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ORIGINAL ARTICLE |
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| Year : 2022 | Volume
: 47
| Issue : 4 | Page : 352-361 |
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Commissioning validation of a brachytherapy treatment planning system with egsnrc monte carlo code and EBT3 GAFChromic film
Mohammed Lahlabou1, Rachid Khelifi2
1 Department of Radiotherapy, Central Hospital of the Army, Kouba, Algiers
2 PTHIRM Laboratory, Department of Physics, Faculty of Science, Saad Dahlab University Blida 1, Blida, Algeria
| Date of Submission | 17-Apr-2022 |
| Date of Decision | 21-Sep-2022 |
| Date of Acceptance | 22-Sep-2022 |
| Date of Web Publication | 10-Jan-2023 |
Correspondence Address:
Dr. Mohammed Lahlabou
Department of Radiotherapy, Central Hospital of the Army, B. P. 244, Kouba, Algiers
Algiers
 Source of Support: None, Conflict of Interest: None  | Check |
DOI: 10.4103/jmp.jmp_30_22
 Abstract | | |
Aims: Most brachytherapy treatment planning system (TPS) commissioning requires data input based on the American Association of Physicists in Medicine Task Group-43 formalism. The commissioning accuracy is very important for dose calculation. The aim of this study is the implementation of a brachytherapy TPS into a clinical environment and check the TPS calculated dose accuracy. Subjects and Methods: After introducing data of the different catheters (CIS Bio International, Saclay, France), composed of several Cesium-137 Eckert and Ziegler BEBIG CSM-11 radioactive sources; for XiO (CMS, St. Louis) brachytherapy TPS, the TPS dose calculation accuracy was investigated by comparing between the TPS calculated dose distribution (DD) for all the catheters with (1) the measuring DD using EBT3 GAFChromic film and (2) calculating DD by egs_brachy (Electron Gamma Shower, National Research Council of Canada) Monte Carlo simulation. The phantom used for this study consists of six PTW slabs 30 cm × 30 cm × 1 cm of polymethyl methacrylate with a Delouche MEDpro applicator on the top. The TPS DD was calculated on the computed tomography scan of this phantom. Statistical Analysis Used: PTW VeriSoft version 6.0.1.7 (PTW-Freiburg, Germany) software was used for analyzing scanned films and to perform the comparison based on the gamma index distribution. Results: For each catheter, the gamma index distribution showed agreement >95% of all pixels in both verification methods, with gamma ≤1. Conclusions: We confirm the commissioning accuracy and that the TPS can be used for clinical purposes.
Keywords: Brachytherapy, EBT3, egs_brachy, gamma index, TG43
How to cite this article:
Lahlabou M, Khelifi R. Commissioning validation of a brachytherapy treatment planning system with egsnrc monte carlo code and EBT3 GAFChromic film. J Med Phys 2022;47:352-61 |
How to cite this URL:
Lahlabou M, Khelifi R. Commissioning validation of a brachytherapy treatment planning system with egsnrc monte carlo code and EBT3 GAFChromic film. J Med Phys [serial online] 2022 [cited 2023 Mar 31];47:352-61. Available from: https://www.jmp.org.in/text.asp?2022/47/4/352/367421 |
| Introduction | |  |
Brachytherapy has proven to be a highly successful radiation treatment in the management of different types of cancers. A Cs137 source is being still used for low-dose-rate (LDR) brachytherapy treatment, mainly addressing gynecological lesions. The brachytherapy treatment planning system (TPS) is an essential component of the treatment process. Hence, accurate commissioning of TPS is important to ensure accurate dose delivery.[1] Brachytherapy TPS requires data for the radioactive source used for clinical practice. The radiation sources are well characterized and documented in the scientific literature based on the most employed algorithm the American Association of Physicists in Medicine (AAPM) Task Group (TG)-43 approaches. The aim of the present study is the implementation of a brachytherapy TPS into a clinical environment and to verify the dose calculation accuracy. The verification is performed by comparing the dose distribution (DD) generated by the TPS with: (1) that experimentally measured using EBT3 GAFChromic film and (2) that calculated using egs_brachy Monte Carlo (MC) code by simulating the same experimental conditions. Before the comparison, the routine validation procedure for any MC code was performed to check the efficacy of egs_brachy by calculating the dose rate constant, along–away dose rate data, radial dose function, and anisotropy function around the radioactive source in water phantom and compared them with the introduced data into the TPS. The GAFChromic films were calibrated according to the recommended protocols.
