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.     

Monte Carlo dose calculations in homogeneous media and at interfaces: a comparison between GEPTS,EGSnrc, MCNP,and measurements

Three Monte Carlo photon/electron transport codes (GEPTS, EGSnrc, and MCNP) are bench-marked against dose measurements in homogeneous (both low- and high-Z) media as well as at interfaces. A brief overview on physical models used by each code for photon and electron (positron) transport is given. Absolute calorimetric dose measurements for 0.5 and 1 MeV electron beams incident on homogeneous and multilayer media are compared with the predictions of the three codes. Comparison with dose measurements in two-layer media exposed to a 60Co gamma source is also performed. In addition, comparisons between the codes (including the EGS4 code) are done for (a) 0.05 to 10 MeV electron beams and positron point sources in lead, (b) high-energy photons (10 and 20 MeV) irradiating a multilayer phantom (water/steel/air), and (c) simulation of a 90Sr/90Y brachytherapy source. A good agreement is observed between the calorimetric electron dose measurements and predictions of GEPTS and EGSnrc in both homogeneous and multilayer media. MCNP outputs are found to be dependent on the energy-indexing method (Default/ITS style). This dependence is significant in homogeneous media as well as at interfaces. MCNP(ITS) fits more closely the experimental data than MCNP(DEF), except for the case of Be. At low energy (0.05 and 0.1 MeV), MCNP(ITS) dose distributions in lead show higher maximums in comparison with GEPTS and EGSnrc. EGS4 produces too penetrating electron-dose distributions in high-Z media, especially at low energy (<0.1 MeV). For positrons, differences between GEPTS and EGSnrc are observed in lead because GEPTS distinguishes positrons from electrons for both elastic multiple scattering and bremsstrahlung emission models. For the 60Co source, a quite good agreement between calculations and measurements is observed with regards to the experimental uncertainty. For the other cases (10 and 20 MeV photon sources and the 90Sr/90Y beta source), a good agreement is found between the three codes. In conclusion, differences between GEPTS and EGSnrc results are found to be very small for almost all media and energies studied. MCNP results depend significantly on the electron energy-indexing method.     

IVBTMC,a Monte Carlo dose calculation tool for intravascular brachytherapy

A new Monte Carlo code (IVBTMC) is developed for accurate dose calculations in intravascular brachytherapy (IVBT). IVBTMC calculates the dose distribution of a brachytherapy source with arbitrary size and curvature in a general three-dimensional heterogeneous medium. Both beta and gamma sources are considered. IVBTMC is based on a modified version of the EGSNRC code. A voxel-based geometry is used to describe the target medium incorporating heterogeneities with arbitrary composition and shape. The source term is modeled using appropriate phase-space data. The phase-space data are calculated for three widely used sources (32P, 90Sr/90Y, and 192Ir). To speed up dose calculations for gamma sources, a special version of IVBTMC based on the kerma approximation is developed. The accuracy of the phase-space data model is verified and IVBTMC is validated against other Monte Carlo codes and against reported measurements using radio-chromic films. To illustrate the IVBTMC capabilities, a variety of examples are treated. 32P, 90Sr/90Y, and 192Ir sources with different lengths and degrees of curvature are considered. Calcified plaques with regular and irregular shapes are modeled. The dose distributions are calculated with a spatial resolution ranging between 0.1 and 0.5 mm. They are presented in terms of isodose contour plots. The dosimetric effects of the source curvature and/or the presence of calcified plaques are discussed. In conclusion, IVBTMC has the capability to perform high-precision IVBT dose calculations taking into account the realistic configurations of both the source and the target medium.     

