TG-43 Dosimetric Parameters Research Articles

Monte Carlo Study on Dose Distributions Around 192Ir, 169Yb, and 125I Brachytherapy Sources Using EGSnrc-based egs_brachy User-code.

As per the recommendations of the American Association of Physicists in Medicine Task Group 43, Monte Carlo (MC) investigators should reproduce previously published dose distributions whenever new features of the code are explored. The purpose of the present study is to benchmark the TG-43 dosimetric parameters calculated using the new MC user-code egs_brachy of EGSnrc code system for three different radionuclides 192Ir, 169Yb, and 125I which represent high-, intermediate-, and low-energy sources, respectively. Brachytherapy sources investigated in this study are high-dose rate (HDR) 192Ir VariSource (Model VS2000), 169Yb HDR (Model 4140), and 125I -low-dose-rate (LDR) (Model OcuProsta). The TG-43 dosimetric parameters such as air-kerma strength, S k, dose rate constant, Λ, radial dose function, g(r) and anisotropy function, F(r,θ) and two-dimensional (2D) absorbed dose rate data (along-away table) are calculated in a cylindrical water phantom of mass density 0.998 g/cm3 using the MC code egs_brachy. Dimensions of phantom considered for 192Ir VS2000 and 169Yb sources are 80 cm diameter ×80 cm height, whereas for 125I OcuProsta source, 30 cm diameter ×30 cm height cylindrical water phantom is considered for MC calculations. The dosimetric parameters calculated using egs_brachy are compared against the values published in the literature. The calculated values of dose rate constants from this study agree with the published values within statistical uncertainties for all investigated sources. Good agreement is found between the egs_brachy calculated radial dose functions, g(r), anisotropy functions, and 2D dose rate data with the published values (within 2%) for the same phantom dimensions. For 192Ir VS2000 source, difference of about 28% is observed in g(r) value at 18 cm from the source which is due to differences in the phantom dimensions. The study validates TG-43 dose parameters calculated using egs_brachy for 192Ir, 169Yb, and 125I brachytherapy sources with the values published in the literature.

Determination of TG-43 Dosimetric Parameters for Photon Emitting Brachytherapy Sources.

Objective:Brachytherapy sources are widely used for the treatment of cancer. The report of Task Group No. 43 (TG-43) of American Association of Physicists in Medicine is known as the most common method for the determination of dosimetric parameters for brachytherapy sources. The aim of this study is to obtain TG-43 dosimetric parameters for 60Co, 137Cs, 192Ir and 103Pd brachytherapy sources by Monte Carlo simulation. Methods:In this study, 60Co (model Co0.A86), 137Cs (model 6520-67), 192Ir (model BEBIG) and 103Pd (model OptiSeed) brachytherapy sources were simulated using MCNPX Monte Carlo code. To simulate the sources, the exact geometric characterization of each source was defined in Monte Carlo input programs. Dosimetric parameters including air kerma strength, dose rate constant, radial dose function and anisotropy function were calculated for each source. Each input program was run with sufficient number of particle histories. The maximum type A statistical uncertainty in the simulation of the 60Co, 137Cs, 192Ir and 103Pd sources, were equal to 4%, 4%, 3.19% and 6.50%, respectively. Results:The results for dosimetry parameters of dose rate constant, radial dose function and anisotropy function for the 60Co, 137Cs, 192Ir and 103Pd sources in this study demonstrated good agreement with other studies. Conclusion:Based on the good agreement between the results of this study and other studies, the TG-43 results for Co0.A86 60Co, 67-65200 137Cs, BEBIG 192Ir and OptiSeed 103Pd sources are validated and can be used as input data in treatment planning systems (TPSs) and to validate the TPS calculations.

Determination of dosimetric parameters for shielded 153Gd source in prostate cancer brachytherapy.

