THE SIMULATION OF THE 4 MV VARIAN LINAC WITH EXPERIMENTAL VALIDATION

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1 2007 International Nuclear Atlantic Conference - INAC 2007 Santos, SP, Brazil, September 30 to October 5, 2007 ASSOCIAÇÃO BRASILEIRA DE ENERGIA NUCLEAR - ABEN ISBN: THE SIMULATION OF THE 4 MV VARIAN LINAC WITH EXPERIMENTAL VALIDATION Renato J. Reis and Tarcísio P. R. Campos Programa de Pós-graduação em Ciências e Técnicas Nucleares (PCTN/UFMG) Av. Antônio Carlos, 6627, Prédio PCA Belo Horizonte, MG renatojulio@gmail.com campos@nuclear.ufmg.br ABSTRACT The Radiation Therapy is a technique that uses ionizing radiation for controling tumors. The main equipments for shallow tumors are the bombs of cobalt (Co-60) and in greater scale the linear accelerators megavoltage 4MV. These equipment delivers a high dose rate relatively on the range of 0,5-4Gy/min, in situ, superficial or in moderate depth. Thus, they become a good option for the treatment of some types of cancerous injuries. The simulation of the entrances of the beams in physical simulator and isodoses curves estimating previously are effected. However, on the computational planning, equipment structure simulations are not used in the routine of the planning maded by The technician of the Radiation Therapy. In the present paper, a computational model of a Linear Accelerator RF of 4MV was developed, based on an equipment of the Varian manufacturer installed in the Clinic Son Francisco in Belo Horizonte. The Monte Carlo Code, MCNP5, was applied to reproduce the profile of the beam, and the irradiation of a water phantom box. The depth dose profile (PDP) was produced on the simulation and compared with experimental calibration data made on the existing equipment of the Clinic Sao Francisco. The results of dose in depth will be presented, matching the experimental data with a correlation factor of 0,9986 (on the range of 0.5 to 15cm) that prove the efficiency of the computational model developed. 1. INTRODUCTION The project of the Linear Accelerators for X-ray generations varies in according to the beam output requirements. For low energies, that is, of 4 to 6 MeV, these accelerators are used exclusively for production of X-rays. For the Salvajolli et al, to generate x-rays of higher energies, uses one technique of electron acceleration, without necessity of high differences of potential between two electrodes. This is the basic principle of functioning of the linear accelerators, using itself, however waves of radio frequency (3000 RF) of MHz, that as all the electromagnetic radiations, are alternating fields - electric and magnetic. As an electric field applies a force in a placed loaded particle in it, if an electron, or electron grouping, is injected in a beam of waves RF in an appropriate place and certain time, it will be subject to this force and will tend to be taken by the wave. These waves special RF proceeding from valves, calls or klystron are microwaves generated in small pulses that they are sent, through a wave guide, to a cylindrical pipe that possess in its interior some metallic records with a small orifice in the way. This tube has the name of accelerating tube and it is where the electrons are sped up until the desired energy.

2 When these sped up electrons leave the accelerating tube, normally on a parallel beam of approximately 3 mm of diameter, they are directed toward a metallic target (normally tungsten) in order to product breemstrahlung radiation. The electron beam collides with the target, and part of the kinetic energy is converted into heat and the other part in x-rays. An example of the project of a linear accelerator, from Varian Clinac manufacturer, is presented in figure 1. This model shows the head of the accelerator with the accelerator structure incorporated., In this version, there are magnetos that make shunting lines in the trajectories of the sped up electrons. Figure 1 Ilustration of a real 4MV linac accelerator 2. MODELING BASED ON A MONTE CARLO CODE The Monte Carlo Technique, presented on the MCNP5 or GEANT4 codes, is recognized as an accurate method to predict the absorbed dose into the patients [1]. Moreover, it can also be used in the simulations of nuclear particle accelerators [2, 3]. On the MCNP5 code, the random nature of the neutral nuclear particle interactions with the matter is simulated. The trajectory of the particle is simulated until its energy is worthless or that it has left the geometry of interest. The exactness of the Monte Carlo Technique, applied on MCNP5 code, depends mainly on the information of the initial condition of the radiation transport, the materials and the geometry of the installation. In order to apply the Monte Carlo method in the x-ray beam generation, first it is necessary to know the profile of the radiation beam produced by the linear accelerator; and, second, the distribution of dose in the patient or water fantoma. Those data should be used to calibrate the LINAC simulation. Thus, the experimental accurally characterization of the entrance beam on a water phantom and its PDP will be important to validate any simulation Development of the linear accelerator model Using Code MCNP5, that understands a statistical method for the forecast of interaction taxes, transference of energy for interaction and the trajectory of incident particles. [4]

