A COMPARISON BETWEEN GEANT4 AND MCNPX ON THE DOSIMETRY OF THE 192IR MICROSELECTRON V2 HDR BRACHYTHERAPY

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1 2011 International Nuclear Atlantic Conference - INAC 2011 Belo Horizonte,MG, Brazil, October 24-28, 2011 ASSOCIAÇÃO BRASILEIRA DE ENERGIA NUCLEAR - ABEN ISBN: A COMPARISON BETWEEN GEANT4 AND MCNPX ON THE DOSIMETRY OF THE 192IR MICROSELECTRON V2 HDR BRACHYTHERAPY Maximiano C. Martins 1, Artur F. de Menezes 1, Ademir X. da Silva 1, Denison de S. Santos 2 e Pedro P. de Queiroz Filho 2, Helio Salmon 3 and Guilherme Pavan 3 1 Programa de Engenharia Nuclear PEN \ COPPE Universidade Federal do estado do Rio de Janeiro Centro de Tecnologia, Cidade Universitária, Ilha do Fundão CEP Rio de Janeiro, RJ, Brasil maximiano@gmail.com ademir@con.ufrj.br arturfismed@yahoo.com.br 2 Instituto de Radioproteção e Dosimetria (IRD / CNEN - RJ) Av. Salvador Allende s/n - Jacarepaguá CEP Rio de Janeiro RJ, Brasil santosd@ird.gov.br queiroz@ird.gov.br 3 Clínicas Oncológicas Integradas (COI) MD.X Barra Medical Center Av. das Américas, Loja E - Barra da Tijuca CEP Rio de Janeiro - RJ guilhermepavan@coinet.com.br heliosalmon@coinet.com.br ABSTRACT The purpose of this work is to study the dosimetric parameters of the 192Ir microselectron V2 High Dose Rate (HDR) Brachytherapy source. Two Monte Carlo codes, Geant4 and MCNPX, were used to simulate the dose quantities recommended by American Association of Physicists in Medicine (AAPM) Task Group 43 (TG-43). The geometry source, detectors to collect the information of interest and the water phantom were simulated. The absorbed dose and the air-kerma strength were calculated to obtain the dose rate constant. The electrons influence are evaluated for different cutoffs of energy and in the vicinity of the source where there is a lack of electronic equilibrium. 1. INTRODUCTION The radiotherapy is a therapeutic procedure capable of destroying a tumor cell using a radiation beam. A radiation dose previously calculated is applied, at a given time, to a tissue volume encompassing the tumor to destroy the tumor tissue while producing the least possible damage to adjacent normal tissues [1].

2 The brachytherapy consists of a radiotherapy technique in which one or more radioactive sources consisting of an outer encapsulation of high atomic number are positioned inside or near the tumor volume allowing the treatment optimization while providing the lowest dose to normal adjacent tissues [1]. The radioisotope Iridium-192 has been widely used for high dose rate brachytherapy (HDR) allowing the treatment of small tumor volumes effectively and accurately reaching the clinical requirements necessary for the total destruction of tumor tissue [2,3,4]. There are currently several manufacturers of Iridium-192 sources which allowed, due to its favorable dosimetric properties, its consolidation as a complementary tool to external beam radiotherapy treatment for different types of tumors such as prostate tumors, breast, anus, cervix, ovary, among others. Another advantage of the brachytherapy is the use of the afterloading system remotely, which allows the collection of the source after treatment without the presence of the staff and other workers in the treatment room, avoiding undesirable occupational exposure. The activity of the sources can range from 1 to 10 Ci [4], allowing the treatment be performed in a few minutes enabling the quickly release of the treated patient. Recently, several studies in brachytherapy have been directed to the study of dose distributions around the sources. In parallel to experimental work the American Association of Physicists in Medicine (AAPM TG - 43) [5,6] recommends the study and characterization of the dosimetric parameters of brachytherapy sources using Monte Carlos Code in heterogeneous media which allows the reduction of uncertainties associated with the value of the dose delivered to the target volume, always trying to minimize the damage to the patient undergoing these techniques [7,8,9,10,11,12]. 2. METHODOLOGY 2.1. GEANT4 The Geant4 (Geometry and Tracking) [13] is a package of computational tools freely distributed by CERN (European Nuclear Research Center) to simulate the passage of the radiation (particle or electromagnetic wave) in the matter. In this study we used the Monte Carlo code Geant4 Standard version 9.4. We considered the following interaction processes: Compton Scattering, Gamma Conversion, Photo Electric Effect for photons, Multiple Scattering, Ionisation and Bremsstrahlung for electrons and positron and the latter was also used eplusannihilation MCNPX The particle transport code MCNPX (version 2.5) [14] used in this work is a combination version of the code MCNP4C LAHET (Los Alamos High-Energy Transport), which

