ELECTRON AND NEUTRON CONTAMINATION IN VARIAN TRILOGY CLINAC iX PHOTON BEAM

• Main Author: YANI (NIM : 30212004), SITTI
• Format: Dissertations
• Language: Indonesia
• Online Access: https://digilib.itb.ac.id/gdl/view/24341
• Institution: Institut Teknologi Bandung

Contaminating particles (electrons and neutrons) are produced in a head linear accelerator (Linac) by photon-material interactions with head linac components. These particles contamination influence the dose distribution in target and become major contribution in surface dose. To investigate the characteristics of particles contamination, the detailed information about the geometries and material from each component in the head Linac should be provided. From some literatures, there are three methods to determine electron contamination, e.g. direct measurement with a magnet, analytical method and Monte Carlo (MC) simulation. Nowadays, MC simulation provides an accurate and good method to investigate the particle contamination. Therefore, the aim of this study is to determine the characteristics of contaminant electrons and neutrons from head of Varian Clinac iX 6 MV photon beam using MC simulation for small and large field sizes. <br /> <br /> <br /> This research consist of 4 steps. The First step was the commissioning process. This process was to compare the dose distribution between simulation and measurement data from Tan Tock Seng Hospital (TTSH) Singapore to obtain the incident electron energy for 6 and 10 MV photon beam. MC simulation for commissioning head Linac was divided into three stages are design head Linac model using BEAMnrc, characterize this model using BEAMDP and analyze the difference between simulation and measurement data using DOSXYZnrc. In the first step, to reduce simulation time, a virtual head linac was built in two parts (patient-dependent components and patient-independent components). The incident electron energy varied between 6,1; 6,2; 6,3 and 6,4 MeV for 6 MV photon beam and 10,1; 10,2; 10,3 and 10,4 MeV for 10 MV with FWHM (full width at half maximum) of source was 1 mm. Phase-space file was from the virtual model characterized using BEAMDP. The results of MC calculations using DOSXYZnrc in water phantom were percent depth doses (PDDs) and beam profiles at depths 10 cm were compared with measurements. This process has been completed if the dose difference of measured and calculated relative depth-dose data along the central axis and dose profile at depths 10 cm was &#8804; 5%. The effect of beam width on percentage depth doses and beam profiles was studied. Results of the virtual model were in close agreement with measurements in incident energy electron 6.4 MeV and 10.3 MeV for 6 and 10 MV, respectively. This results showed that photon beam width could be tuned using large field beam profile at the depth of maximum dose. MC model developed in this study accurately represents the Varian Clinac iX with millennium MLC 120 leaf and can be used for reliable patient dose calculations. <br /> <br /> <br /> The second step was simulating electron contamination using EGSnrc code system. Electron contamination plays an important role in surface dose. Dose of the contamination for a 6 MV photon beam was at 39,3%; 28,2% and 16,4% respectively at depths of 0,3; 0,5 and 0,7 cm from the surface of the phantom for a wide field of 10×10 cm2. This value was obtained by comparing the dose of the contamination of electrons to a dose of all particles from phsp file. Doses due to contamination of electrons in cases with extensive grounds were small (less than 6 cm) has a small value. For the field of 4×4 cm2 was found that the dose of the contamination was 13.6% on the surface of the water phantom. <br /> <br /> <br /> The penumbra width (region at the edge of field size which dose rate changed rapidly from 80% to 20%) for all small field size for 10 MV photon beam was around 0,4 – 0,6 cm. It is important to point out that the depth with dose maximum (dmaks) for PDD curve has a little bit shifted. For the smallest field size 1×1 cm2, it was found to have a maximum depth of 2,7 cm, whereas for the largest field size 5×5 cm2, the depth has increased to 2,1 cm. This shift in the maximum depth, corresponded with the number of scatter particle. <br /> <br /> <br /> Third, comparing the spectral distribution, statistical uncertainty and simulation time between EGSnrc and MCNPX code system. Comparison between EGSnrc and MCNPX on simple geometry (X-ray target 6 MV photon beam) was also performed to compare the spectral distribution of photon, statistical uncertainty and simulation time. The output particles analyzed in this simulation was photon. The results showed that the spectrum from EGSnrc and MCNPX was not well-matched. It was under 5% differences, especially in the build-up region. The spectral distribution chart was a visual representation of the X-ray spectrum produced by a 6 MV photon beam target. The 100% peak of EGSnrc and MCNPX spectral distribution were 0,245 and 0,250 MeV, respectively. Meanwhile, the spectrum has the same shape and value in the tail region with the difference not more than 2%. <br /> <br /> <br /> The last, neutron contamination simulation for Varian Clinac iX 6, 10 dan 15 MV photon beam was performed using MCNPX. Neutron contamination was not found in 6 MV photon beam. These contaminants were found only in 10 and 15 MV formed mainly on X-ray target, primary collimator, vacuum window, flattening filter (FF), JAWS X and Y and MLC. Contamination neutrons contributed to the photon beam although most of them were scattered to treatment room. Neutrons energy in scoring plane within 100 cm from the target were 2,239 and 4,467 MeV for 10 and 15 MV photon beam, respectively. Neutrons with these energy is dangerous for patients and radiation workers since neutrons have a high relative biological effectiveness (RBE) and cannot be ignored. This study increased our knowledge of the clinical photon beams and associated contaminant electrons and neutrons for large and small field sizes. It demonstrated the accuracy of the Monte Carlo technique in simulating these contaminations. The higher the energy head Linac and filed sizes will be even greater, which means more dangerous for patients and radiation workers. <br />