BackgroundInterstitial rotating shield brachytherapy (I-RSBT) is a recently developed method for treatment of prostate cancer. In the present study TG-43 dosimetric parameters of a 153Gd source were obtained for use in I-RSBT.Materials and methodsA 153Gd source located inside a needle including a Pt shield and an aluminum window was simulated using MCNPX Monte Carlo code. Dosimetric parameters of this source model, including air kerma strength, dose rate constant, radial dose function and 2D anisotropy function, with and without the shields were calculated according to the TG-43 report.ResultsThe air kerma strength was found to be 6.71 U for the non-shielded source with 1 GBq activity. This value was found to be 0.04 U and 6.19 U for the Pt shield and Al window cases, respectively. Dose rate constant for the non-shielded source was found to be 1.20 cGy/(hU). However, for a shielded source with Pt and aluminum window, dose rate constants were found to be 0.07 cGy/(hU) and 0.96 cGy/(hU), on the shielded and window sides, respectively. The values of radial dose function and anisotropy function were tabulated for these sources. Additionally, isodose curves were drawn for sources with and without shield, in order to evaluate the effect of shield on dose distribution.ConclusionsExistence of the Pt shield may greatly reduce the dose to organs at risk and normal tissues which are located toward the shielded side. The calculated air kerma strength, dose rate constant, radial dose function and 2D anisotropy function data for the 153Gd source for the non-shielded and the shielded sources can be used in the treatment planning system (TPS).

A modern Monte Carlo investigation of the TG‐43 dosimetry parameters for an 125I seed already having AAPM consensus data

To investigate potential causes for differences in TG-43 brachytherapy dosimetry parameters in the existent literature for the model IAI-125A(125)I seed and to propose new standard dosimetry parameters. The MCNP5 code was used for Monte Carlo (MC) simulations. Sensitivity of dose distributions, and subsequently TG-43 dosimetry parameters, was explored to reproduce historical methods upon which American Association of Physicists in Medicine (AAPM) consensus data are based. Twelve simulation conditions varying(125)I coating thickness, coating mass density, photon interaction cross-section library, and photon emission spectrum were examined. Varying(125)I coating thickness, coating mass density, photon cross-section library, and photon emission spectrum for the model IAI-125A seed changed the dose-rate constant by up to 0.9%, about 1%, about 3%, and 3%, respectively, in comparison to the proposed standard value of 0.922 cGy h(-1) U(-1). The dose-rate constant values by Solberg et al. ["Dosimetric parameters of three new solid core (125)I brachytherapy sources," J. Appl. Clin. Med. Phys. 3, 119-134 (2002)], Meigooni et al. ["Experimental and theoretical determination of dosimetric characteristics of IsoAid ADVANTAGE™ (125)I brachytherapy source," Med. Phys. 29, 2152-2158 (2002)], and Taylor and Rogers ["An EGSnrc Monte Carlo-calculated database of TG-43 parameters," Med. Phys. 35, 4228-4241 (2008)] for the model IAI-125A seed and Kennedy et al. ["Experimental and Monte Carlo determination of the TG-43 dosimetric parameters for the model 9011 THINSeed™ brachytherapy source," Med. Phys. 37, 1681-1688 (2010)] for the model 6711 seed were +4.3% (0.962 cGy h(-1) U(-1)), +6.2% (0.98 cGy h(-1) U(-1)), +0.3% (0.925 cGy h(-1) U(-1)), and -0.2% (0.921 cGy h(-1) U(-1)), respectively, in comparison to the proposed standard value. Differences in the radial dose functions between the current study and both Solberg et al. and Meigooni et al. were <10% for r ≤ 5 cm, and increased for r > 5 cm with a maximum difference of 29% at r = 9 cm. In comparison to Taylor and Rogers, these differences were lower (maximum of 2% at r = 9 cm). For the similarly designed model 6711 (125)I seed, differences did not exceed 0.5% for 0.5 ≤ r ≤ 10 cm. Radial dose function values varied by 1% as coating thickness and coating density were changed. Varying the cross-section library and source spectrum altered the radial dose function by 25% and 12%, respectively, but these differences occurred at r = 10 cm where the dose rates were very low. The 2D anisotropy function results were most similar to those of Solberg et al. and most different to those of Meigooni et al. The observed order of simulation condition variables from most to least important for influencing the 2D anisotropy function was spectrum, coating thickness, coating density, and cross-section library. Several MC radiation transport codes are available for calculation of the TG-43 dosimetry parameters for brachytherapy seeds. The physics models in these codes and their related cross-section libraries have been updated and improved since publication of the 2007 AAPM TG-43U1S1 report. Results using modern data indicated statistically significant differences in these dosimetry parameters in comparison to data recommended in the TG-43U1S1 report. Therefore, it seems that professional societies such as the AAPM should consider reevaluating the consensus data for this and others seeds and establishing a process of regular evaluations in which consensus data are based upon methods that remain state-of-the-art.

