125I Brachytherapy Sources Research Articles

Calculation of photon scattering and transmission correction factors for a free air ionization chamber at Nuclear Science and Technology Research Institute in Iran

In primary standard dosimetry laboratories, the air kerma strength of low energy brachytherapy sources 125I and 103Pd is measured by standard free air ionization chambers. At present, one of these free air ionization chambers is constructed in Radiation Application Research School, in Nuclear Science and Technology Research Institute in Iran. The transmission and scattering correction factors are needed for the absolute determination of air kerma strength.Transmission and scattering correction factors were calculated for a monoenergetic point isotropic source with energies from 5keV to 100keV for ionization chamber with 4cm and 6cm aperture radii by using the MCNP 4C code. The calculated scattering correction factors for 125I and 103Pd brachytherapy sources are 0.9987 and 0.9992, respectively and determined transmission correction is one for our free air ionization chamber of aperture radius 4cm.

Dosimetric characteristics of three new design 125I brachytherapy sources

Abstract For roughly 25 years, 125I sources have been used in the treatment of various malignant diseases such as prostate cancer. Three new brachytherapy sources, IR01-125I, IR02-125I and IR03-125I, have been developed and are designed for permanent implant application. The Monte Carlo radiation transport code version MCNP 5 was used to calculate the dosimetry parameters around the sources in accordance with the updated report of the American Association of Physicists in Medicine (AAPM), Task Group No. 43. For each source, the dose rate constant Λ, the radial dose function gL(r), and the anisotropy function F(r, θ), were obtained. The results indicated a dose rate constant of 0.932 ± 0.01, 0.934 ± 0.01 and 0.939 ± 0.01 Gy h−1 U−1 for the IR01-125I, IR02-125I and IR03-125I sources respectively. With the goal of determining an optimal design for a 125I source, each seed's parameters were compared with other seeds. In this study, the optimal source IR03-125I provides the most isotropic dose distribution in water. Finally, the results for optimal source were compared with published results for those of other commercial sources.

Studies on the adsorption of 125I on bromide coated silver rods for the preparation of 125I-seeds for brachytherapy applications

This paper describes a method for the preparation of 125I brachytherapy sources. The process consist of surface oxidation of a silver rods [0.5 mm (ϕ) × 3 mm (1)], formation of a thin silver bromide over layer and finally replacement of bromide ions by 125I− ions. The effect of various parameters such as 125I feed activity, iodide carrier concentration and reaction time were examined and optimized for maximum impregnation of 125I activity on the substrate. Sources containing 74–111 MBq (2–3 mCi) 125I on a silver matrix could be prepared and encapsulated in titanium capsules [0.8 mm (ϕ) × 4.75 mm (1)] using Nd: YAG laser. Quality assurance tests to ensure nonleachability, uniform distribution of activity and leak tightness that are necessary before application were evaluated.

SU‐GG‐T‐438: Dose to Medium or Dose to a Water Cavity Embedded in Medium? A Monte Carlo Study

Purpose: The target for radiotherapy is to sterilize cells by imposing damage to their DNA content. It is therefore of interest to study the relations between the dose calculated to a tissue medium, Dm,m, with an representative average atomic composition versus the dose Dn,m specifically absorbed by the cell nuclei, and the dose Dw,m to a cell nuclei surrogate represented as a water cavity embedded into tissue. The simulations were performed for six types of tissues and three different brachytherapy sources. Methods and Materials: Absorbed dose calculations were performed by GEANT4 MC code version 9.2 using the Penelope physics package. Three different sources, 192Ir to represent high energy conditions, 169Yb for intermediate energies and a low energy brachytherapy source 125I, were simulated. The photon spectra used in this study were taken from http://www.physics.carleton.ca/clrp/seed_database. The 192Ir spectra was from Nucletron, microSelectron‐HDR v2 and for 125I from Nucletron, SelectSeed, 130.002. Particle spectra for 192Ir and energy spectra for 192I were also scored to calculate the dose with different cavity theories and compare it with MC calculated doses. Results: The MC calculated Dm,m, Dw,m and Dn,m ratio shows the largest value for 125I brachytherapy source and the cortical bone material, adipose tissue and prostate tissues(ICRU). The difference between Dw,m and Dn,m is also largest for the cortical bone and 125I source. For the 192Ir source, the ratio was 1 for the soft tissues while for the cortical bone it was higher. Conclusion: Monte Carlo calculations made it possible to report the dose as absorbed dose to medium. Accurate reporting of treatment planning requires a stringent standard for dose definition. MC calculations can be used to develop conversion factors to convert dose according to different definitions for different brachytherapy sources.