3 simulates the transport and interaction of nucleons, muons, pions and light ions in complex geometries and extends the capabilities of MCNP. This code contains all the capabilities of the MCNP4C and MCNP5 codes and can simulate the transport of photons, electrons, neutrons, protons, light ions, and processes of interaction of charged particles with matter to the wide range of energies [15] TLD For this work 15 TLD-100 Harshaw dosimeters and the reading equipment belonging to the Semex-IRD-CNEN were used. The calibration, reading and evaluation were performed by the person responsible for the external dosimetry sector. The TLDs were separated into five groups of three units (Figure 1). Figure 1. Badge containing 3 TLDs Source HDR from Clínicas Oncológicas Integradas (COI) The brachytherapy source used in this study was the Ir-192 HDR MicroSlectron V2 (Figure 2), which has 0.9 mm diameter of encapsulation and 4.5 mm in length and an iridium core of 0.65 mm in diameter and 3.6 mm in length. The source used in this work belongs to the IOC (Integrated Clinical Oncology) located in Barra da Tijuca Rio de Janeiro. At the time of the simulation the source activity was Ci (Figure 3).

Figure 2. 192 Ir MicroSelectron V2 HDR Source Figure 3. The Nucleotron AfterLoading System. 2.5.

acrylic wall thickness of 8 mm (Figure 4).

4 Figure Ir MicroSelectron V2 HDR Source Figure 3. The Nucleotron AfterLoading System Water Phantom The experimental measurements were performed using a cubic phantom, filled with water, with dimensions of 40x40x40 cm ³ consisting of an acrylic wall thickness of 8 mm (Figure 4). For the experimental measures 3 TLDs were positioned in the central region of each lateral side of the phantom mentioned. The irradiation was performed with Ir-192 source positioned within a catheter fixed by three nylon wire in the central region of the phantom filled with water. We used an irradiation time of 452 s.

5 Figure 4. View of the experimental apparatus used for the correct positioning of the Ir-192 source within the water phantom Validation of IR-192 Source Benchmark with Geant4 and MCNPX to obtain the dose rate constant dosimetric parameter of the HDR source 192 Ir MicroSelectron V2. In this work we used the GEANT4 and MCNPX codes based on the Monte Carlo method to simulate the 192 Ir MicroSelectron V2 HDR brachytherapy source. From the modeling of the geometry of the source and using the emission spectrum of the 192 Ir was possible to obtain the dose rate constant dosimetric parameters, which is a crucial for treatment planning using high dose rate brachytherapy (HDR). The motivation for this work were based on the recommendations of the Task Group No. 43 Protocol of "American Association of Physicists in Medicine" AAPM regarding the importance of using the Monte Carlo method for reducing uncertainties associated with the delivered dose to the brachytherapy treatment. The results obtained by both codes agree with other results in the literature using the Monte Carlo method.

6 Simulation of the dose rate Constant dosimetric parameter Λ = D (ro,θ S k o ) The dose rate constant depends of the radionuclides and the source model used and is influenced by the internal geometry of the source and the experimental methodology used for the determination of SK. Unit standard: cgy / HU. The determination of the dose rate constant ( Λ ) using the Monte Carlo Method is obtained by the ratio between the dose rate (ro,θo ) obtained at 1 cm from the transverse axis of the source P(ro,θo ) by the amount of ar kerma Sk at 100 cm of the source (Figure 5). D Figure 5. Coordinate system used for dosimetric calculations in brachytherapy. The intensity of kerma is defined as the product of kerma rate K ar (d), at a distance d, measured along the transverse axis of the source multiplied by the square of the distance d according the following equation: S k = K ar (d) d 2 (1) Its numerical value is identical to the reference rate of air kerma (Reference Air Kerma Rate, or AKR). Unit standard: 1 Gy m 2 h -1 = 1U The distance measurement should be large enough so that the distance d is greater than the linear dimension of the source so that Sk is independent of the distance d, thus the air kerma can be given in terms of the reference distance, which is commonly used as the distance of 1 m.