The scope of this work is to perform the commissioning of XiO brachytherapy TPS, for catheters composed of 137Cs CSM11 LDR source model. XiO is based on the AAPM TG-43 dosimetry approach, in which the absorbed dose is calculated by superimposing precalculated single-source DDs in water. The TG-43 method leads to errors in dose calculation as it does not take into account for problems such as heterogeneity, capsule attenuation, and applicator radiation interaction. A review of the literature concluded that accepted clinical dose parameters can be overestimated or underestimated by at least 5%.[2] Hence, we have employed the MC Egs_brachy code simulation and GAFChromic films to investigate that the TPS dose calculation accuracy does not exceed 5%.
| Subjects and Methods | | |
GAFChromic films calibration
A 6-MV photon beam from Synergy® linear accelerator (Elekta, Stockholm, Sweden) was used to irradiate the EBT3® GAFChromic (Ashland Specialty Ingredients, Bridgewater, NJ, USA) films of sheet dimensions 20.32 cm × 25.4 cm (8 × 10”). Each film was manipulated according to the procedure described in the AAPM TG-55 report. An EPSON GT200000 Flatbed Scanner and the PTW Verisoft® version 6.0.1.7 (PTW-Freiburg, Germany) software were employed for image scanning and analyzing[3], with all filters and image enhancement options disabled. All films (calibration and measurement) were scanned in landscape orientation, 24-h postirradiation. Images were collected at 24 bits per color channel with a spatial resolution of 72 dpi and saved as tagged image file format (TIFF) image files.[4] Film pieces of dimensions 3 cm × 3.2 cm were centrally located and irradiated at a depth of 1 cm in a Solid Water slab phantom composed of six PTW slabs of polymethyl methacrylate with a dimension of 30 cm × 30 cm × 1 cm [Figure 1] at doses to water in the range of 0.10–10 Gy. | Figure 1: (a) Synergy Elekta linear accelerator, (b) centrally located EBT3 film on the slab, (c) absolute dose calculation with an ionization chamber, (d) Epson GT200000 Flatbed Scanner, and (e) irradiated EBT3 films
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EBT3 films were considered to have no relative energy dependence according to the manufacturer (<5% difference in net optical density when exposed at 100 KeV and 18 MeV[5]) and the experimental studies for 192Ir in the literature (Palmer et al. 2013c, 2013b, Moura et al. 2015).[6],[7] A PTW Farmer 0.6 cm3 ionization chamber was used to measure the absolute dose according to the protocol of the Technical Report Series 398. The output of the linac was measured before and after the irradiations, and a variation of 0.07% was observed.
To check the accuracy of the measurement with the films, a comparison is realized between the measurement of the unknown doses with the films and the ionization chamber placed and the same depth. The results were in good agreement with a difference <0.1%.
