are generally higher with MCref than with TG43sim, for example, by 22 ± 13% for skin D_1cm2, 82 ± 7% for ribs D_max, and 71 ± 23% for heart D_1cm3. Discrepancies between MCref and TG43sim doses vary over the patient cohort, as well as with the tissue and metric considered. Skin doses are particularly sensitive to seed orientation, with average difference of 4% (maximum 28%) in D_1cm2 for seeds modeled vertically (egs_brachy default) compared with those aligned with implant angle.ConclusionsTG-43 dose evaluations generally underestimate doses to critical normal organs/tissues while overestimating target doses. There is considerable variation in MCref and TG43sim on a patient-by-patient basis, motivating clinical adoption of patient-specific MC dose calculations. The MCref framework presented herein provides a consistent modeling approach for clinical implementation of advanced TG-186 dose calculations.     

Dosimetric characterization of a newly designed encapsulated interstitial brachytherapy source of iodine-125—model LS-1 BrachyseedTM

A newly designed encapsulated ¹²⁵I source has been introduced (Model LS-1 BrachySeed^TM manufactured by DRAXIMAGE Inc.) for interstitial brachytherapy . In this source ¹²⁵I radionuclide is contained in two ceramic beads positioned at each end of a titanium capsule. The source contains a rod of Pt–Ir, which serves as a radiographic marker for source localization in the patient. Principle photon emissions are 27.4 and 31.0 keV X-rays and a 35.5 keV gamma-ray. The 22.2 and 25.5 keV silver X-rays produced by fluorescence of the silver dopant in the ceramic bead radioisotope carriers, are also emitted. In this work, the dosimetric characteristics of the ¹²⁵I source were measured with micro LiF TLD chips and dosimetry parameters were characterized based upon the American Association of Physicists in Medicine, Task Group, No. 43 formalism. The corrected 1999 National Institute of Standards and Technology standard for low energy interstitial brachytherapy sources was used to specify the air kerma strength of the sources used in this study. The dose rate constant of the sources was determined to be 1.02±0.07 cGyh⁻¹ U⁻¹. The radial dose function was measured and was found to be similar to that of the silver-based model 6711 ¹²⁵I source. However, the anisotropy function of the Model LS-1 BrachySeed^TM source is considerably better than that of model 6711 ¹²⁵I source, especially on the points along and close to the longitudinal axis of the source. The BrachySeed^TM model LS-1 provides more isotropic angular dose distribution in tissue than model 6711 ¹²⁵I source. The anisotropy constant for the model LS-1 source was determined to be 1.006, which is considerably better than the value of 0.93 for the model 6711 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.     

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.     

Experimental determination of the Task Group-43 dosimetric parameters of the new I25.S17plus 125I brachytherapy source

PurposeTo present experimental dosimetry results for the new IsoSeed I25.S17plus ¹²⁵I brachytherapy source, in fulfillment of the American Association of Physicists in Medicine recommendation for, at least one, experimental dosimetry characterization of new low-energy seeds before their clinical implementation.Methods and MaterialsA batch of 100 LiF thermoluminescent dosimeter (TLD)-100 microcubes was used for the experimental determination of the dose-rate constant, radial dose, and anisotropy functions, in irradiations performed using two Solid Water phantoms. Monte Carlo (MC) simulations were used to determine appropriate correction factors that account for the use of Solid Water as a phantom material instead of liquid water and for the different energy response of the TLD dosimeters in the experimental ¹²⁵I photon energies relative to the 6 MV x-ray photon beam used for the TLD calibration. Measurements were performed for four I25.S17plus seeds; one with direct traceability of air-kerma strength calibration to National Institute of Standards and Technology and three with secondary National Institute of Standards and Technology traceability.ResultsA mean dose-rate constant, Λ, of 0.956 ± 0.043 cGy h⁻¹ U⁻¹ was experimentally determined for the I25.S17plus source, which agrees within uncertainties with the MC result of 0.925 ± 0.013 cGy h⁻¹ U⁻¹ calculated independently for the same seed model in a previous study. Agreement was also observed between the measured and the MC-calculated radial dose and anisotropy function values.ConclusionsExperimental dosimetry results for the I25.S17plus ¹²⁵I source verify corresponding independent MC results in the form of Task Group-43 dosimetry parameters. The latter are found in agreement within uncertainties with sources of similar design incorporating a silver marker, such as the Oncura OncoSeed Model 6711.     

6711型125I种子源剂量学参量的蒙特卡罗模拟研究

目的 计算6711型125I种子源的剂量特性.方法 使用EGSnrc蒙特卡罗模拟程序对种子源的各向异性函数、径向剂量函数和剂量率常数进行计算,并给出了不同介质中的空间剂量率分布,将计算结果与美国医学物理师协会(AAPM)TG43-U1号报告中的推荐值和其他已发表的相关数据进行了比较.结果 各向异性函数与其他最新发表的数据符合较好;径向剂量函数与TG43-U1推荐值符合较好;剂量率常数为0.951 cGy·h-1·U-1,与TG43-U1推荐值在1.45%内吻合.结论 6711型125I种子源剂量场具有低剂量率,高梯度的特点;各向异性函数在近距离小角度处存在小突起的结构.     

组织成分及密度对125I粒子植入剂量分布影响的研究

目的 分析组织成分及密度对¹²⁵I粒子植入剂量分布影响,为临床放射性粒子植入剂量计算和评估提供参考。方法 应用egs_brachy软件建立OncoSeed 6711型¹²⁵I放射性粒子物理模型,计算剂量率常数和水中径向剂量函数g（r）,以验证计算模型准确性;根据不同组织的元素组成及密度表,分别计算¹²⁵I粒子在水、前列腺、乳腺、肌肉、骨等不同介质中g（r）和剂量分布。结果 计算得到的剂量率常数为0.950 cGy·h^-1·U^-1、水中g（r）和吸收剂量均与文献数据近似。在同一径向位置,¹²⁵I粒子在前列腺、肌肉中吸收剂量与水中差异<5%;在距源中心0.05 cm处,骨中吸收剂量是水中的6.042倍;在近源1.7 cm内,乳腺与水中吸收剂量差异均>10%。结论 ¹²⁵I粒子在部分介质中的剂量分布与水有着较明显区别,在临床剂量计算中需考虑组织成分及密度对吸收剂量的影响。     

携带125I粒子导管和徒手粒子植入后剂量学差异的模体研究

目的对比在实时针道计划下携带¹²⁵I粒子的可降解导管植入和徒手粒子植入后靶区各项剂量学参数的差异。方法本实验模拟病灶42个,分为徒手组21个和导管组21个。根据治疗计划系统进行粒子植入。分别记录术前、术后的最小剂量(D_min)、最大剂量(D_max)、平均剂量(D_mean)、适形指数(CI)、靶外体积指数(EI)、均匀性指数(HI)、覆盖90%靶体积的剂量(D90)、90%处方剂量覆盖靶体积的百分比(V90)。通过Bland-Altman法分析术前、术后剂量参数的一致性,组间比较采用秩和检验。结果经Bland-Altman法分析,两组术前、术后大部分参数均具有较好的一致性,仅徒手组D_min和V₉₀的一致性欠佳。但是导管组在D_max(Z=-3.824,P<0.005)、CI(Z=-1.962,P<0.005)、HI(Z=-2.352,P<0.005)、D90(Z=-2.453,P<0.005)、V90(Z=-2.845,P<0.005)的参数误差范围更小。结论实时针道计划下携带¹²⁵I粒子的可降解导管植入术前、术后剂量学参数均具有较好的一致性,且剂量参数值误差范围更小。     

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.