Application of Monte Carlo calculations for validation of a treatment planning system in high dose rate brachytherapy

AimThe accuracy of treatment planning systems is of vital importance in treatment outcomes in brachytherapy. In the current study the accuracy of dose calculations of a high dose rate (HDR) brachytherapy treatment planning system (TPS) was validated using the Monte Carlo method.Materials and methodsThree ⁶⁰Co sources of the GZP6 afterloading brachytherapy system were modelled using MCNP4C Monte Carlo (MC) code. The dose distribution around all the sources was calculated by MC and a dedicated treatment planning system. The results of both methods were compared.ResultsThere was good agreement (<2%) between TPS and MC calculated dose distributions except at a point near the sources (<1 cm) and beyond the tip of the sources.ConclusionsOur study confirmed the accuracy of TPS calculated dose distributions for clinical use in HDR brachytherapy.     

A Feasibility Study of Fricke Dosimetry as an Absorbed Dose to Water Standard for 192Ir HDR Sources

High dose rate brachytherapy (HDR) using ¹⁹²Ir sources is well accepted as an important treatment option and thus requires an accurate dosimetry standard. However, a dosimetry standard for the direct measurement of the absolute dose to water for this particular source type is currently not available. An improved standard for the absorbed dose to water based on Fricke dosimetry of HDR ¹⁹²Ir brachytherapy sources is presented in this study. The main goal of this paper is to demonstrate the potential usefulness of the Fricke dosimetry technique for the standardization of the quantity absorbed dose to water for ¹⁹²Ir sources. A molded, double-walled, spherical vessel for water containing the Fricke solution was constructed based on the Fricke system. The authors measured the absorbed dose to water and compared it with the doses calculated using the AAPM TG-43 report. The overall combined uncertainty associated with the measurements using Fricke dosimetry was 1.4% for k = 1, which is better than the uncertainties reported in previous studies. These results are promising; hence, the use of Fricke dosimetry to measure the absorbed dose to water as a standard for HDR ¹⁹²Ir may be possible in the future.     

Monte Carlo Dosimetry of the 60Co BEBIG High Dose Rate for Brachytherapy

Introduction

The use of high-dose-rate brachytherapy is currently a widespread practice worldwide. The most common isotope source is ¹⁹²Ir, but ⁶⁰Co is also becoming available for HDR. One of main advantages of ⁶⁰Co compared to ¹⁹²Ir is the economic and practical benefit because of its longer half-live, which is 5.27 years. Recently, Eckert & Ziegler BEBIG, Germany, introduced a new afterloading brachytherapy machine (MultiSource^®); it has the option to use either the ⁶⁰Co or ¹⁹²Ir HDR source. The source for the Monte Carlo calculations is the new ⁶⁰Co source (model Co0.A86), which is referred to as the new BEBIG ⁶⁰Co HDR source and is a modified version of the ⁶⁰Co source (model GK60M21), which is also from BEBIG.

Objective and Methods

The purpose of this work is to obtain the dosimetry parameters in accordance with the AAPM TG-43U1 formalism with Monte Carlo calculations regarding the BEBIG ⁶⁰Co high-dose-rate brachytherapy to investigate the required treatment-planning parameters. The geometric design and material details of the source was provided by the manufacturer and was used to define the Monte Carlo geometry. To validate the source geometry, a few dosimetry parameters had to be calculated according to the AAPM TG-43U1 formalism. The dosimetry studies included the calculation of the air kerma strength S _k, collision kerma in water along the transverse axis with an unbounded phantom, dose rate constant and radial dose function. The Monte Carlo code system that was used was EGSnrc with a new cavity code, which is a part of EGS++ that allows calculating the radial dose function around the source. The spectrum to simulate ⁶⁰Co was composed of two photon energies, 1.17 and 1.33 MeV. Only the gamma part of the spectrum was used; the contribution of the electrons to the dose is negligible because of the full absorption by the stainless-steel wall around the metallic ⁶⁰Co. The XCOM photon cross-section library was used in subsequent simulations, and the photoelectric effect, pair production, Rayleigh scattering and bound Compton scattering were included in the simulation. Variance reduction techniques were used to speed up the calculation and to considerably reduce the computer time. The cut-off energy was 10 keV for electrons and photons. To obtain the dose rate distributions of the source in an unbounded liquid water phantom, the source was immersed at the center of a cube phantom of 100 cm³. The liquid water density was 0.998 g/cm³, and photon histories of up to 10¹⁰ were used to obtain the results with a standard deviation of less than 0.5% (k = 1). The obtained dose rate constant for the BEBIG ⁶⁰Co source was 1.108±0.001 cGyh^-1U^-1, which is consistent with the values in the literature. The radial dose functions were compared with the values of the consensus data set in the literature, and they are consistent with the published data for this energy range.     