Treatment planning system implementation
The XiO® 4.8 (CMS, St. Louis) brachytherapy TPS was used to achieve the commissioning of the different catheters (CIS Bio International, Saclay, France) composed of Eckert and Ziegler BEBIG Cesium-137 Type CSM11 radioactive sources. The TPS calculation algorithm assumes that the entire calculation volume consists of a homogenous water-like medium. Dose calculation for brachytherapy sources in XiO is based on interpolation of precomputed dose rate tables. They are precomputed in the TPS library based on physics data entered directly by users. It supports two methods of dose rate table generation: TG43 (based on the report of TG-43 of the AAPM) and Sievert integral (A variant of the Sievert integral scheme that is similar to the model described by J. F. Williamson).[8],[9] The radioactive source data required by TPS based on the AAPM TG-43 approach were taken from AAPM and ESTRO reports.[10] [Figure 2] shows the reference polar coordinate system for radioactive sources adapted from the TG-43 report where the general two-dimensional dose rate equation is retained: | Figure 2: Reference polar coordinate system for radioactive sources adapted from the TG-43 report
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where r denotes the distance from the center of the active source to the point of interest, r0 denotes the reference distance (r0 = 1 cm), and θ denotes the polar angle specifying the point of interest. The reference angle, θ0 is specified to be θ0 = 90° [Figure 2]. SK: air-kerma strength, Λ: dose rate constant, GL (r, θ): geometry factor, gL (r): radial dose function, and F (r, θ): anisotropy functions.[11],[12]
Experimentally measurement
A Curietron (BEBIG Eckert and Ziegler Company) source projector was used to carry out the measurements [Figure 3]. It contains four channels: Two vaginal and two uterine. The Curie stock (where the radioactive sources are stocked) holds 10catheters with five different lengths (17, 40, 50, 60, and 70 mm). Each catheter contains two or several sources of CSM-11 according to its length. [Table 1] and [Table 2] resume their features and radioactive source layout scheme, respectively. | Figure 3: (a) BEBIG Eckert and Ziegler Curietron, (b) Curiestock, and (c) Delouche MEDPro applicator
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The Delouche MEDPro applicator [Figure 3] was used to be loaded with the catheters. It is made of plastic material and it consists of a set of three applicators. Each applicator consists of an intrauterine probe and two cylindrical vaginal kites, in the middle of which are implanted hollow tubes. These tubes have an external diameter of 6 mm and an internal diameter of 3.5 mm. The same phantom used for the film calibration was used for verification. It consists of six slabs with thicknesses of 1 cm with the intrauterine probe on the top [Figure 4]. For the five different catheters, a film was centrally located between the sixth and the fifth slab and was irradiated for 10 h. | Figure 4: PTW slabs phantom and intrauterine probe setup (a) Front view (b) profile view
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Treatment planning system calculated dose
The TPS DD was calculated on the computed tomography (CT) scan of the phantom with the probe obtained with 2 mm slice thickness by PHILIPS Big Bore Brilliance CT scanner [Figure 5]. Five treatment plans were calculated with Xio brachytherapy TPS for the five different catheter lengths. A plan consist of one catheter centrally located and was prescribed to deliver a DD for a duration of 10 h. [Figure 6] shows the calculated DD for catheter 70. The planned dose matrix in the film plane was exported from the TPS for comparison with the measurement using the gamma index method. A gamma value >1 was considered outside the tolerance range. For a catheter, if the percentage of passing points is >95% it is considered a validated catheter.[13] | Figure 5: (a) PHILIPS Big Bore Brilliance CT scanner, (b) phantom transversal slice, and (c) phantom sagittal slice. CT: Computed tomography
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 | Figure 6: Catheter_70 TPS calculated dose distribution. TPS: Treatment planning system
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Monte Carlo code efficacy
The egs_brachy MC application was used to obtain calculated DD by simulation. It is a versatile and fast EGSnrc application designed for brachytherapy, employing the egs++ library for modeling geometries and sources.[14] The algorithm of the dose calculation by the code for LDR brachytherapy scores the collision kerma per history in a geometric region voxel through a track length estimator. Due to the low energies involved, charged particle equilibrium can be assumed and collision kerma can be considered equal to the absorbed dose to the medium. For the calculations in this study, electrons are not transported with the default MC parameters for low-energy sources (electron cutoff = 1.5 MeV and photon cutoff = 1 KeV).[15] Rayleigh scattering, bound Compton scattering, photoelectric absorption, and fluorescent emission of characteristic X-rays are all simulated. All calculations used photon cross sections from the XCOM database. The photon energy spectrum file (Cs137_NNDC_2.6_line.spectrum) of the National Nuclear Data Center has been used for these simulations.[14]
A routine validation procedure was performed before DD calculation. It consists of calculating the dose rate constant, along–away dose rate data, radial dose function, and anisotropy function around the radioactive source in water phantom and comparing them with the reference data (introduced data into the TPS).
