Monte Carlo calculations and experimental measurements of dosimetry parameters of a new 103Pd source

Permanent prostate implantation using 125I (iodine) or 103Pd (palladium) sources is a popular treatment option in the management of early prostate cancer. As sources of new designs are developed and marketed for application in permanent prostate implantations, their dosimetric characteristics must be carefully determined in order to maintain the accuracy of patient treatment. This report presents the results of experimental measurements and Monte Carlo calculations of the dosimetric parameters performed for a newly available 103Pd seed source. The measurements were performed in a large scanning water phantom using a diode detector. The positioning of the source and detector was achieved by a computer-controlled positioning mechanism in the scanning water phantom. The dose rate constant in water for the new 103Pd source was determined from measurements with the diode detector calibrated with 125I sources of known air-kerma strength. The radial dose function values for the source were measured using the diode detector. Monte Carlo photon transport calculations were then used to calculate the dosimetric parameters of dose rate constant, radial dose function, and anisotropy function using an accurate geometric model of the source. The measured dose rate constant of 0.693 cGy/U-hr compares well with the Monte Carlo calculated value of 0.677 cGy/U-hr. These results are further compared with data on existing 103Pd sources.     

Monte Carlo modeling of the transverse-axis dose distribution of the model 200 103Pd interstitial brachytherapy source

This study presents the first theoretical analysis of the absolute dose-rate distribution about the Model 200 103Pd interstitial brachytherapy source. Monte Carlo photon-transport (MCPT) simulation techniques have been used to evaluate the transverse-axis dose-rate distribution of the Model 200 source as a function of thickness of the Pd metal coating (containing the 103Pd) plated onto the surfaces of right cylindrical graphite pellets contained within the seed. The dose-rate constant, A, was realistically estimated by simulating the wide-angle, free-air chamber (WAFAC) calibration geometry. The WAFAC is the experimental realization of NIST's (National Institute of Standards and Technology) recently implemented primary standard of air-kerma strength (S(K)). Our results show that polar angle- and distance-dependent oblique filtration and shielding effects induce significant and unexpected photon fluence anisotropy near the transverse-axis and inverse square law deviations at typical calibration distances. Any source consisting of radioactivity deposited on a highly attenuating surface with sharp edges may exhibit such effects. In the case of the Model 200 seed, the Pd metal thickness does not significantly influence the relative dose distribution in water at distances less than 5 cm, but does make A sensitive to the S(K) measurement geometry. Fortunately, the WAFAC averages fluence over a sufficiently large aperture that the resultant A, 0.68 +/- 0.02 cGy x h(-1) x U(-1), is almost independent of Pd metal layer thickness and in close agreement with recent measurements and calculations. This value is 20% higher than that of the renormalized Task Group 43 A value.     

Monte carlo investigation of the dosimetric properties of the new 103Pd BrachySeedPd-103 Model Pd-1 source

Recently, 103Pd brachytherapy sources have been increasingly used for interstitial implants as an alternative to 125I sources. The BrachySeedPd-103 Model Pd-1 seed is one of the latest in a series of new brachytherapy sources that have become available commercially. The dosimetric properties of the seed were investigated by Monte Carlo simulation, which was performed using the Integrated Tiger Series CYLTRAN code. Following the AAPM Task Group 43 formalism, the dose rate constant, radial dose function, and anisotropy parameters were determined. The dose rate constant, A, was calculated to be 0.613 +/- 3% cGy h(-1) U(-1). This air kerma strength was derived from Monte Carlo simulation using the point extrapolation method. The radial dose function, g(r), was computed at distances from 0.15 to 10 cm. The anisotropy function, F(r,theta), and anisotropy factor, phi(an)(r), were calculated at distances from 0.5 to 7 cm. The anisotropy constant, phi(an), was determined to be 0.978, which is closer to unity than most other 103Pd seeds, indicating a high degree of uniformity in dose distribution. The dose rate constant and the radial dose function were also investigated by analytical modeling, which served as an independent evaluation of the Monte Carlo data, and found to be in good agreement with the Monte Carlo results.     