Air kerma strength characterization of a GZP6 Cobalt-60 brachytherapy source

Background

Task group number 40 (TG-40) of the American Association of Physicists in Medicine (AAPM) has recommended calibration of any brachytherapy source before its clinical use. GZP6 afterloading brachytherapy unit is a ⁶⁰Co high dose rate (HDR) system recently being used in some of the Iranian radiotherapy centers.

Aim

In this study air kerma strength (AKS) of ⁶⁰Co source number three of this unit was estimated by Monte Carlo simulation and in air measurements.

Materials and methods

Simulation was performed by employing the MCNP-4C Monte Carlo code. Self-absorption of the source core and its capsule were taken into account when calculating air kerma strength. In-air measurements were performed according to the multiple distance method; where a specially designed jig and a 0.6 cm³ Farmer type ionization chamber were used for the measurements. Monte Carlo simulation, in air measurement and GZP6 treatment planning results were compared for primary air kerma strength (as for November 8th 2005).

Results

Monte Carlo calculated and in air measured air kerma strength were respectively equal to 17240.01 μGym² h⁻¹ and 16991.83 μGym² h⁻¹. The value provided by the GZP6 treatment planning system (TPS) was “15355 μGym² h⁻¹”.

Conclusion

The calculated and measured AKS values are in good agreement. Calculated-TPS and measured-TPS AKS values are also in agreement within the uncertainties related to our calculation, measurements and those certified by the GZP6 manufacturer. Considering the uncertainties, the TPS value for AKS is validated by our calculations and measurements, however, it is incorporated with a large uncertainty.     

Dosimetric investigation of a new high dose rate 192Ir brachytherapy source,IRAsource, by Monte Carlo method

PurposeThe purpose of the present study was to perform an independent calculation of dosimetric parameters associated with a new ¹⁹²Ir brachytherapy source model, IRAsource.Materials and methodsThe parameters of air kerma strength (AKS), dose rate constant (DRC), geometry function (GF), radial dose function (RDF), as well as two-dimensional (2D) anisotropy function (AF) of IRAsource ¹⁹²Ir source model were calculated in this study. The MC n-particle extended (MCNPX) code was also employed for simulating high dose rate (HDR), IRAsource and ¹⁹²Ir source; and formalism was used for calculating dosimetry parameters based on task group number 43 updated report (TG-43 U1).ResultsThe results of this study were consistent with the ones reported about the IRAsource source by Sarabiasl et al. The AKS per 1 mCi activity and the DRC values were also equal to 3.65 cGycm² h^–1 mCi^–1 and 1.094 cGyh^–1U^–1; respectively. The comparison of the results of the DRC and the RDF reported by Sarabiasl et al. also validated the ¹⁹²Ir IRAsource simulation in this study. Moreover, the AFs of IRAsource source model were in a good agreement with those of Sarabiasl et al. at different distances, which could be attributed to identical geometries.ConclusionIn line with those reported by Sarabiasl et al., the results of this study confirmed the IRAsource ¹⁹²Ir source for clinical uses. The calculated dosimetric parameters of the IRAsource source could be utilized in clinical practices as input data sets or for validation of treatment planning system calculations.     

Dosimetric characterization of an 192Ir brachytherapy source with the Monte Carlo code PENELOPE

Monte Carlo calculations are highly spread and settled practice to calculate brachytherapy sources dosimetric parameters. In this study, recommendations of the AAPM TG-43U1 report have been followed to characterize the Varisource VS2000 ¹⁹²Ir high dose rate source, provided by Varian Oncology Systems.In order to obtain dosimetric parameters for this source, Monte Carlo calculations with PENELOPE code have been carried out. TG-43 formalism parameters have been presented, i.e., air kerma strength, dose rate constant, radial dose function and anisotropy function. Besides, a 2D Cartesian coordinates dose rate in water table has been calculated. These quantities are compared to this source reference data, finding results in good agreement with them.The data in the present study complement published data in the next aspects: (i) TG-43U1 recommendations are followed regarding to phantom ambient conditions and to uncertainty analysis, including statistical (type A) and systematic (type B) contributions; (ii) PENELOPE code is benchmarked for this source; (iii) Monte Carlo calculation methodology differs from that usually published in the way to estimate absorbed dose, leaving out the track-length estimator; (iv) the results of the present work comply with the most recent AAPM and ESTRO physics committee recommendations about Monte Carlo techniques, in regards to dose rate uncertainty values and established differences between our results and reference data.The results stated in this paper provide a complete parameter collection, which can be used for dosimetric calculations as well as a means of comparison with other datasets from this source.     