Dimensions for the BEBIG CSM-11 source are taken from the study by Ballester et al.[16] It consists of a pollucite cylindrical active core (elemental composition by mass: 26.18% Si, 3.00% Ti, 1.59% Al, 3.73% B, 1.21% Mg, 2.86% Ca, 12.61% Na, 0.94% Cs, and 47.89% O, with a density of 2.9 g/cm3). The active core with 0.85 mm in diameter and 3.2 mm in height is housed in a cylindrical capsule of AISI_316 L stainless steel (density of 7.8 g/cm3) with a diameter of 1.65 mm. The tip of the capsule is a hemisphere of a radius of 0.825 mm with its center offset of 0.225 mm from the center of the source. A cylindrical air gap of a thickness of 0.225 mm and diameter of 0.85 mm is located on the distal end of the core. The cylindrical air gap is capped by a hemispherical air gap of a radius of 0.0425 mm whose center is offset by 1.825 mm from the center of the core.[16],[17] The schematic egs_brachy modeled source is shown in [Figure 7]. | Figure 7: Eckert and Ziegler BEBIG LDR 137Cs model CSM-11 source. LDR: Low-dose-rate
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Dose rate constants Λ is calculated by dividing the dose to water per history in a (0.1 mm) 3 voxel centered on the reference position in an 80 cm × 80 cm × 80 cm water phantom with a density of 0.998 g/cm3 (as recommended by TG43U1), by the air-kerma strength per history factor (SKhist). The voxel resolutions used to calculate the radial and anisotropy functions are: 0.1 mm for r ≤1 cm, 0.5 mm for 1 cm < r ≤5 cm, 1 mm for 5 cm < r ≤10 cm, and 2 mm for r >10 cm.[18]
Monte Carlo simulation calculated dose
A catheter contains several CSM-11 sources separated by steel balls with a diameter of 1.6 mm [Table 2]. The whole is housed in a cylindrical capsule of SS_AISI 316 L (density of 4.08 g/cm3) filled with air with an outer diameter of 3.2 mm and inner diameter of 1.68 mm. [Figure 8] shows catheter_17 simulation geometry. Dose calculations are done with the catheter positioned at the center of a rectilinear water phantom (0.998 g/cm3) with dimensions of 80 cm × 80 cm × 80 cm. The scoring region is three-dimensional water phantom 15 cm × 15 cm × 15 cm with 5 mm voxel resolution. The DD is extracted from the two-dimensional plane at 1 cm away from the catheter using a Matlab code that read “.3ddose” files.[19] To get the DD in gray, the result was multiplied by the actual air-kerma strength in MBq (corresponds to the film irradiation date) and by the irradiation duration in second (10 h). A comparison was performed with the planned dose matrix extracted from the TPS in the same conditions using the gamma index method. A gamma value >1 was considered outside the tolerance range. For a catheter, if the percentage of passed points is >95%, it is considered a validated catheter.[13]
| Results | | |
Monte Carlo code efficacy
The dose rate constant calculated with EGS_brachy for 1E8 iteration is 1.103 cGy/(h U) which is about 0.8% higher than the value introduces on TPS of 1.094 cGy/(h U) from the ESTRO/AAPM 229 report. [Figure 9] shows the comparison between the reference radial dose function and anisotropy function around the radioactive source in water phantom for (r = 0.5 cm, r = 1 cm, and r = 4 cm) and those calculated by egs_brachy.