A segmented 32P source Monte Carlo model to derive AAPM TG-60 dosimetric parameters used for intravascular brachytherapy

A new high dose rate 20 mm 32P intravascular brachytherapy (IVB) beta source used with automated stepping has recently been introduced. The AAPM Task Group 60 recommends that beta IVB sources should have well characterized dosimetric parameters in water. In this study, Monte Carlo simulations (MCNPX v 2.4) were used to derive these parameters for a 2 mm source segment rather than the entire 20 mm source to ensure the correct formulation using the traditional TG-60 and TG-43 polar coordinate system (r, theta) parameters. The dose rate at the reference depth of 2 mm, the radial dose function, and the anisotropy function were generated for the 2 mm 32P source segment at the mid-plane, distal edge and proximal edge of the original 20 mm source. Our results indicate that the anisotropy of the 2 mm distal and proximal segments are the same, but differ from that of the mid-plane segment due to the perturbation of the adjacent tungsten marker. Using the TG-60 formulation of the mid-plane and edge segments resulted in dose distributions similar to those obtained for a 20 mm linear beta source model. The segmented formulation provides a method consistent with the familiar TG-60 formulation and ability to calculate the dose-distribution inside curved vessels.     

Monte Carlo calculations of dosimetry parameters of the urocor prostaseed 125I source

This report presents the results of Monte Carlo calculations of the dosimetric parameters of the Urocor ProstaSeed 1251 seed source. This source contains five spherical silver markers, of 0.5 mm diameter, with 1251 deposited on the spheres through ion exchange. The silver spheres are then encapsulated within a 0.8 mm in diameter and 4.5 mm long cylindrical shell of titanium, as is common to this type of sources. Physical dimensions of the source are confirmed by measurement. Four (4) geometric models of the source, based on different assumptions on the locations of the silver spheres within the seed, were used in dose rate calculations. Monte Carlo photon transport simulation was used to calculate the dosimetric parameters of dose rate constant, radial dose function, and anisotropy function using these geometric models of the source. The Monte Carlo calculated dose rate constant of the ProstaSeed source was found equal to 0.925 cGy/Uh with approximate uncertainties of 5%. Radial dose function values and anisotropy function values were derived from the relative dose distribution around the source calculated by the Monte Carlo code. The calculated values of these dosimetric parameters agree with previously published thermoluminescence-dosimeter-measured values for this source after consideration of measurement and calculation uncertainties.     

Dose rate table for a 32P intravascular brachytherapy source from Monte Carlo calculations

Studies of intravascular brachytherapy to prevent restenosis following angioplasty have shown many promising results. Accurate dose rate tables based on detailed models of the brachytherapy sources are necessary for treatment planning. This work will present an away and along dose rate table for a 27 mm long catheter based 32P beta source. MD-55-2 radiochromic film has been exposed at five different depths (0.5 mm-4 mm) in a polystyrene phantom using a 27 mm long Guidant 32P beta source. The total dose to the active region of the film was determined using the absolute detector response of the MD-55-2 radiochromic film. The Monte Carlo code MCNP4B2 was also used to calculate the dose to the active region of the film using a detailed model of the source, encapsulation, and radiochromic film. The dose to film calculations showed good agreement with the measurements presented in this work with an average difference of 7%. The Monte Carlo calculations were also verified against previously published depth dose in water measurements determined using radiochromic film and plastic scintillator. The depth dose calculations in water showed good agreement with the previously published measurements with the calculations being about 2.5% lower than the film measurements and about 2.5% higher than the scintillator measurements. This work then uses the verified Monte Carlo code to present a dose rate table for the 32P intravascular beta source.     

A Monte Carlo investigation of the dosimetric characteristics of the VariSource 192Ir high dose rate brachytherapy source.