A dosimetry method in the transverse plane of HDR Ir-192 brachytherapy source using gafchromic EBT2 film

Radiochromic film dosimetry is increasingly used in brachytherapy applications for its higher resolution ability as compared to other experimental methods. The present study was aimed to assess the accuracy and suitability of use of the improved radiochromic film model, Gafchromic EBT2, to evaluate the dose distribution in the transverse plane of microselectron HDR ¹⁹²Ir source.A specially designed and locally fabricated Polymethyl methacrylate (PMMA) phantom was used in this work for the experimental measurement of dose distribution around the source in its transverse plane. The AAPM TG-43U1 recommended radial dose function, g (r), and dose rate constant, Λ, for the source were measured using Gafchromic EBT2 film and thermoluminescent dosimeters (TLD). The EBT2 film measured dosimetric quantities were validated against their values obtained from the TLD measurements and previously published values for the same source available in literature.The dose rate constant and radial dose function for microselectron HDR ¹⁹²Ir source obtained from Gafchromic EBT2 film measurements are in agreement with their TLD measured results within 3.9% and 2.8% respectively. They also agree within the accepted range of uncertainty with their experimental and Monte Carlo calculated results reported in literature.This work demonstrates the suitability of using Gafchromic EBT2 film dosimetry in characterization of dose distribution in the transverse plane of HDR Ir-192 source. This is a more efficient method than TLD dosimetry at discrete and distant positions. Relative to TLD dosimetry, it is found to be better reproducible, easy to use and a less expensive method of dosimetry.     

Dosimetric and radiobiological comparison of TG-43 and Monte Carlo calculations in 192Ir breast brachytherapy applications

PurposeTo investigate the clinical significance of introducing model based dose calculation algorithms (MBDCAs) as an alternative to TG-43 in ¹⁹²Ir interstitial breast brachytherapy.Materials and methodsA 57 patient cohort was used in a retrospective comparison between TG-43 based dosimetry data exported from a treatment planning system and Monte Carlo (MC) dosimetry performed using MCNP v. 6.1 with plan and anatomy information in DICOM-RT format. Comparison was performed for the target, ipsilateral lung, heart, skin, breast and ribs, using dose distributions, dose-volume histograms (DVH) and plan quality indices clinically used for plan evaluation, as well as radiobiological parameters.ResultsTG-43 overestimation of target DVH parameters is statistically significant but small (less than 2% for the target coverage indices and 4% for homogeneity indices, on average). Significant dose differences (>5%) were observed close to the skin and at relatively large distances from the implant leading to a TG-43 dose overestimation for the organs at risk. These differences correspond to low dose regions (<50% of the prescribed dose), being less than 2% of the prescribed dose. Detected dosimetric differences did not induce clinically significant differences in calculated tumor control probabilities (mean absolute difference <0.2%) and normal tissue complication probabilities.ConclusionWhile TG-43 shows a statistically significant overestimation of most indices used for plan evaluation, differences are small and therefore not clinically significant. Improved MBDCA dosimetry could be important for re-irradiation, technique inter-comparison and/or the assessment of secondary cancer induction risk, where accurate dosimetry in the whole patient anatomy is of the essence.     

Dosimetric characterization of the 60Co BEBIG Co0.A86 high dose rate brachytherapy source using PENELOPE

⁶⁰Co sources are being used as an alternative to ¹⁹²Ir sources in high dose rate brachytherapy treatments. In a recent document from AAPM and ESTRO, a consensus dataset for the ⁶⁰Co BEBIG (model Co0.A86) high dose rate source was prepared by using results taken from different publications due to discrepancies observed among them. The aim of the present work is to provide a new calculation of the dosimetric characteristics of that ⁶⁰Co source according to the recommendations of the AAPM and ESTRO report. Radial dose function, anisotropy function, air-kerma strength, dose rate constant and absorbed dose rate in water have been calculated and compared to the results of previous works. Simulations using the two different geometries considered by other authors have been carried out and the effect of the cable density and length has been studied.     