Monte Carlo dose voxel kernel calculations of beta-emitting and Auger-emitting radionuclides for internal dosimetry: A comparison between EGSnrcMP and EGS4

Dose-point kernels (DPKs) can be widely applied to therapeutic nuclear medicine to obtain more accurate absorbed dose assessments in internal dosimetry assuming a spherical geometry. Recently, EGSnrc-the latest in the family of EGS Monte Carlo codes--has been tested for isotropic monoenergetic electrons and Y-90 beta spectrum in spherical geometry. The availability of SPECT images allows one to take into account heterogeneities in activity distribution within tumors, and to perform dose calculations using voxel dosimetry based on Monte Carlo simulations in a Cartesian geometry. The purpose of this study is to evaluate the differences of dose distributions scored in Cartesian voxels also known as Dose Voxel Kernels (DVKs) for five beta-emitting (131I, 89Sr, 153Sm, 186Re, and 90Y) and Auger-emitting (111In) radionuclides, when their computation is made using these two Monte Carlo codes from the same family to check if the new physics in EGSnrc simulation system produces DVK very different from those calculated with EGS4. We have calculated the DVKs for point and voxel sources in Cartesian scoring grids of different spatial resolutions. Our results for the point source, scored in the finer spatial resolution, show a poor agreement between EGSnrc and EGS4 (up to about 20%) for voxels closer to the origin, and a better agreement (below 5%) for longer distances for all radionuclides. For the voxel source, where doses were scored in the coarser spatial resolution, dose deposition in the central voxel is in good agreement for all the radionuclides; while surrounding voxels exhibit a slightly worse agreement.     

Monte Carlo dose calculations using MCNP4C and EGSnrc/BEAMnrc codes to study the energy dependence of the radiochromic film response to beta-emitting sources

The energy dependence of the radiochromic film (RCF) response to beta-emitting sources was studied by dose theoretical calculations, employing the MCNP4C and EGSnrc/BEAMnrc Monte Carlo codes. Irradiations with virtual monochromatic electron sources, electron and photon clinical beams, a (32)P intravascular brachytherapy (IVB) source and other beta-emitting radioisotopes ((188)Re, (90)Y, (90)Sr/(90)Y,(32)P) were simulated. The MD-55-2 and HS radiochromic films (RCFs) were considered, in a planar or cylindrical irradiation geometry, with water or polystyrene as the surrounding medium. For virtual monochromatic sources, a monotonic decrease with energy of the dose absorbed to the film, with respect to that absorbed to the surrounding medium, was evidenced. Considering the IVB (32)P source and the MD-55-2 in a cylindrical geometry, the calibration with a 6 MeV electron beam would yield dose underestimations from 14 to 23%, increasing the source-to-film radial distance from 1 to 6 mm. For the planar beta-emitting sources in water, calibrations with photon or electron clinical beams would yield dose underestimations between 5 and 12%. Calibrating the RCF with (90)Sr/(90)Y, the MD-55-2 would yield dose underestimations between 3 and 5% for (32)P and discrepancies within +/-2% for (188)Re and (90)Y, whereas for the HS the dose underestimation would reach 4% with (188)Re and 6% with (32)P.     

Monte Carlo modeling of the response of NRC's 90Sr/90Y primary beta standard

The BEAMnrc/EGSnrc Monte Carlo code system is employed to develop a model of the National Research Council of Canada primary standard of absorbed dose to tissue in a beta radiation field, comprising an extrapolation chamber and 90Sr/90Y beta source. We benchmark the model against the measured response of the chamber in terms of absorbed dose to air, for three different experimental setups when irradiated by the 90Sr/90Y source. For the first setup, the chamber cavity depth is fixed at 0.2 cm and the source-to-chamber distance varied between 11 and 60 cm. In the other two cases, the source-to-chamber distance is fixed at 30 cm. In one case the response for different chamber depths is studied, while in the other case the chamber depth is fixed at 0.2 cm as different thicknesses of Mylar are added to the front surface of the extrapolation chamber. The agreement as a function of distance between the calculated and measured responses is within 0.37% for a variation in response of a factor of 29. In the case of dose versus chamber depth, the agreement is within 0.4% for the ISO-recommended nominal depths of 0.025-0.25 cm. Agreement between calculated and measured responses is very good (between 0.02% and 0.2%) for added Mylar foils of thicknesses up to 10.8 mg cm(-2). For larger Mylar thicknesses, deviations of 0.6%-1.2% are observed, which are possibly due to the systematic uncertainties associated with the restricted collisional stopping powers of air or Mylar used in the calculations. We conclude that our simulation model represents the extrapolation chamber and 90Sr/90Y source with adequate accuracy to calculate correction factors for accurate realization of dose rate to tissue at a depth of 7 mg cm(-2) in an ICRU tissue phantom, despite the fact that the uncertainties in the physical characteristics of the source leave some uncertainty in certain calculated quantities.     

