MAGNETIC FIELD INFLUENCE TOWARDS THE PHOTON BEAM DOSE DISTRIBUTION OF LINAC SIEMENS ONCOR 6 MV USING MONTE CARLO SIMULATION (TOPAS)

• Main Author: Faizatul Azimah, Alfia
• Format: Theses
• Language: Indonesia
• Online Access: https://digilib.itb.ac.id/gdl/view/71735
• Institution: Institut Teknologi Bandung

The influence of strong magnetic field towards particle radiation in MRI-Linac causing the changes of charged particle trajectories with the re-entering of secondary electron into the medium, known as electron returning effects (ERE) hence affecting to the changes of dose deposition inside the irradiate medium. The major objective of this study is to investigate the influence of magnetic field towards particles of radiation beam and the dose deposited by the particles in water phantom. The magnetic field and radiation beam were modeled as the representation of MRI-Linac, based on Monte Carlo simulation. Tool for Particle Simulation (TOPAS) was used as the Monte Carlo based software to perform the simulation. The 6 MV X-Ray from Linac Siemens Oncor treatment head modelling were simulate to radiate the 40 cm x 40 cm x 40 cm water phantom. Magnetic field was set in transversal and longitudinal direction concerning the beam and influenced the water phantom with 1.5 T, 3 T and 7 T of strength. The simulation was executed with photon beam as phase space consisted of 4×109 particles with 10 cm x 10 cm field size. Electron trajectories were visualized with the TOPAS graphic window. The PDD curve and dose profile curve were plotted using Matlab R2021b to analyze changes in electron trajectories and radiation dose due to the influence of a magnetic field compared to linac without a magnetic field. The visualization of electron trajectories consisted of the electron as the particle contamination in the photon beam towards the water fantom, and the electron as the secondary particles generated by photon particles interaction with the water phantom. The electron trajectories of linac with transversal magnetic field simulation was found visualized with circular mtion with some of the electron contaminations were deflected, while the ERE was found in the secondary electron as the range of electron trajectories towards the depth of water phantom were decreased. In contrast, the longitudinal magnetic field was found refering the electron contamination towards the water phantom, and did not influence the secondary electron becoming ERE inside the water phantom. The surface dose (Ds) and the depth at maximum dose (zmax) of water phantom irradiation by linac without magnetic field were 44.057% and 1.7 cm. The Ds were found to be increase linearly -0.1756 of gradient and ???? ? = 0.8471, whereas the zmaz were decreased with 6.709 of gradient amd linearity ???? ? = 0.9238 due to the transversal magnetic field strength increment. The Ds for each 1.5 T, 3 T, and 7 T transversal magnetic field changed into 41.132%, 58.9506%, and 86.8493% while the zmax shifted to be 0.9 cm, 0.9 cm, and 0.3 cm. Whereas the longitudinal magnetic field strenght increment did not affect the Ds with 1.0529 of gradient and ???? ? = 0.3997 along with unaffected linearly of zmax with -0.0202 of gradient and ???? ? = 0.2782. The increment of Ds for 1.5 T, 3 T and 7 T longitudinal magnetic field was due to the electron contamination, becoming 54.1854%, 53.9892%, and 54.0715%, with 1.5 cm, 1.7 cm, and 1.5 cm of zmax position. Compared to the height of dose profile by water phantom irradiation with linac without magnetic field, the height of dose profile in water phantom with transversal magnetic field influence increased at the surface, and at the 1.5 cm depth, whereas 10 cm depth the height was decreased. Otherwise, the longitudinal magnetic field causing the increment of the dose profile height at the surface and at the 1.5 cm depth, but did not varied due to the magnetic field streght, beside the dose profile at 10 cm depth did not change. These results indicate the presence of ERE as the effect of the trajectories changes by secondary particles affecting the dose distribution on the radiation target. The linac with transverse magnetic field influenced substantially compared to the longitudinal magnetic field configuration of MRI-Linac.