An analytical Monte Carlo simulation code, incorporating in detail source construction and dimensions, was used to investigate the dosimetric characteristics of the VariSource 192Ir high dose rate brachytherapy source. Dose-rate constant, radial dose functions, and anisotropy functions, utilized in the AAPM Task Group 43 dose estimation formalism, were calculated with the source centered in a spherical water phantom of 30 cm in diameter. The results, which are in agreement with the corresponding data available in the literature, are compared to those obtained in our previous study for the widely used microSelectron 192Ir high dose-rate brachytherapy source. The dose-rate constant was found to be equal to 1.043 +/- 0.005 cGy h(-1) U(-1) for the VariSource, compared to a value of 1.116 +/- 0.006 cGy h(-1) U(-1) calculated for the microSelectron. Significant differences in the anisotropy of the two sources are observed only for polar angles close to their long axis and are due to their different dimensions.     

Monte Carlo and experimental derivation of TG43 dosimetric parameters for CSM-type Cs-137 sources

In this study, complete dosimetric datasets for the CSM2 and CSM3 Cs-137 sources were obtained using the Monte Carlo GEANT4 code. The application of this calculation method was experimentally validated with thermoluminescent dosimetry (TLD). Functions and parameters following the TG43 formalism are presented: the dose rate constant, the radial dose functional, and the anisotropy function. In addition, to aid the quality control process on treatment planning systems, a two-dimensional (2D) rectangular dose rate table (the traditional along-away table), coherent with the TG43 dose calculation formalism, is given. The data given in this study complement existing information for both sources on the following aspects: (i) the source asymmetries were considered explicitly in the Monte Carlo calculations, (ii) TG43 data were derived directly from Monte Carlo calculations, (iii) the radial range of the different tables was increased as well as the angular resolution in the anisotropy function, including angles close to the longitudinal source axis. The CSM2 source TG-43 data of Liu et al. [Med. Phys. 31, 477-483 (2004)] are not consistent with the Williamson 2D along-away data [Int. J. Radiat. Oncol., Biol., Phys. 15, 227-237 (1988)] at distances closer than approximately 2 cm from the source. The data presented here for this source are consistent with this 2D along-away table, and are suitable for use in clinical practice.     

EchoSeed Model 6733 Iodine-125 brachytherapy source: improved dosimetric characterization using the MCNP5 Monte Carlo code

This study primarily aimed to obtain the dosimetric characteristics of the Model 6733 (125)I seed (EchoSeed) with improved precision and accuracy using a more up-to-date Monte-Carlo code and data (MCNP5) compared to previously published results, including an uncertainty analysis. Its secondary aim was to compare the results obtained using the MCNP5, MCNP4c2, and PTRAN codes for simulation of this low-energy photon-emitting source. The EchoSeed geometry and chemical compositions together with a published (125)I spectrum were used to perform dosimetric characterization of this source as per the updated AAPM TG-43 protocol. These simulations were performed in liquid water material in order to obtain the clinically applicable dosimetric parameters for this source model. Dose rate constants in liquid water, derived from MCNP4c2 and MCNP5 simulations, were found to be 0.993 cGyh(-1)?U(-1) (±1.73%) and 0.965 cGyh(-1)?U(-1) (±1.68%), respectively. Overall, the MCNP5 derived radial dose and 2D anisotropy functions results were generally closer to the measured data (within ±4%) than MCNP4c and the published data for PTRAN code (Version 7.43), while the opposite was seen for dose rate constant. The generally improved MCNP5 Monte Carlo simulation may be attributed to a more recent and accurate cross-section library. However, some of the data points in the results obtained from the above-mentioned Monte Carlo codes showed no statistically significant differences. Derived dosimetric characteristics in liquid water are provided for clinical applications of this source model.     

Monte Carlo dosimetric study of the BEBIG Co-60 HDR source

Although not as widespread as Ir-192, Co-60 is also available on afterloading equipment devoted to high dose rate brachytherapy, mainly addressed to the treatment of gynaecological lesions. The purpose of this study is to obtain the dosimetric parameters of the Co-60 source used by the BEBIG MultiSource remote afterloader (BEBIG GmbH, Germany) for which there are no dosimetric data available in the literature. The Monte Carlo code GEANT4 has been used to obtain the TG43 parameters and the 2D dose rate table in Cartesian coordinates of the BEBIG Co-60 HDR source. The dose rate constant, radial dose function and anisotropy function have been calculated and are presented in a tabular form as well as a detailed 2D dose rate table in Cartesian coordinates. These dosimetric datasets can be used as input data and to validate the treatment planning system calculations.     

