Calculation of the dosimetric parameters of ۱۸ MV photon beam from flattening filter (FF) and flattening filter free (FFF) of linear accelerator with and without the magnetic deflector (MD) and lead filter

Background or Introduction: The presence of charged particles in therapeutic photon beams can cause contamination, leading to a shift in maximum dose depth, an increase in surface dose, and a reduction in skin-sparing effectiveness. Electrons and positrons are generated through photon interactions within the LINAC head and air volume. The flattening filter (FF) and beam monitor chamber are primary sources of electron contamination. Removing the FF can decrease head scatter, increase dose rate, and reduce electron and neutron contamination. Reducing field size, using magnetic fields, replacing air with helium, and using lead filters can reduce electron contamination. Using the Monte Carlo technique, dosimetric properties were compared in FF and flattening filter-free (FFF) modes of ۱۸ MV- ۲۱۰۰ C/D Varian LINAC’s head. The magnetic deflector and lead filter were evaluated for electron contaminant reduction in FF and FFF modes.Material and Methods: The Monte Carlo (MC) code, MCNP version ۶.۱.۰, was utilized to simulate the ۱۸ MV photon beam of a ۲۱۰۰ C/D-Varian linear accelerator (LINAC) for both the flattening filter (FF) and flattening filter free (FFF) modes. The simulation included modeling a magnetic field of ۱ T and a lead filter with a thickness of ۱ mm to remove contaminant electrons. The study aimed to calculate the dosimetric parameters for various scenarios of the LINAC's head.Results & Discussion: The study found that removing the flattening filter in the flattening filter free (FFF) mode resulted in an increase in the dose rate, electron contamination, skin dose, and un-flatness compared to the flattening filter (FF) mode. However, the off-axis dose was reduced. Using a lead filter significantly reduced contaminant electrons, while the magnetic deflector (MD) removed all secondary electrons from the beam line. In the FF mode, the surface dose was decreased by ۴.۵% and ۶.۱% for the MD and lead filter, respectively. In the FFF mode, the surface dose was reduced by ۵.۹% and ۴.۴% for the MD and lead filter, respectively. The MD and lead filter also reduced penumbra by ۱۵.۵% and ۱۱.۵% compared to the FFF mode. These findings suggest that using a lead filter or MD can effectively reduce electron contamination and improve dosimetric parameters, particularly in the FFF mode.Conclusion(s): The study evaluated methods to reduce electron contamination in therapeutic photon beams. Results suggest that using a lead filter or magnetic deflector can reduce electron contamination and improve dosimetric parameters.

Keywords:

Magnetic field deflector , Electron contamination , Flattening filter free , Radiotherapy , MCNP ۶.۱