Background:The free flattening filter (FFF) beam can affect the characteristics of the linac output such as the maximum dose depth, surface dose, dose in the fall-off area, and doses outside the field because the beam hardening effect does not occur in the FFF linac head. Therefore, the present study aimed to investigate the influence of the FFF beam on the dose distribution in an inhomogeneous phantom using the EGSnrc/DOSXYZnrc Monte Carlo package. Materials and Methods: In the present study, an Elekta Infinity 10 MV photon beam equipped with a multileaf collimator Agility linear accelerator was used. Two types of virtual inhomogeneous phantoms were built for percent depth doses (PDDs) and dose profiles measurement. The first phantom comprised four layers: water (4 cm thickness), bone (2 cm thickness), lung tissue (5 cm thickness), and water (19 cm thickness). The second phantom had a half-lung tissue slab and a half-bone slab (10 cm thickness) on the left side of the water. Results: The PDD curves in the inhomogeneous phantom considerably decreased in the lung area for small exposure fields because the charged particle equilibrium was not achieved. The dose in the lung was higher than the dose in the water when the charged particle equilibrium was reached. Meanwhile, the dose in the bone is always lower than the dose in the water. Conclusions: The dose distribution of flattening filter (FF) and FFF beams in the inhomogeneous phantom was the same in the small field of exposure. However, differences in dose distribution are increasingly apparent for larger field sizes.
Target is one of the important components in a linear accelerator (linac). Target materials depend on the energy of linac so that they have different spectrums produced. This study investigates the spectrum characteristic of target material for some linacs using Monte Carlo codes. The target geometry and material simulated was collected from Varian Medical System. The simulation was carried out on two different Monte Carlo codes, namely PHITS and EGSnrc. In the simulation process, the cut-off energy of electron and photon are set in the same value at 0.521 MeV and 0.01 MeV, respectively. The SLAB geometry is implemented to model the target. The spectrum of photon and electron was collected at the end of the target. The deviation of electron and photon spectrum collected from PHITS and EGSnrc was calculated. The results show that the spectrum of photon and electron from PHITS and EGSnrc have the same trend with a deviation of more than 25%. The maximum differences between the codes were obtained at energy 0.04 MeV<E<0.3 MeV for 15 MeV sources. Differences between PHITS and EGSnrc are more than 17% in electron energy 18 and 20 MeV. The big deviation is caused by the cross-section data applied in PHITS and EGSnrc being different in some photon and electron interaction cross-sections.
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