Influence of Photon Energy Spectra from 192Ir and 125I Brachytherapy Sources on Kerma and Dose Rates in Water and Air

Purpose: Several different 192Ir photon energy spectra have been used for both Monte Carlo (MC) simulations of dose deposition and for MC and measured media-energy corrections. However, the dosimetric influence of spectrum choice was not known, and recommendations for the field of medical physics to use a particular 192Ir spectrum are outdated. Furthermore, dosimetric differences between the 125I photon spectrum recommended in the 2004 AAPM TG-43U1 report and that recommended by the National Nuclear Data Center (NNDC) are unknown.

A comprehensive dosimetric comparison between 131Cs and 125I brachytherapy sources for COMS eye plaque implant

Purpose To verify the dosimetric characteristics of 131Cs source in the Collaborative Ocular Melanoma Study (COMS) eye plaque brachytherapy, to compare 131Cs with 125I in a sample implant, and to examine the accuracy of treatment planning system in dose calculation. Methods and Materials Monte Carlo (MC) technique was used to generate three-dimensional dose distributions of a 16-mm COMS eye plaque loaded with 131Cs and 125I brachytherapy sources separately. A spherical eyeball, 24.6 mm in diameter, and an ellipsoidal tumor, 6 mm in height and 12 mm in diameter, were used to evaluate the doses delivered. The simulations were carried out both with and without the gold and gold alloy plaque. A water-equivalent seed carrier was used instead of the silastic insert designed for the traditional COMS eye plaque. The 13 sources involved were also individually simulated to evaluate the intersource effect. In addition, a treatment planning system was used to calculate the doses at the central axis for comparison with MC data. Results The gold plaque had significantly reduced the dose in the tumor volume; at the prescription point of this study, that is, 6 mm from the edge of inner sclera, the gold plaque reduced the dose by about 7% for both types of 131Cs and 125I sources, but the gold alloy plaque reduced the dose only by 4% for both types of sources. The intersource effect reduced the dose by 2% for both types of sources. At the same prescription dose, the treatment with the gold plaque applicator tended to create more hot regions for either type of sources than were seen with the homogeneous water phantom. The doses of TPS agree with the MC. Conclusion The 131Cs source is comparable to the 125I source in the eye plaque brachytherapy. The TPS can provide accurate dose calculations for eye plaque implants with either type of sources.

Preliminary Research of Tissue Heterogeneity Correction for Dose Distribution of 125I Brachytherapy Source

Tissue heterogeneity is commonly observed in clinical brachytherapy source implants. In some cases, an obvious variance in dose distribution will be caused by application of the AAPM recommended formulas, which are only valid for a homogeneous medium. By taking soft tissue as the base and considering adipose, air and bone as the separate interlayers, an evaluation model is established and dose correction formulas are derived for a single 125I source in this study. The applicability of correction formulas is validated at different interlayer thicknesses and angles by Monte Carlo (MCNP 4C) simulations. The results show fairly good agreement between correction formulas and MC simulations with overall maximum variances of 1.62 ± 0.06, 5.93 ± 0.23, and 8.41 ± 0.33% for adipose, air, and bone, respectively.

Preliminary Research of Tissue Heterogeneity Correction for Dose Distribution of 125I Brachytherapy Source

Preliminary Research of Tissue Heterogeneity Correction for Dose Distribution of 125I Brachytherapy Source

SU‐GG‐T‐03: Dosimetric Study of HDR 192Ir and LDR 125I Brachytherapy Sources Using the Penelope Monte Carlo Code