Monte Carlo and experimental dosimetry of an 1251 brachytherapy seed

We have performed a comprehensive dosimetric characterization of the Oncura model 6711 125I seed using both experimental [LiF thermoluminscent dosimetry (TLD)] and theoretical (Monte Carlo photon transport) methods. In addition to determining the dosimetric parameters of the 6711, this report quantified: (1) the angular dependence of LiF TLD energy response functions for both point and volume detectors in water, poly(methylmethacrylate), and solid water media; and (2) the contribution of underlying geometric uncertainties to the overall uncertainty of Monte Carlo derived dosimetric parameters according to the National Institute of Standards and Technology Report 1297 methodology. The theoretical value for the dose rate constant in water was 0.942 cGy U(-1) h(-1)+/-1.76% [combined standard uncertainty (CSU) with coverage factor k=1] and the experimental value was 0.971 cGy U(-1) h(-1)+/-6.1%. Agreement between experimental and theoretical radial dose function values was well within the k= 1 CSU, while agreement between experimental and theoretical anisotropy function values was within the k= 1 CSU only after incorporating the use of polar angle-dependent energy response functions. The angular dependence of the relative energy response was found to have a complex and significant dependence on measurement medium and internal geometry of the source.     

Monte Carlo dosimetric study of best industries and Alpha Omega Ir-192 brachytherapy seeds

Ir-192 seeds are widely used in the USA for low dose rate interstitial brachytherapy. There are two commercially available models: those manufactured by Best Industries filtered with stainless steel, and those manufactured by Alpha-Omega seeds filtered with Pt. Newly developed 3D correction algorithms for brachytherapy are based on dosimetry data obtained on unbounded phantom size, allowing corrections for heterogeneities and actual tissue boundaries. Published dosimetric datasets for both seeds have been obtained under bounded conditions. The aim of the present study is to obtain dosimetric datasets for these seeds under full scatter conditions. The Monte Carlo GEANT4 code has been used to estimate air-kerma strength and dose rate in water around the Ir-192 seeds. Functions and parameters following the TG43 formalism are obtained and presented in tabular forms: the dose rate constant, the radial dose function, and the anisotropy function. Tables for the anisotropy factor have been obtained in order to apply punctual approximation. Differences between dose rate distributions for both seeds show that specific dataset must be used for each type of seed in clinical dosimetry. The data in the present study improve on published data in the following aspects: (i) dosimetric data were obtained under full scatter conditions, which affect dose values at distances greater than 4-5 cm from the source; (ii) the dose rate tables are given at greater distances from the source; and (iii) the spatial resolution in high dose gradient areas, such as those near the longitudinal source axis, has been improved.     

Monte Carlo-aided dosimetry of the new Bebig IsoSeed 103Pd interstitial brachytherapy seed.

A new model 103Pd interstitial brachytherapy source, the IsoSeed 103Pd, was recently introduced by Bebig Isotopentechnik und Umweltdiagnostik GmbH for permanent implant applications. This study presents the first quantitative theoretical study of the seed's dosimetric quantities. Monte Carlo photon transport (MCPT) simulation techniques have been used to evaluate the dose-rate distributions around the model IsoSeed 103Pd source in liquid water and air phantoms. These results have been used to calculate and tabulate the anisotropy function, F(r, theta), radial dose function, g(r), and anisotropy factors, phi(r), and dose-rate constant as defined by AAPM Task Group 43 (TG-43) Report. Cartesian "away" and "along" tables, giving the dose rates per unit air-kerma strength in water in the range 0.1-3 cm distance around the seed have also been tabulated. The dose-rate constant, lambda, was evaluated by simulating the wide-angle, free-air chamber (WAFAC) calibration geometry recently implemented by NIST (National Institute of Standards and Technology) to realize the primary standard of air-kerma strength (SK,N99) for low-energy photon-emitting brachytherapy sources. The dose-rate constant has been found to be lambda=0.660+/-0.017 in units of dose-rate per unit air-kerma strength (cGy x h(-1) x U(-1)).     