Characterization of a high-dose-rate 90Sr-90Y source for intravascular brachytherapy by using the Monte Carlo code PENELOPE

Radiation treatment with catheter-based beta-emitter sources is currently under clinical trial to prevent restenosis. In the present paper, we address the characterization of the high-dose-rate 90Sr-90Y seeds of the Beta-Cath system supplied by Novoste Corporation, one of the commercially available sources for intravascular brachytherapy. The Monte Carlo code PENELOPE has been used to simulate the transport of electrons emitted by the encapsulated 90Sr-90Y seeds. The calculated radial dose function and anisotropy function for a single seed in water are compared with simulation results from other authors. Regarding g(r), the present result lies between the ITS3 and EGS4 curves, being somewhat closer to ITS3, while in the case of F(r, theta) some differences appear for certain angular intervals and radial distances. In order to put the observed differences into perspective, we have calculated radial doses for point isotropic sources in water. Our results for 0.5 and 1 MeV electrons are in good agreement with simulations using EGSnrc, and an excellent agreement is obtained with ITS for point 90Sr-90Y emitters. Dose distributions in water are calculated for source 'trains' consisting of 1, 2, 3, 4, 5, 9 and 12 seeds. The dose at the source midplane is enhanced if the number of seeds is up to 4, and saturates for trains with 5 or more seeds. We also compare the dose distribution obtained by simply adding the contributions of individual seeds with the simulation of the complete source train. It is found that both calculation procedures yield essentially the same result for distances greater than about 2 mm. Finally, the contribution of bremsstrahlung photons to the dose is briefly analysed.     

The radial dose function of low-energy brachytherapy seeds in different solid phantoms: comparison between calculations with the EGSnrc and MCNP4C Monte Carlo codes and measurements

The use of low-energy photon emitters for brachytherapy applications, as in the treatment of the prostate or of eye tumours, has drastically increased in the last few years. New seed models for 103Pd and 125I have recently been introduced. The American Association of Physicists in Medicine recommends that measurements are made to obtain the dose rate constant, the radial dose function and the anisotropy function. These results must then be compared with Monte Carlo calculations to finally obtain the dosimetric parameters in liquid water. We have used the results obtained during the characterization of the new InterSource (furnished by IBt, Seneffe, Belgium) palladium and iodine sources to compare two Monte Carlo codes against experiment for these low energies. The measurements have been performed in three different media: two solid water plastics, WT1 and RW1, and polymethylmetacrylate. The Monte Carlo calculations were made using two different codes: MCNP4C and EGSnrc. These codes use photon cross-section data of a different origin. Differences were observed between both sets of input data below 100 keV, especially for the photoelectric effect. We obtained differences in the radial dose functions calculated with each code, which can be explained by the difference between the input data. New cross-section data were then tested for both codes. The agreement between the calculations using these new libraries is excellent. The differences are within the statistical uncertainties of the calculations. These results were compared with the experimental data. A good agreement is reached for both isotopes and in the three phantoms when the measured values are corrected for the presence of the TLDs in the phantom.     