Purpose: To determine the dosimetric parameters of two HDR models 192Ir (microSelectron, and M‐19), and LDR 125I brachytherapy sources. Method and Materials: In this study we used the microSelectron and M‐19 models, and also source (Amersham OncoSeed model 6711). Penelope Monte Carlo transport code was used. All the photonic interactions were taking into account. AAPM TG‐43 was followed to determine the dosimetric properties1. 2D dose distribution, dose rate constant, anisotropy and radial dose functions were determined. Results: For both Iridium models, the 2D‐dose distribution agrees very well with published data for radius 1 ⩽ r ⩽ 5 cm, the difference was less than 3 %. The dose rate constant is 1.11654 cGy h−1 U−1 for microSelectron and 1.12456 cGy h−1 U−1 M‐19 model. Less than 3 % difference was observed for the radial dose function for radius 1 < r < 5 cm for both Iridium models. The DRC for was found to be 0.984 cGy h−1 U−1, and less than 3 % difference for the radial dose function in comparison with published data in TG‐43. Conclusion: Extensive work had been done on and brachytherapy sources. Daskalov et al2, Medich et al3, and Granero et al4, used different source models and different Monte Carlo transport codes, but the difference for the dosimetric parameters between all these studies was less than 5 %. In this study we used Penelope Monte Carlo code to determine the dosimetric parameters. The results showed 3 % difference for 2D dose distribution, DRC, and radial dose function for Iridium sources and 3 % difference for Iodine source.

SU‐FF‐T‐441: Updated Solid Water™ to Water Conversion Factors for 125I and 103Pd Brachytherapy Sources

Purpose: The updated TG‐43U1 recommends determination of the consensus of low energy brachytherapy source parameters using experimental and Monte Carlo Simulated data. As per this recommendation, the measured parameters in Solid Water™ are converted into water data for clinical application. However, the published conversion factors may have been obtained using Monte Carlo simulations with a cross section files that may have errors for low energy photons, and also for inaccurate chemical composition of the phantom material. In this project impact of different calcium composition in Solid Water™ phantom on dose rate constant, radial dose function of 103Pd and 125I brachytherapy sources have been investigated. Method and Materials: Conversion factor of Solid water™ to water has been determined using MCNP5 Monte Carlo code. In these simulations two different calcium compositions in Solid Water™ of 2.3% (SWa) and 1.7% (SWb) were utilized. Monte Carlo simulations were performed for 125I sources, with and without silver marker, in order to investigate the impact of characteristic x‐rays (emitted by silver marker) on dosimetric parameters. However, the simulation for 103Pd was performed only on one type of the source. Each simulation was run for a total of 20M histories to keep statistical fluctuation less than 0.3%. Results: For 2.3% calcium content phantom material the calculated conversion factor was found to be 1.027 and 1.045 for 125I and 103Pd sources respectively. However, for 1.7% calcium these values were found to be 0.989 and 0.963, respectively. Conclusion: The Solid water‐to‐water conversion factors for 125I and 103Pd sources are found to be 0.989 and 0.963 respectively as compared to the published value of 1.05 and 1.048. In addition, presence of silver marker had no significant impact on conversion factor for I‐125 sources. Moreover, conversion for g(r) was also introduced for clinical application of measured data.

SU‐FF‐T‐110: Benchmarking MCNP Low‐Energy Bremsstrahlung Modeling for Electronic Brachytherapy Simulations

Purpose: Electronic brachytherapy (eBx) sources have been used clinically for over a decade; however, dosimetric characterization methods using measurements or calculations are not well‐established. Monte Carlo methods for simulating electron transport, and subsequently photon production, have not been benchmarked to the same degree as for photon‐emitting HDR 192Ir or LDR 125I brachytherapy sources. Materials & Methods: Towards better understanding the capabilities of MCNP5 to simulate radiation transport for the Xoft Axxent eBx source, this study presents a comparison of calculated MCNP5 results obtained using coupled electron:photon transport with measured bremsstrahlung spectra from the literature. Given the electron energy and target material, MCNP5 bremsstrahlung modeling accounts for photon energy, angle, and probability based on the cross‐sections and angular distributions from NIST (Seltzer and Berger, 1985). The Axxent eBx source currently operates at 50 kV with electrons bombarding a ∼ 1 μm thick high/low Z target. Pertinent high/low Z comparisons for thin targets, defined as materials thin enough to produce negligible electron absorption in the target, were available from Motz and Placious (1958) using 50 kV on 5 nm Au and 63 nm Al, from Cosslett and Dyson (1957) using 10 kV on 25 nm Au, and from Doffin and Kuhlenkampff (1957) using 34 kV on 25 nm Al. Results: Comparisons of calculations and experimental data indicate that bremsstrahlung angular peak, relativistically shifted forward, agreed within a few degrees with measurements in the literature. However, the overall simulated distribution exhibited angularly invariant regions in the forward direction, attributed to MCNP low‐energy physics simplifications of the NIST dataset. Given that the brachytherapy target is ∼ 50 times thicker, with resultant smearing of the energy/angular distributions, the practical impact of this effect is under investigation, and complementary EGSnrc simulations are in progress. Conflict of Interest: This research was sponsored in part by Xoft, Inc.