7 Calculation of Dose Rate constant and Kerma Intensity The dose rate D (ro,θo ), using the Geant4 code, was obtained considering detectors in the form of concentric rings arranged at 1 cm from the source positioned at the origin of the coordinate system instead of single voxel detector positioned at P(ro,θo ) as illustrated in Figure 6. The simulation was performed considering 10 9 events. This methodology was used to optimize the computer simulation time. Considering that the source is isotropic, it is possible to obtain a greater number of events with detectors arranged in a ring shape and positioned in the region of interest. The intensity of air kerma was calculated taking into account the recommendations of Taylor et al 2008 [12] and Rogers et al 2007 [9] with the detector positioned 1 m from the source. Figure 6. View of the methodology used to calculate the dose rate at 1 cm from the source. The particle count for the fluency calculation were made on 4 detectors in the form of voxels with dimensions of 10 x 10 x 0.05 cm 3 positioned 1 m symmetrically arranged around the source as illustrated in Figure 7.

Figure 7. View of the geometry used to determine the intensity of kerma Sk. 2.6.4.

8 Figure 7. View of the geometry used to determine the intensity of kerma Sk Dose rate constant using the MCNP To determine the dose rate constant using the MCNP code was used an idealized methodology. The dose rate constant was obtained by the ratio between the dose rate, obtained by a point detector (tally F5) positioned at 1 cm from the source considering the simulator medium consisting of water (Figure 8), by the kerma intensity measured at 100 cm from the irradiation source considering the medium simulator as vacuum as show in figure 9. Figure 8. Illustration of geometry used to determine the dose rate at 1 cm from the transverse axis of the source in a simulator consisting of water.

Figure 9. Illustration of geometry used to determine the kerma intensity at 100 cm from the transverse axis of the source in a simulator consisting of vacuum. 2.6.5.

9 Figure 9. Illustration of geometry used to determine the kerma intensity at 100 cm from the transverse axis of the source in a simulator consisting of vacuum Spectrum of Ir-192 source using the code MCNP The 192 Ir spectrum was determined using the tally F4 on a spherical surface of radius 5 cm considering the medium simulator consisting of vacuum with the source positioned at the origin of the coordinate system. 3. RESULTS Figures 9 and 10 show the images, obtained using the graphic viewer Moritz and OpenInventor, for sources modeled using Geant4 and MCNPX Monte Carlo codes respectively. Figure 9: View, using the OpenInventor graphic viewer, of the 192 Ir source modeled using the Geant4 code.

10 Figure 10. View, using the Moritz graphic viewer, of the Ir-192 source modeled using the MCNP code Benchmarking of Simulated Spectra The energy spectrum of photons (Figure 11) was weighted according to the following equation 2: (dr (Ej) / de) / R = Σ Ei / (ΔE R) ( 2) Where: Ej is the energy of the middle of the bin, Ei is the energy of the ith photon that escape of the encapsulation or cable with energies between Ej ± ΔE / 2 and ΔE is the bin width. The spectrum is normalized to the total energy radiated by the source, R (total energy of the photons to escape the encapsulation or cable) per unit MeV -1. Results of both codes were compared with those obtained by Rogers et al using the EGSnrc code.

11 Figure 11: Graphs showing the spectra comparison obtained from different Monte Carlo codes.

12 3.2. Comparative Study of Dose Rate Constant Simulated Using the Geant4 and MCNP Table 1 presents the results of dose rate constant obtained for a distance of 1 cm from the 192 Ir source compared with different results presented in the literature. Also are presented a comparative study of the ratio Dose/Sk, recently used as a reference in the literature, obtained for the distance of 0.2 cm from the source. Table 1. The dose rate constant. Author Method Λ (cgy h-1 U-1) Abs. Uncertainty R. E. P. Taylor, D. W. O. Rogers [10,9] ro= 1 cm; voxel at 100 cm ro = 0.2 cm; voxel at 100 cm G. M. Daskalov et al [3,2] extrap Ruqing Wang, X. Allen Li [11] ro = 0.2 cm; voxel at 100 cm This Work Geant4 ro = 1 cm; voxel at 100 cm This Work Geant4 ro = 0.2 cm; voxel at 100 cm This Work MCNPX ro = 1 cm; voxel at 100 cm This Work MCNPX ro = 0.2 cm; voxel at 100 cm Experimental measurements using water phantom and TLDs dosimeters We also simulated a geometry of a cubic water phantom (as shown in Figure 4), with 4 sets of TLDs and results were compared with experimental measurements performed in the radiotherapy service belonging to COI (Clínica Oncológica Integrada). The results of the air kerma obtained with the Geant4 code were compared with experimental results obtained with TLD dosimeters.