Application of imaging-derived parameters to dosimetry of intravascular brachytherapy sources: perturbation effects of residual plaque burden

The dosimetric effect of geometric and material heterogeneities on intravascular brachytherapy dose delivery has been studied recently. Residual plaque within the coronary vessel appears to have an impact on the uniform delivery of radiation dose to the arterial tissue. In this study, we have examined the effect of residual plaque burden and post-PCI (percutaneous coronary intervention) plaque configuration on the dose to the arterial wall from clinical intravascular brachytherapy beta-emitting sources containing 32P and 90Sr/90Y. Monte Carlo simulations using the MCNP4B code were performed for these catheter-based sources with residual plaque burden ranging between 25% and 50%. The residual plaque burden values were derived from post-PCI data provided in several recent clinical studies. Dose calculations were performed for three different values of plaque density (1.45 g cm(-3), 2.20 g cm(-3), and 3.1 g cm(-3)) and three different plaque morphologies for the same residual plaque burden. The dose perturbation factor (DPF), defined as the ratio of dose at 2 mm radial distance for a given case to the dose at the same radial distance in homogeneous water medium, was determined for each of the three different plaque densities. The range of DPF values was 0.81-1.01, 0.62-0.99, and 0.41-0.97 for different plaque densities for the 32P source. Corresponding DPF values for the 90Sr/90Y source were 0.90-1.01, 0.84-1.01, and 0.62-1.01. The results indicate the need for accurate assessment of post-PCI clinical measurements such as minimal lumen diameter and residual plaque burden and incorporation of these values into dose calculations.     

Calculation of beta-ray dose distributions from ophthalmic applicators and comparison with measurements in a model eye

Dose distributions throughout the eye, from three types of beta-ray ophthalmic applicators, were calculated using the EGS4, ACCEPT 3.0, and other Monte Carlo codes. The applicators were those for which doses were measured in a recent international intercomparison [Med. Phys. 28, 1373 (2001)], planar applicators of 106Ru-106Rh and 90Sr-90Y and a concave 106Ru-106Rh applicator. The main purpose was to compare the results of the various codes with average experimental values. For the planar applicators, calculated and measured doses on the source axis agreed within the experimental errors (<10%) to a depth of 7 mm for 106Ru-106Rh and 5 mm for 90Sr-90Y. At greater distances the measured values are larger than those calculated. For the concave 106Ru-106Rh applicator, there was poor agreement among available calculations and only those calculated by ACCEPT 3.0 agreed with measured values. In the past, attempts have been made to derive such dose distributions simply, by integrating the appropriate point-source dose function over the source. Here, we investigated the accuracy of this procedure for encapsulated sources, by comparing such results with values calculated by Monte Carlo. An attempt was made to allow for the effects of the silver source window but no corrections were made for scattering from the source backing. In these circumstances, at 6 mm depth, the difference in the results of the two calculations was 14%-18% for a planar 106Ru-l06Rh applicator and up to 30% for the concave applicator. It becomes worse at greater depths. These errors are probably caused mainly by differences between the spectrum of beta particles transmitted by the silver window and those transmitted by a thickness of water having the same attenuation properties.     

Monte Carlo calculations of dose distributions around 32P and 198Au stents for intravascular brachytherapy.

3D dose distributions are calculated for a 32P impregnated stent and a 198Au stent for intravascular brachytherapy with the EGS4 Monte Carlo simulation code. The stents were modeled as a combination of eight helicoidal struts. This allowed investigation of the effect of the stent geometry and the electron absorption in the strut material on the dose distributions. Absorbed dose to water was calculated at radial distances ranging from 50 microm to 5 mm from the stent surface. The dose distributions around the stents are compared to the dose distribution around an intravascular brachy-therapy 192Ir source, also calculated with the EGS4 Monte Carlo code. The dose profiles near the struts show hot spots. At 50 microm distance a peak to valley ratio of 3 for 32P and 6 for 198Au in the dose distribution is obtained. For both the isotopes the inhomogeneities decrease with distance and at a radial depth of 350 microm the effect becomes negligible. The calculations showed the importance of the effect of the absorption in the stent material as this leads to a dose decrease to 67% for the 198Au stent and to 77% for 32P near the stent at a distance of 2 mm from the stent axis. It is concluded that from the dosimetric point of view, the 198Au stent is inferior to the 32P stent and the 192Ir source. Application of the 198Au stent in clinical practice requires further investigation of the importance of the adventitia in the restenosis process, and the tolerance dose of the intima.     