Dose verification with Monte Carlo technique for prostate brachytherapy implants with 125I sources

The Monte Carlo technique has been implemented to generate the dose distributions in a model prostate patient, implanted with iodine-125 ( 125I) brachytherapy sources. The results of these calculations are also compared with the dose distributions calculated by a commercially available treatment planning system. The comparison shows that with the source strength suggested by the Monte Carlo technique, the current clinical planning system is found to provide 100% coverage of the prostate with the prescription dose for the same implant pattern. However, the dose-volume histogram of this investigation shows that the VariSeed™ treatment planning system has a 29% and 136% larger dose coverage for the 150% and 200% isodose lines, respectively, than the Monte Carlo simulation. These differences are attributed to the oversimplification of the current planning system using the point source approximation, and also to the interseed effects from multisources that are neglected in the conventional planning systems. The results of this study provide evidence that supports the use of the Monte Carlo technique in treatment planning systems to provide accurate dose calculations in brachytherapy implants.

Development of a rat solid tumor model for continuous low-dose-rate irradiation studies using 125I and 103Pd sources

Purpose To develop an experimental technique for studying the radiobiology of continuous low-dose-rate irradiation (CLDRI) using clinical brachytherapy sources emitting low energy photons for a rat solid tumor model. Methods and materials BA1112 tumors were grown between the ears of 14-week-old male WAG/Rij rats by interdermal inoculation. A radioactive source afterloading system, which consists of a lightweight helmet sutured to the rat and a nine-source polystyrene applicator, was fabricated for in vivo tumor irradiation by 125I and 103Pd brachytherapy sources. This system has a 12 × 12 mm opening in the center to accommodate the tumor and its growth during irradiation (the diameter of a typical BA1112 tumor was about 6 mm when radiation was applied). The spatial locations of the nine sources were optimized to produce an as uniform as possible three-dimensional dose distribution to the central portion of the applicator for both the 125I and 103Pd sources. Absolute dose delivered by the applicator was verified by point dose measurements using calibrated TLD in a polystyrene phantom that mimics the scattering environment of the tumor on the rat. Results The feasibility of tumor cure experiments using the experimental technique presented in this work was demonstrated. The technique was used to study the influence of initial dose rate on the in vivo tumor cure probability of BA1112 tumors irradiated by 125I and 103Pd sources at dose rates varying from 8–20 cGy/h. The technique was also used for studying the in vitro tumor cell survival following in vivo CLDRI irradiation of the tumor. Conclusion An experimental technique using an in vivo tumor model has been developed for studying the radiobiological effects of continuous low-dose-rate irradiations using 125I sources alone, 103Pd sources alone, or a mixture of 125I and 103Pd sources.

Comparison of methods for preparation of 125I brachytherapy source cores for the treatment of eye cancer

Two procedures for fixing the 125I activity on different matrices were explored. In the first method, anodic deposition of 125I on silver wire was used. In the second, 125I was adsorbed as iodate (IO − 3) on alumina microspheres by using a solid-solution interface technique. While the 125I uptake was ∼84% on the silver wires by electrodeposition method, the microspheres showed ∼97% by the solid-solution interface technique. The low leachable (0.05%) wires, and comparatively high leachable (∼4%) spheres were trial encapsulated and laser welded. The encapsulated sources were measured for source strength and were found to be suitable for the treatment of eye cancer.

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% cGy h −1 U −1 in liquid water, which is in good agreement with 1.02±8% cGy h −1 U −1 reported by Nath and Yue, 2002. The anisotropy constant was found to be 0.98 in liquid water.