EBT3 measurement versus treatment planning system calculated dose
The comparison results between the measured and TPS planned planar DDs for the different catheters with the gamma index for different criteria of dose difference and distance criteria are presented in [Table 3]. | Table 3: Comparison measured and treatment planning system calculated dose distribution with gamma index for different criterions
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It summarizes the percentage of points that pass the criterion. A 5-mm acceptance distance and 3% difference in reference dose with the value of the local dose is the minimum criterion that shows agreement for >95% of all points, with gamma ≤1 for all the catheters. [Table 4] contains comparison details for dose difference and distance criteria 3% and 5 mm, respectively. | Table 4: Comparison measured and treatment planning system calculated dose distribution with gamma index for dose difference and distance criteria 3% and 5 mm, respectively
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[Figure 10] shows the measured, TPS calculated DD and gamma index distribution for catheter_70 (it was chosen because it contains the maximum number of radioactive sources). A comparison between different profiles that pass through the origin is illustrated in [Figure 11]. | Figure 10: Comparison measured and TPS calculated dose distribution with gamma index distribution for dose difference and distance criteria 3% and 5 mm, respectively, for catheter_70. TPS: Treatment planning system
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 | Figure 11: Comparison measured and TPS calculated dose distribution with different profiles for catheter_70. TPS: Treatment planning system
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Monte Carlo simulation versus treatment planning system calculated dose
[Table 5] summarizes the percentage of points that pass the gamma index criterion for the comparison between the egs_brachy and TPS planned planar DDs for the different catheters for different criteria. A 2-mm acceptance distance and 3% difference in reference dose with the value of the local dose is the minimum criterion that shows agreement for >95% of all points, with gamma ≤1 for all the catheters. [Table 6] contains comparison details for 3% dose difference and 5 mm distance criteria. | Table 5: Comparison of egs_brachy and treatment planning system calculated dose distribution with gamma index for different criterions
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 | Table 6: Comparison of egs_brachy and treatment planning system calculated dose distribution with gamma index for dose difference and distance criteria 3% and 2 mm, respectively
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[Figure 12] shows the simulated, TPS calculated DD and gamma index distribution for catheter_70. A comparison between different profiles that pass through the origin is illustrated in [Figure 13]. | Figure 12: Comparison of egs_brachy and TPS calculated dose distribution with the gamma index distribution for dose difference and distance criteria 3% and 2 mm, respectively, for catheter_70. TPS: Treatment planning system
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 | Figure 13: Comparison of egs_brachy and TPS calculated dose distribution with different profiles for catheter_70. TPS: Treatment planning system. LR: Left-Right for lateral direction, TG: Target-Gun for longitudinal direction
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| Discussion | | |
EBT3 Measurement versus treatment planning system calculated dose
A catheter is considered validated if the point percentage that passes the criterion of the gamma index is >95%. According to the obtained results in [Table 3], all the catheters are validated for 3% dose difference and 5 mm distance criteria. Five millimeter seems a great distance but it is expected if taking into account the applicator positioning errors on the phantom. For these criteria, [Table 4] shows that the maximums gamma index >1 is found for catheter_17 and catheter_60. They equals to 1.915 and 3.139, respectively, and they are located at (LR= −5, TG =40) and (LR = −5, TG = 15). For the absolute dose difference, all the maximums are at 5 mm close to the radioactive source axe. By analyzing the profiles in [Figure 11], the absolute dose difference increases to 1.6 Gy when approaching the radioactive source axis and it decreases by moving away. The results are logical as the TPS miscalculates in the region close to the radioactive source.
Monte Carlo simulation versus treatment planning system calculated dose
According to [Table 5] results, all the catheters are considered validated catheters for 3% dose difference and 2 mm distance criteria as their gamma index comparison shows agreement for >95%. These criteria are acceptable as they are similar to intensity-modulated radiation therapy plan verification. For these criteria, [Table 6] shows that the maximum gamma index >1 is found for catheter_40 and catheter_60. They equals to 1694 and 2796, respectively, and they are located at (LR = 0, TG = 25) and (LR = 0, TG = 30). For the absolute dose difference, all the maximums are at 0 or 5 mm close to the radioactive source axe. By analyzing the profiles in [Figure 13], the absolute dose difference shows the same behavior as above with less difference their maximum is 0.97 Gy. The results are logical for the same reason above.
Thus, we confirm the previous results of the comparison. These verification results demonstrate the commissioning accuracy, the catheters can be used for the clinical purpose and the two verification methods can be used for brachytherapy plans delivery quality assurance.
Financial support and sponsorship
Nil.
Conflicts of interest
There are no conflicts of interest.
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[Figure 1], [Figure 2], [Figure 3], [Figure 4], [Figure 5], [Figure 6], [Figure 7], [Figure 8], [Figure 9], [Figure 10], [Figure 11], [Figure 12], [Figure 13]
[Table 1], [Table 2], [Table 3], [Table 4], [Table 5], [Table 6]
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