Calculation of the X-ray dose in the electron beams of NEPTUN 10PC linac using Monte Carlo method

The X-ray dose in a clinical electron beam is due to the bremsstrahlung rays presented in the incident beam and to a less extent the X-rays generated in the patient. The major part of this X-ray, however, is produced by the scattering foils, x-ray collimators, electron applicators etc. . The X-ray dose can be influenced by the geometry and construction of these components in a linac. Thus, it can be expected that the amount of X-ray dose be varied in different linacs even with the same electron beam energies. The aims of this study was to simulate the NEPTUN 10PC linac electron beams and calculate the relevant X-ray dose produced in electron beams using Monte Carlo method . Materials and methods: NEPTUN 10PC linac was simulated for the electron mode using the BEAMnrc code. This linac provides 6, 8 and 10 MeV electron beam energies. Detailed information regarding the geometry and materials of various components of the linac treatment head was provided by the vendor. For all of the simulations, the energy cut-off for the particle transport was set at ECUT=0.521 MeV and PCUT=0.010 MeV. The number of source electrons used for the X-ray dose calculation was 900 million. In order to benchmark the simulated models, the PDD curves and dose profiles derived from the calculated data were compared with the experimental data using the KS test. The central axis depth dose curves and dose profiles of the three electron beams were calculated using a simulated water phantom in DOSXYZnrc code for the reference field size (10×10 cm2) with the applicators in place. Diodedetectors of the Scanditronix dosimetry system and an RFA 300 water phantom were used for experimental measurements. After benchmarking the simulated models, the X-ray doses of the three electron beams in the simulated linac were calculated for the reference field size using Monte Carlo method . Results: The calculated and measured PDD curves and dose profiles of NEPTUN 10PC linac were compared for the three electron beams at the reference field size. The agreement between them was very good. The results showed that the amount of X-ray dose at the phantom surface from the components of the simulated linac was between 0.2-0.5% of the maximum dose for the three electron bean energies. In addition, it was found that the Xray dose increases with the increase of electron beam energy . Conclusion: The good agreement noted between the measured and simulated data indicates acceptable accuracy and precision of the simulation method used in this study. So, the values of the X-ray dose calculated with the Monte Carlo method have had a good level of accuracy and precision. The results of this study indicate that the X-ray dose from the electron beams of NEPTUN 10PC is not significant .

کلیدواژه ها:

Electron beams ، X-ray dose ، Monte Carlo method ، Linac