SU-E-T-501: Perturbation of TG-43 Parameters of the Brachytherapy Sources under Insufficient Scattering Materials.

According to the TG-43 recommendations of American Association of Physicist in Medicine (AAPM), the dosimetry parameters of brachytherapy sources are obtained in a water phantom with full scattering conditions. However, in many actual clinical treatments the source are not surrounded with sufficient tissue in all directions to provide the full scattering condition. HDR brachytherapy of the breast or 125I eye-plaque treatment of the ocular melanoma are among the treatment sites that fits this category. In this project, the impact of insufficient phantom material surrounding the 137Cs, 192Ir, and 103Pd brachytherapy source on their TG43 dosimetric characteristics has been investigated. In this study, the effect of the insufficient tissue (referred here as missing tissues) around the brachytherapy sources on their TG-43 dosimetric parameters have been investigated using MCNP5 Monte Carlo code. The brachytherapy sources were simulated in different locations inside a cubical water phantom with the dimensions of 30*30*30 cm3 . The variation of the dosimetric parameters of the three sources (i.e. Î>, g(r), and F(r,θ)) were compared to the values from full scattering conditions (i.e. source at the center of the phantom). The results of this study indicate the variation of g(r)and 2D anisotropy function of the brachytherapy sources as a function of the missing tissue thickness. These changes increase by decreasing the energy of the photons emitted by the brachytherapy sources. These differences are mainly due to the lack of full scattering condition for the points near the phantom boundary. Unlike the published data for symmetric, but insufficient phantom material around the source, the impact of asymmetric phantom materials have been evaluated on dosimetric characteristics of the brachytherapy sources.

Monte Carlo derivation of AAPM TG-43 dosimetric parameters for GZP6 Co-60 HDR sources

Cobalt 60 source is generally available on high dose rate (HDR) afterloading equipment especially for treatment of gynecological lesions. The GZP6 remote afterloader (Nuclear Power Institute of China) utilizes 60Co sources for treatment of intracavitary and intraluminal malignancies. In this study, the AAPM TG-43 dosimetric parameters of three sources in GZP6 system have been studied using MCNP4C Monte Carlo (MC) code; and the results are compared with other available 60Co HDR sources. The presented parameters consist of air kerma strength, dose rate constant, radial dose function and anisotropy function. They show less than 1% uncertainty. The TG-43 based dosimetry data can be used not only to validate the dedicated treatment planning software (TPS), but also to introduce new complementary software to enhance the system performance in gynecological treatments.

Evaluation of interpolation methods for TG-43 dosimetric parameters based on comparison with Monte Carlo data for high-energy brachytherapy sources.