Monte Carlo-aided dosimetry of the theragenics TheraSeed model 200 103Pd interstitial brachytherapy seed

A dosimetric study of a 103Pd seed for permanent interstitial brachytherapy, the Theragenics Corporation Model 200 (TheraSeed), has been undertaken utilizing Monte Carlo photon-transport (MCPT) simulations. All dosimetric quantities recommended by the American Association of Physicists in Medicine (AAPM) Task Group 43 (TG-43) [Med. Phys. 22, 209-234 (1995)] report have been calculated. This source contains graphite pellets coated with palladium metal, within which the radioactive 103Pd is distributed. Due to the significant influence of this metal coating thickness on the dose distribution, two coating thicknesses, 2.2 microm (light seed, representing currently available seeds) and 10.5 microm (heavy seed, representing reactor-produced seeds available before 1994), were analyzed. Quantities determined are the following: dose rate constant, radial dose function, anisotropy function, anisotropy factor, anisotropy constant, and "along and away" dose tables. The National Institute of Standards and Technology (NIST) Wide Angle Free Air Chamber (WAFAC) standard for air-kerma strength (SK,N99) was simulated, allowing a comparison to measured dosimetry data normalized to SKN99. The calculated dose-rate constants are 0.691 (light seed) and 0.694 cGy h(-1) U(-1) (heavy seed), where 1 U= 1 microGy x m2 x h(-1), in contrast to the recommended TG-43 value of 0.74 cGy h(-1) U(-1) and the value, 0.665 cGy h(-1) U(-1), recommended by AAPM report 69 [Med. Phys. 27, 634-642 (2000)]. Anisotropy constants (1/r2 weighted average, r > or = 1 cm) are 0.862 and 0.884 for the light seed and heavy seed, respectively. A generalization of the AAPM formalism [Med. Phys. 27, 634-642] for evaluating the time-dependent ratio of an administered-to-prescribed dose is presented. The findings of this study, in combination with 5% corrections applied to WAFAC measurements performed in 1999, imply that changes in the AAPM's recommended ratios as large as 6%, are indicated.     

A Monte Carlo calculation of dosimetric parameters of 90Sr/90Y and 192Ir SS sources for intravascular brachytherapy

The AAPM TG-60 report has proposed various dose calculation parameters for intravascular brachytherapy (IVBT). These parameters include the dose rate constant (or the dose rate at a reference position for a beta-particle emitting source), the radial dose function, and the anisotropy function. In this work, we have used a modified EGS4 Monte Carlo system to calculate these parameters for the two most commonly used IVBT sources (the beta-particle emitting 90Sr/90Y source and the photon emitting 192Ir SS source). To ensure the calculation accuracy, the present calculation was compared with several measurements and calculations reported by other authors. Excellent agreement was found for the results with the photon source. For beta-particle source calculation, the present results for a variety of point sources agree very well with a previous work. The presently calculated radial dose functions for the 90Sr/90Y source are consistent with those of a published work for intermediate radial distances. The dose uniformity in the axial direction was also studied. The contributions of bremsstrahlung photons to total doses for the 90Sr/90Y beta source, and the influence of ignoring electron transport on calculated doses for the 192Ir SS photon source are discussed.     

Brachytherapy dosimetry parameters calculated for a new 103Pd source

A new brachytherapy source having 103Pd adsorbed onto silver beads has been designed. The dose distributions of this source have been characterized using version 5 of the MCNP Monte Carlo radiation transport code available from Oak Ridge National Laboratory. These results are presented in terms of the updated AAPM Task Group No. 43 (TG-43U1) formalism, dosimetry parameters, and recommended calculation methodology.