Dosimetric characteristics of a linear array of gamma or beta-emitting seeds in intravascular irradiation: Monte Carlo studies for the AAPM TG-43/60 formalism

In principle, the AAPM TG-43/60 formalism for intravascular brachytherapy (IVBT) dosimetry of catheter-based sources is fully valid with a single seed of cylindrical symmetry and in the region comparable to or larger than the mean-free path of emitting radiation. However, for the geometry of a linear array of seeds within the few millimeter range of interest in IVBT, the suitability of the AAPM TG-43/60 formalism has not been fully addressed yet. We have meticulously investigated the dosimetric characteristics of catheter-based gamma (192Ir) and beta (90Sr/Y) sources using Monte Carlo methods before applying the AAPM TG-43/60 formalism. The dosimetric perturbation due to radiation interactions with neighboring seeds is at most 2% over the entire region of interest for the 192Ir source, while it increases to about 5% for the 90Sr/Y source. As the transaxial distance (y) increases beyond 3 mm, the sum of the dose contributions from neighboring seeds exceeds the dose contribution from the center seed for both sources. However, it continues to increase with the increasing y for 192Ir but is saturated beyond y = 5 mm for 9Sr/Y. Even within a few millimeters from the seeds, the dose from the low-energy betas of 192Ir is still less than 1% of the total dose. The radial dose and anisotropy functions are reformulated in reduced cylindrical coordinate with the reference point at y = 2 mm. The dose rate constant of 192Ir and the dose rate of 90Sr/Y at the reference point showed a fairly good agreement (within +/- 2%) with earlier studies and the NIST-traceable value, respectively. We conclude that the dosimetric perturbation caused by close proximity of neighboring seeds is nearly negligible so that the AAPM TG-43/60 formalism can be applied to a linear array of seeds.     

Validation of GEANT4, an object-oriented Monte Carlo toolkit, for simulations in medical physics

GEANT4 (GEometry ANd Tracking 4) is an object-oriented Monte Carlo simulation toolkit that has been developed by a worldwide collaboration of scientists. It simulates the passage of particles through matter. In order to validate GEANT4 for medical physics applications, different simulations are conducted. The results are compared to published results based on three Monte Carlo codes widely used in medical physics: MCNP, EGS4, and EGSnrc. When possible, the simulation results are also compared to experimental data. Different geometries are tested (multilayer and homogeneous phantoms), different sources considered (point-source and broad parallel beam), and different primary particles simulated (photons and electrons) at different energies. For the heterogeneous media, there are notable differences between the Monte Carlo codes reaching up to over 5% in relative difference. For the monoenergetic electrons in a homogeneous medium, the difference between GEANT4 and the experimental measurements is similar to the difference between EGSnrc and the experimental measurements; for the depth-dose curves, the difference expressed as a fraction of the peak dose is always smaller than 4%. We conclude that GEANT4 is a promising Monte Carlo simulation toolkit for low-energy medical applications.     