PurposeThe aim of this work was to determine dose distributions for high-energy brachytherapy sources at spatial locations not included in the radial dose function gL(r) and 2D anisotropy function F(r,θ) table entries for radial distance r and polar angle θ. The objectives of this study are as follows: 1) to evaluate interpolation methods in order to accurately derive gL(r) and F(r,θ) from the reported data; 2) to determine the minimum number of entries in gL(r) and F(r,θ) that allow reproduction of dose distributions with sufficient accuracy.Material and methodsFour high-energy photon-emitting brachytherapy sources were studied: 60Co model Co0.A86, 137Cs model CSM-3, 192Ir model Ir2.A85-2, and 169Yb hypothetical model. The mesh used for r was: 0.25, 0.5, 0.75, 1, 1.5, 2–8 (integer steps) and 10 cm. Four different angular steps were evaluated for F(r,θ): 1°, 2°, 5° and 10°. Linear-linear and logarithmic-linear interpolation was evaluated for gL(r). Linear-linear interpolation was used to obtain F(r,θ) with resolution of 0.05 cm and 1°. Results were compared with values obtained from the Monte Carlo (MC) calculations for the four sources with the same grid.ResultsLinear interpolation of g L(r) provided differences ≤ 0.5% compared to MC for all four sources. Bilinear interpolation of F(r,θ) using 1° and 2° angular steps resulted in agreement ≤ 0.5% with MC for 60Co, 192Ir, and 169Yb, while 137Cs agreement was ≤ 1.5% for θ < 15°.ConclusionsThe radial mesh studied was adequate for interpolating gL(r) for high-energy brachytherapy sources, and was similar to commonly found examples in the published literature. For F(r,θ) close to the source longitudinal-axis, polar angle step sizes of 1°-2° were sufficient to provide 2% accuracy for all sources.

SU-FF-T-17: Impact of Applicator and Dummy Pellets On the TG 43 Parameters of Selectron Cs-137 Source

Introduction In this study, dose rate distribution around a spherical pellet source, from a low dose rate (LDR) Selectron remote afterloading system, have been determined using Experimental and Monte Carlo Simulation techniques. Materials and Methods: Monte Carlo simulations were performed using MCNP4C code, for a single pellet source in water medium and Plexiglas and measurements were performed in Plexiglas phantom material using LiF TLD chips. Absolute dose rate distribution and the TG-43 dosimetric parameters such as dose rate constant, radial dose functions and anisotropy functions were obtained for a single pellet source. In order to investigate the effect of the applicator, and surrounding pellets on dosimetric parameters of the source, the simulations were repeated for 6 different arrangements with a single active source and five non-active pellets inside central metallic tubing of a vaginal cylindrical applicator. In commercial treatment planning systems (TPS) the attenuation effects of the applicator and inactive spacers on total dose are neglected. Results The results indicate that this effect could lead to overestimation of the calculated anisotropy function by up to 7% along the longitudinal axis of the applicator especially beyond the applicator tip. The radial dose function and anisotropy function of the new source design have been tabulated for each configuration. Conclusions Impact of the Applicator and Dummy Pellets on the TG-43 Parameters of Selectron Cs-137 Source has been determined using TLD and Monte Carlo calculations. The results indicate neglecting the applicator and dummy pellets could cause significant errors in F(r, θ).

SU-FF-T-15: Comparison of Various Monte Carlo Codes for Brachytherapy Source Dosimetry

Purpose: Monte Carlo simulations are used for dosimetric evaluation of brachytherapy sources. Various investigators had demonstrated good agreement between the measured and simulated data while some others have shown discrepancies between the two methods. These discrepancies were attributed to the errors in cross section data, differences in phantom and source geometry and composition. However, the exact causes of these differences have not yet fully explored. The goal of this project is to verify the differences among the widely available Monte Carlo codes for brachytherapy source dosimetry. Material and method: These investigations are based on VariSource , Model VS2000, IsoAid Advantage Model IA1-125A, and Best Industries Model Best2335 sources. TG-43 parameters of these sources have been determined using 6 different commonly utilized Monte Carlo codes. All the simulations would be performed in water and number of photons would be selected to provide statistical fluctuations better than better than 1%. Dose rate constant, radial dose functions and anisotropy function at 2 cm distance will be simulated for each source by each Monte Carlo code. Results: The results of these investigations indicate the more recent Monte Carlo code with the updated cross section data are in excellent agreement with each other for producing all of the TG-43 parameters except 2D anisotropy functions at small angles (<10 degree). The older Monte Carlo codes or older cross section data have larger uncertainty in their results. Conclusion: The TG-43 dosimetric parameters of the brachytherapy source can be simulated with the one of the Monte Carlo codes. It would be recommended to use one of the well established codes as a benchmark code for establishment of other codes in parameterization of the brachytherapy sources.