13 Because of the TL dosimeters used, belonging to the Institute of Radioprotection and Dosimetry (IRD), were calibrated in a Cesium-137 source, It was necessary to apply a correction factor for energy dependence on the simulated results. After correcting the results, the Geant4 value was closer to the experiments with 9.19 % difference, as shown in Table 2. Table 2. Monte Carlo X Experimental Arrangement Experimental Kerma (cgy) TLD (cgy) Error (%) With energy dependence correction (Geant4) Without correction (Geant4) MCNPX CONCLUSION The results for the modeling of the Ir192 Nucletron MicroSelectron V2 HDR sources using the Geant4 code obeying the recommendations of the TG43 dosimetry protocol proved to be satisfactory given the small discrepancy with the results obtained by other studies in the literature. The results obtained for the MCNPX code showed a greater discrepancy when compared with the literature. Possible differences to the results were already expected, since a different methodology was adopted. However a deeper analysis of the geometry and the methodology used should be performed. The spectrum obtained using the code Geant4 showed satisfactory results with the literature. The spectra obtained using the MCNP code showed a slightly larger discrepancy, however, there was a coincidence of major peaks of emission and the dose rate constant present in the order of the literature results. In general, the results obtained using the Geant4 code had minor discrepancies when compared with the results obtained in the literature. The results obtained using the MCNP code showed a major discrepancy, which shows that the idealized methodology used in this work may have contributed to the largest discrepancies in the results when compared with the literature.

14 REFERENCES 1. INCA (2011) 2. G.M. Daskalov, Erratum: "Monte Carlo-aided dosimetry of a new high dose-rate brachytherapy source" [Med. Phys. 25, (1998)], Med. Phys., 27, , G.M. Daskalov et al, Monte Carlo-aided dosimetry of a new high dose-rate brachytherapy source, Med. Phys., 25, , J.F. Williamson and Zuofeng Li, Monte Carlo aided dosimetry of the MicroSelectron pulsed and high dose-rate 192Ir sources, Med. Phys., 22, No 6 (1995) 5. R. NATH, et al., Dosimetry of Interstitial Brachytherapy Sources, Published for the American Association of Physicists in Medicine by the American institute of Physics, AAPM Report No. 51, Report Of AAPM Radiation Therapy Committee Task Group M.J. RIVARD, et al., Update of AAPM Task Group No. 43 Report: A revised AAPM protocol for brachytherapy dose calculations, Med. Phys. 31 (3), , March F. Lliso et al, Fitted dosimetric parameters of high dose-rate 192Ir sources according to the AAPM TG43 formalism, Med. Phys., 28, , F. Lliso et al, Erratum: "Fitted dosimetric parameters of high dose-rate 192-Ir sources according to the AAPM TG43 formalism" [Med. Phys. 28(4), (2001)], Med. Phys., 28, , R. E. P. Taylor et al, Benchmarking BrachyDose: voxel-based EGSnrc Monte Carlo calculations of TG-43 dosimetry parameters, Med. Phys., 34, , R. E. P. Taylor, D. W. O. Rogers, EGSnrc Monte Carlo calculated dosimetry parameters for 192Ir and 169Yb brachytherapy sources, Med. Phys., 35, , Ruqing Wang, X. Allen Li, Dose characterization in the near-source region for two high dose rate brachytherapy sources, Med. Phys., 29, , R. E. P. Taylor and D. W. O. Rogers Nucletron, MicroSelectron- V2, The CLRP TG43 Database HDR Carleton Laboratory for Radiotherapy Physics (2008) AGOSTINELLI, S., PIA, M. G.,NIEMINIE, P., The application of Geant4 Simulation code for Brachytherapy Treatment. Proceedings of the IXth International Conference on Calorimetry in High Energy Physics, Calor 2000, Annecy, Briesmeister, J. F., MCNP: A General Monte Carlo code for neutron and photon transport. Version 4C. Los Alamos National Laboratory report LA M, March Bozkurt, A., Assessment of Organ Doses from Expousure to Neutrons Using the Monte Carlo Technique and an Image-Based Anatomical Model, Doctor Thesis, Rensselaer Polytechnic Institute Troy, New York, 2000.

Monte Carlo simulation of 192 Ir radioactive source in a phantom designed for brachytherapy dosimetry and source position evaluation.

Monte Carlo simulation of 192 Ir radioactive source in a phantom designed for brachytherapy dosimetry and source position evaluation Samuel Chiquita 1 1 University of Porto, Porto, Portugal Abstract In