Feasibility study of the Fricke chemical dosimeter as an independent dosimetric system for the small animal radiation research platform (SARRP)

For the small animal radiation research platform (SARRP) with X-ray beams in the medium energy range (tube operating voltage at 220 kVp), reference dosimetry is based on the AAPM TG-61 recommendations following the in-phantom method. The objective of this study was to evaluate the feasibility of the Fricke solution as a dosimeter to determine the absorbed dose to water. Feasibility studies at this X-ray energy range are not widely available. We evaluated the accuracy, dose linearity and dose rate dependence in a comparison with an NE 2571 Farmer ionization chamber (IC) and measurements in water. The G(Fe³⁺) factor was calculated from the curve fitting of the chemical yields for two radioactive sources (¹⁹²Ir and ⁶⁰Co) and one X-ray system with a tube operating at 150 and 250 kVp. The same methodology was followed for the dependence of the G(Fe³⁺) value on the energy and the dose agreement assessment for 180 and 200 kVp in the SARRP. The Fricke system exhibits a good linear response over the range of 5–70 Gy and an accuracy better than 2% for a 2 Gy/min dose rate. The dose rate dependence is smaller than 1% for dose rates greater than 1 Gy/min. The dependence of the G(Fe³⁺) value on the energy is smaller than 0.41%, with dose agreements better than 2%. The feasibility of the dosimeter for measurements at high doses and high dose rates makes it a suitable tool for dosimetric verifications in several preclinical irradiation configurations.     

Tissue composition and density impact on the clinical parameters for 125I prostate implants dosimetry

The MCNPX code was used to calculate the TG-43U1 recommended parameters in water and prostate tissue in order to quantify the dosimetric impact in 30 patients treated with ¹²⁵I prostate implants when replacing the TG-43U1 formalism parameters calculated in water by a prostate-like medium in the planning system (PS) and to evaluate the uncertainties associated with Monte Carlo (MC) calculations. The prostate density was obtained from the CT of 100 patients with prostate cancer. The deviations between our results for water and the TG-43U1 consensus dataset values were −2.6% for prostate V₁₀₀, −13.0% for V₁₅₀, and −5.8% for D₉₀; −2.0% for rectum V₁₀₀, and −5.1% for D_0.1; −5.0% for urethra D₁₀, and −5.1% for D₃₀. The same differences between our water and prostate results were all under 0.3%. Uncertainties estimations were up to 2.9% for the g_L(r) function, 13.4% for the F(r,θ) function and 7.0% for Λ, mainly due to seed geometry uncertainties. Uncertainties in extracting the TG-43U1 parameters in the MC simulations as well as in the literature comparison are of the same order of magnitude as the differences between dose distributions computed for water and prostate-like medium. The selection of the parameters for the PS should be done carefully, as it may considerably affect the dose distributions. The seeds internal geometry uncertainties are a major limiting factor in the MC parameters deduction.     

Comparison of planned and measured rectal dose in-vivo during high dose rate Cobalt-60 brachytherapy of cervical cancer

Cobalt-60 (Co-60) is a relatively new source for the application of high-dose rate (HDR) brachytherapy. Radiation dose to the rectum is often a limiting factor in achieving the full prescribed dose to the target during brachytherapy of cervical cancer. The aim of this study was to measure radiation doses to the rectum in-vivo during HDR Co-60 brachytherapy. A total of eleven HDR brachytherapy treatments of cervical cancer were recruited in this study. A series of diodes incorporated in a rectal probe was inserted into the patient's rectum during each brachytherapy procedure. Real-time measured rectal doses were compared to calculated doses by the treatment planning system (TPS). The differences between calculated and measured dose ranged from 8.5% to 41.2%. This corresponds to absolute dose differences ranging from 0.3 Gy to 1.5 Gy. A linear relationship was observed between calculated and measured doses with linear regression R² value of 0.88, indicating close association between the measured and calculated doses. In general, absorbed doses for the rectum as calculated by TPS were observed to be higher than the doses measured using the diode probe. In-vivo dosimetry is an important quality assurance method for HDR brachytherapy of cervical cancer. It provides information that can contribute to the reduction of errors and discrepancies in dose delivery. Our study has shown that in-vivo dosimetry is feasible and can be performed to estimate the dose to the rectum during HDR brachytherapy using Co-60.     