CSnrc: correlated sampling Monte Carlo calculations using EGSnrc

CSnrc, a new user-code for the EGSnrc Monte Carlo system is described. This user-code improves the efficiency when calculating ratios of doses from similar geometries. It uses a correlated sampling variance reduction technique. CSnrc is developed from an existing EGSnrc user-code CAVRZnrc and improves upon the correlated sampling algorithm used in an earlier version of the code written for the EGS4 Monte Carlo system. Improvements over the EGS4 version of the algorithm avoid repetition of sections of particle tracks. The new code includes a rectangular phantom geometry not available in other EGSnrc cylindrical codes. Comparison to CAVRZnrc shows gains in efficiency of up to a factor of 64 for a variety of test geometries when computing the ratio of doses to the cavity for two geometries. CSnrc is well suited to in-phantom calculations and is used to calculate the central electrode correction factor Pcel in high-energy photon and electron beams. Current dosimetry protocols base the value of Pcel on earlier Monte Carlo calculations. The current CSnrc calculations achieve 0.02% statistical uncertainties on Pcel, much lower than those previously published. The current values of Pcel compare well with the values used in dosimetry protocols for photon beams. For electrons beams, CSnrc calculations are reported at the reference depth used in recent protocols and show up to a 0.2% correction for a graphite electrode, a correction currently ignored by dosimetry protocols. The calculations show that for a 1 mm diameter aluminum central electrode, the correction factor differs somewhat from the values used in both the IAEA TRS-398 code of practice and the AAPM's TG-51 protocol.     

Comparison of EGS4 and MCNP4b Monte Carlo codes for generation of photon phase space distributions for a Varian 2100C

Monte Carlo based dose calculation algorithms require input data or distributions describing the phase space of the photons and secondary electrons prior to the patient-dependent part of the beam-line geometry. The accuracy of the treatment plan itself is dependent upon the accuracy of this distribution. The purpose of this work is to compare phase space distributions (PSDs) generated with the MCNP4b and EGS4 Monte Carlo codes for the 6 and 18 MV photon modes of the Varian 2100C and determine if differences relevant to Monte Carlo based patient dose calculations exist. Calculations are performed with the same energy transport cut-off values. At 6 MV, target bremsstrahlung production for MCNP4b is approximately 10% less than for EGS4, while at 18 MV the difference is about 5%. These differences are due to the different bremsstrahlung cross sections used in the codes. Although the absolute bremsstrahlung production differs between MCNP4b and EGS4, normalized PSDs agree at the end of the patient-independent geometry (prior to the jaws), resulting in similar dose distributions in a homogeneous phantom. EGS4 and MCNP4b are equally suitable for the generation of PSDs for Monte Carlo based dose computations.     

Dosimetry characterization of a 32P source wire used for intravascular brachytherapy with automated stepping

Depth-dose curve measurements and Monte Carlo simulations for a catheter-based 32P intravascular brachytherapy source wire are described. The measured dose rates were obtained using both radiochromic-dye film and an extrapolation chamber (EC). Calibrated radiochromic-dye films were irradiated at distances between 0.5 and 5 mm from the source axis in polystyrene phantoms, and scanned with high-resolution densitometers. Measurements with an automated EC with a 1 mm diameter collecting electrode were also performed at a distance of 2 mm from the source in polystyrene. The measured dose rates obtained from the film and EC were divided by the measured source activity to obtain measured values of dose rate per unit contained activity. Dosimetric calculations of the catheter-based 32P wire geometry were also obtained using several Monte Carlo codes (CYLTRAN, MCNP, PENELOPE, and EGS). The measured and calculated values of dose rate per unit contained activity are in good agreement (<10%) within the relevant treatment distances (1 to 4 mm). With carefully selected input parameters, the calculated depth-dose curves using these codes were within 5% at 4 mm depth. At greater depths the discrepancies between the codes increase. We discuss likely mechanisms for these differences.     

The dose distribution surrounding 192Ir and 137Cs seed sources

Dose distributions in water have been measured using LiF thermoluminescent dosimeters for 192Ir seed sources with stainless steel and with platinum encapsulation to determine the effect of differing encapsulation. The dose distribution has also been measured for a 137Cs seed source. In addition, dose distributions surrounding these sources have been calculated using the EGS4 Monte Carlo code and have been compared to the measured data. The two methods are in good agreement for all three sources. Tables are given which describe the dose distribution surrounding each source as a function of distance and angle. In addition, specific dose constants have been determined from results of Monte Carlo simulation. This work has confirmed the utility of the EGS4 Monte Carlo code in modelling 192Ir and 137Cs seed sources to obtain brachytherapy dose distributions.