Radiochromic film dosimetry for model 6711 I-125 seed with special emphasis on eye plaque applications

Purpose: In the treatment of ocular melanomas using eye plaques, the distances between the radioactive seeds and the interest points in and around the tumor can be less than 0.1 cm. It is important to have accurate dosimetric parameters at such short distances specific to a seed model. However, the near field 2d anisotropy functions for many I-125 seed models have not been available. We report our study of radiochromic EBT film dosimetry in the near field (at distances down to 0.06 cm) to determine the TG-43 dosimetric parameters specific to the model 6711 I-125 seed for use in eye plaque treatment planning calculations.

SU-FF-T-147: Determination of TG43 Parameters for Cs-131 Model CS-1R2 Seed Using Radiochromic EBT Film Dosimetry

Purpose: To measure the 2D dose distributions for Cs‐131 seed model CS‐1R2 for distances from 0.06cm to 4cm using radiochromic EBT film dosimetry. TG43 dosimetric parameters were generated. Method and Materials: Each radiochromic film (GAFCHROMIC® EBT lot ♯35076) was in contact with a model CS‐1R2 seed (IsoRay Medical) at the center of a solid water phantom, 30×30×20cm. A multiple film technique was employed. More than 50 films were separately exposed to 10 seeds, with the product of air kerma strength and exposure time between 14 and 800 Uhr. The seed strength ranged from 10 to 4U (NIST traceable) during the experimental runs. For calibration, 15 films (of the same lot) were exposed to 50kV x‐ray (M50) in air at 100cm SSD with doses ranging from 0.2 to 20Gy. Since the EBT film response is almost identical for M50 and M40 x‐rays, the energy response can be considered to be flat in this region and thus also the same for the Cs‐131 energy. All experimental, calibration and background films were scanned (pixel resolution 0.2mm) using a CCD camera‐based densitometer, with red and green light sources. Conversion from net optical density readings to doses were achieved based on the calibration curve established for each light source used in scanning. The 2D dose values in cylindrical coordinates were converted to polar coordinates, and the TG43 parameters were generated. Results: The dose rate constant, radial dose function and 2D anisotropy function were determined, and compared with those reported by other authors. General agreement was found. Conclusion: It is feasible to determine TG43 parameters for Cs‐131 model CS‐1R2 seed in solid water phantom using radiochromic EBT film. This method is superior to TLDdosimetry (1) in providing data with high spatial resolution for distances down to 0.06 cm and (2) within reasonably achievable time frame.

A Monte Carlo evaluation of the dosimetric characteristics of the EchoSeed™ Model 6733 125I brachytherapy source

Purpose: 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 EchoSeed™Model 6733 125I 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 American Association of Physicists in Medicine (AAPM) TG-43 guidelines. Methods and Materials: As per AAPM recommendation, it is required to perform at least one experimental study and one Monte Carlo simulation, preferably done by two independent investigators. Other investigators have experimentally determined the dosimetric parameters of this new source. In this project, the Monte Carlo simulated dosimetric parameters of the EchoSeed™ Model 6733 source have been provided. Results: The results of this evaluation indicate the value of the dose rate constant of 0.97 ± 3% cGyh −1U −1 in liquid water, which is in good agreement with the measured value of 0.99 ± 8% cGyh −1U −1 reported by Meigooni et al. The anisotropy constant of the EchoSeed™ 125I brachytherapy source was found to be 0.960 in liquid water. Conclusions: The Monte Carlo Simulated TG-43 dosimetric parameters of the EchoSeed ™ were determined and the results were compared with the published measured data.