Experimental determination of the TG-43 dosimetric characteristics of EchoSeed model 6733 I25I brachytherapy source.

Recently an improved design of a 125I brachytherapy source has been introduced for interstitial seed implants, particularly for prostate seed implants. This design improves the in situ ultrasound visualization of the source compared to the conventional seed. In this project, the TG-43 recommended dosimetric characteristics of the new brachytherapy source have been experimentally determined in Solid Water phantom material. The measured dosimetric characteristics of the new source have been compared with data reported in the literature for other source designs. The measured dose rate constant, A, in Solid Water was multiplied by 1.05 to extract the dose rate constant in water. The dose rate constant of the new source in water was found to be 0.99 +/- 8% cGy h(-1) U(-1). The radial dose function was measured at distances between 0.5 and 10 cm using LiF TLDs in Solid Water phantom. The anisotropy function, F(r, theta), was measured at distances of 2, 3, 5, and 7 cm.

Comprehensive Monte Carlo calculations of AAPM Task Group Report No. 43 dosimetry parameters for the Model 3500 I-Plant 125I brachytherapy source

The Model 3500 I-Plant 125I brachytherapy source, produced by Implant Sciences Corporation, is created through a novel technique which utilizes ion implantation of 124Xe into a ceramic matrix followed by neutron activation of the substrate. Clinical dosimetry parameters were calculated using MCNP. For data processing, a 0.376 cm active length was used for the geometry function. The 2-D anisotropy function, F( r, θ), exhibited less anisotropy than the Amersham Health Model 6711 source, and decreased as radial distance, r, increased. The anisotropy constant, φ ̄ an , was 0.933; dose rate constant, Λ, was 1.017±0.005 cGy/h/U; and radial dose function, g( r), for r={0.1, 0.2, 0.5, 2, 5, 10 cm} was 0.990, 1.014, 1.030, 0.872, 0.448, and 0.114, respectively. In general, there was good agreement with experimental results recently obtained by others using thermoluminescent dosimeters.

Monte Carlo dose parameters of the BrachySeed model LS-1 125I 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 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.

Dosimetric characteristics of the bests double-wall 103Pd brachytherapy source.

103Pd and 125I brachytherapy sources are being used for interstitial implants in tumor sites such as the prostate. Recently, a double-wall 103Pd source has been introduced, which has a design different from that of sources presently on the market. Dosimetric characteristics (dose rate constant, radial dose function, and anisotropy function) of this source were experimentally and theoretically determined following the AAPM Task Group 43 recommendations and were related to the October 10, 2000 revision of the NIST 1999 SK Standard for 103Pd. Measurements were performed in a Solid Water phantom using LiF thermoluminescent dosimeters. For these measurements, slabs of Solid Water phantom material were machined to accommodate the source and LiF TLD chips of dimensions (3.1 x 3.1 x 0.8 mm3) and (1.0 x 1.0 x 1.0 mm3). The TLD chips were surrounded by at least 10 cm of Solid Water phantom material to provide full scattering conditions. The Monte Carlo simulations were performed in Solid Water and liquid water using the PTRAN code. The results of this investigation show an excellent agreement (within 5%) between the measured (0.67+/-8% cGy h(-1) U(-1)) and calculated (to be 0.65+/-3% cGy h(-1) U(-1)) dose rate constant in Solid Water. The Monte Carlo calculated dose rate constant of the Best 103Pd in water was found to be 0.67+/-0.02 cGy h(-1) U(-1). The radial dose function, g(r), of the new 103Pd source was measured at distances ranging from 0.5 and 7 cm using LiF TLD in Solid Water phantom material. Moreover, the radial dose function of the new source was calculated in liquid water and Solid Water at distances ranging from 0.1 to 7 cm using the PTRAN Monte Carlo Code. The anisotropy function, F(r, theta), of the new 103Pd source was also measured in Solid Water and calculated in both Solid Water and water phantom material. From the anisotropy functions, the anisotropy factors, and anisotropy constant were calculated for each medium. The results indicated that the measured anisotropy constant of the Best 103Pd source in Solid Water was 0.89+/-5%. Complete dosimetric data are described in this manuscript.