Prevention of Transfusion-Associated Graft-versus-Host Disease by Irradiation: Technical Aspect of a New Ferrous Sulphate Dosimetric System

Irradiation of whole blood and blood components before transfusion is currently the only accepted method to prevent Transfusion-Associated Graft-Versus-Host-Disease (TA-GVHD). However, choosing the appropriate technique to determine the dosimetric parameters associated with blood irradiation remains an issue. We propose a dosimetric system based on the standard Fricke Xylenol Gel (FXG) dosimeter and an appropriate phantom. The modified dosimeter was previously calibrated using a ⁶⁰Co teletherapy unit and its validation was accomplished with a ¹³⁷Cs blood irradiator. An ionization chamber, standard FXG, radiochromic film and thermoluminescent dosimeters (TLDs) were used as reference dosimeters to determine the dose response and dose rate of the ⁶⁰Co unit. The dose distributions in a blood irradiator were determined with the modified FXG, the radiochromic film, and measurements by TLD dosimeters. A linear response for absorbed doses up to 54 Gy was obtained with our system. Additionally, the dose rate uncertainties carried out with gel dosimetry were lower than 5% and differences lower than 4% were noted when the absorbed dose responses were compared with ionization chamber, film and TLDs.     

A Monte Carlo study on dose distribution evaluation of Flexisource 192Ir brachytherapy source

Aim

The aim of this study is to evaluate the dose distribution of the Flexisource ¹⁹²Ir source.

Background

Dosimetric evaluation of brachytherapy sources is recommended by task group number 43 (TG. 43) of American Association of Physicists in Medicine (AAPM).

Materials and methods

MCNPX code was used to simulate Flexisource ¹⁹²Ir source. Dose rate constant and radial dose function were obtained for water and soft tissue phantoms and compared with previous data on this source. Furthermore, dose rate along the transverse axis was obtained by simulation of the Flexisource and a point source and the obtained data were compared with those from Flexiplan treatment planning system (TPS).

Results

The values of dose rate constant obtained for water and soft tissue phantoms were equal to 1.108 and 1.106, respectively. The values of the radial dose function are listed in the form of tabulated data. The values of dose rate (cGy/s) obtained are shown in the form of tabulated data and figures. The maximum difference between TPS and Monte Carlo (MC) dose rate values was 11% in a water phantom at 6.0 cm from the source.

Conclusion

Based on dosimetric parameter comparisons with values previously published, the accuracy of our simulation of Flexisource ¹⁹²Ir was verified. The results of dose rate constant and radial dose function in water and soft tissue phantoms were the same for Flexisource and point sources. For Flexisource ¹⁹²Ir source, the results of TPS calculations in a water phantom were in agreement with the simulations within the calculation uncertainties. Furthermore, the results from the TPS calculation for Flexisource and MC calculation for a point source were practically equal within the calculation uncertainties.     

On the use of a novel Ferrous Xylenol-orange gelatin dosimeter for HDR brachytherapy commissioning and quality assurance testing

PurposeTo evaluate a commercially available Ferrous-Xylenol Orange-Gel (FXG) dosimeter (TrueView™) coupled with Optical-Computed Tomography (OCT) read out, for 3D dose verification in an Ir-192 superficial brachytherapy application.MethodsTwo identical polyethylene containers filled with gel from the same batch were used. One was irradiated with an 18 MeV electron field to examine the dose-response linearity and obtain a calibration curve. A flap surface applicator was attached to the other to simulate treatment of a skin lesion. The dose distribution in the experimental set up was calculated with the TG-43 and the model based dose calculation (MBCA) algorithms of a commercial treatment planning system (TPS), as well as Monte Carlo (MC) simulation using the MCNP code. Measured and calculated dose distributions were spatially registered and compared.ResultsApart from a region close to the container’s neck, where gel measurements exhibited an over-response relative to MC calculations (probably due to stray light perturbation), an excellent agreement was observed between measurements and simulations. More than 97% of points within the 10% isodose line (80 cGy) met the gamma index criteria established from uncertainty analysis (5%/2 mm). The corresponding passing rates for the comparison of experiment to calculations using the TG-43 and MBDCA options of the TPS were 57% and 92%, respectively.ConclusionTrueView™ is suitable for the quality assurance of demanding radiotherapy applications. Experimental results of this work confirm the advantage of the studied MBDCA over TG-43, expected from the improved account of scatter radiation in the treatment geometry.