3 A linear electron accelerator was shaped with energy of 4MV. All the structures had faithfully been represented having as reference the commercial model of the Varian Manufacturer installed in the Institute of São Francisco in Belo Horizonte. The geometry used in the model is represented by figure 2. Figure 2- Ilustration of the internal geometry of the 4MV Linac Acellerator. For development of the computational model, all the geometric data involving the physical structures had been taken from the manual that folloies the equipment. In figure 2, the structures are represented in distinct colors, and all the distance had been evaluated taking the isocentric as origin (0 cm). The target is in blue and locates at 80 cm from table s surface. Below the target, at 78 cm, there is primary collimators, represented in red on Fig.2. After that, at 77cm, there is the flatness filter in green, and finally there are the secondary collimators in yellow, placed at 73 cm Geometry of the computational model The structures of the head of the accelerator had been shaped following the geometry presented for the manufacturer in its manual [Ref]. To better understanding, figure 3 and 4 show the model developed in MCNP5 to be used in the simulation of the PDP on the water phantom. (a) Figura 3- (a) Cross Section view taken from the graphic interface of the MCNP5, (b) three dimensional view of the LINAC modeling. (b)

4 2.3. Simulation of the Deep Dose Profile The Deep Dose Profile (PDP) is nothing more than a percentage relation of the absorbed dose in an arbitrary depth, in relation to the maximum absorbed dose in the depth in which electronic balance occurs. The value of the PDP was taken on the lookup tables used for the Physicists in the routine of treatment for x-ray for photons in the São Francisco Radiotherapy Institute. The simulation data for the PDP on phantom water was plotted together, as shown in Fig.4. The simulation provides data on all points with deviation less than 5%. The first series corresponds the simulation of the beam for a field of 10 x 10 cm 2 in the surface of fanton. Percentage of the Dose Percentage of the dos Teorical Simulation Depth (cm) Figure 4 Deeth Dose Profile (PDP) in function to the depth for the 4MV VARIAN LINAC and PDP adopted to the Sao Francisco Radiotherapic Institute for equivalent equipment. 3. CONCLUSIONS The structures of the geometry of the linear accelerator head has dimensions in agreement to the physical model installed in the São Francisco Hospital and maintain the equivalent geometry and materials. The model can be used as a virtual model for reproducing the photon beam. Results from the simulation have demonstrated that the Deep Dose Profile, with the 10x10 field size, agree with experimental data with a correlation factor R 2 of 0,9986., validating the PDP simulated. The characterization of the beam (the beam parameters produced by the accelerator) was found, and can be applied in other simulations in computational dosimetry. REFERENCES 1. Mohan R, Why Monte Carlo? XII Int. Conf. on the Use of Computers in Radiation Therapy. Salt Lake City- Utah, May 1997, pp McCall R. C., Improvement of linear accelerator depth-dose curves Med Phys

5 3. Sätherberg A, et al. Calculation of photon energy and dose distributions in 50 MV scanned photon beam for differente target configurations and scan patterns Med Phys Rogers et al, Monte Carlo techniques of electron and photon transport for radiation dosimetry. San Diego,CA: Academic Press, pp