ANALYSIS OF NEUTRON BEAM CHARACTERISTICS ON 10 MV LINAC HEAD USING MONTE CARLO SIMULATION (TOPAS)

• Main Author: Walyatalattov Solichin, Alwali
• Format: Theses
• Language: Indonesia
• Online Access: https://digilib.itb.ac.id/gdl/view/80241
• Institution: Institut Teknologi Bandung

The growing number of radiotherapy centres in Indonesia means a more effective and accurate protection measure needs to be implemented. Neutron contamination is one of the side effects of high-energy photon radiation in radiotherapy treatment using linear accelerators. This study aims to comprehend the neutron particle contamination on high-energy linear accelerator head. Monte carlo simulations using TOPAS 3.9 will be used to determine neutron contamination at 10 MV photon radiation. A Siemens Oncor head LINAC will be simulated to generate two phasespace. The first phase space is commissioned above the jaws, and the second phase space is below the MLC for a 10 x 10 cm2 field size. The phase space itself has a 20 x 20 cm size. The first phase space spent 4 hours to simulate a 107 original electron particles and generate 11 x 107 particles that contain 146 neutron particles. The second phase space uses the output from first phase space as the source for simulation, spending 70 hours to simulate 25 x 107 particles and generate 4497 neutron particles. The irradiation field is also varied at 5 x 5 cm2 and 15 x 15 cm2 which produces 4,298 and 4,462 neutron particles, respectively. The graph also illustrates that a small radiation field will cause a sharp spike in the graph in the area around Y = 0 cm. In the particle fluence and energy graphs, it can be seen that the wider the irradiation field, the graph will be more sloping in the same area, this shows that increasing the radiation field area, the beam will be more evenly distributed. In the angular distribution, it can be seen that in a smaller field, it will cause relatively sharp peaks and valleys, while in a larger field the shape will be wider. This can be seen at angles around 800 - 1000 while the angular distribution looks even with peaks at several angles from 70 - 1630 . In the spectral distribution graph, it can be seen that the largest energy range is at 0.178 - 0.676 MeV with the average energy in the small to large field variation successively 0.456; 0.447; 0, 453 MeV. The addition of electron history increases the quantity of particles produced in the second phase space, including neutron particles. Material, photon energy, and electron history all play crucial roles in the creation of neutron contamination on the LINAC head. This illustrates that the neutron beam is dominated by epithermal neutrons. This preliminary data can be used as a basis to simulate further variations of radiotherapy treatment such as dose distribution, field shape variation, and field size variation.