The effect of the presence of the patient on the particles dose estimations in high energy linear accelerator mazes

Purpose: Medical accelerators operating above 10 MV are a source of undesirable neutron radiations which contaminate the therapeutic photon beam. These photo-neutrons which contaminate the therapeutic beam can also generate secondary gamma rays, via inelastic and capture reactions, which increase the undesirable dose to the patient body, the oncology staff and the general public. The purpose of the present work is to investigate, through Monte Carlo simulation, the effect of the presence of the patient on the neutron and gamma rays dose calculations. Methods: To illustrate this effect, the MCNP5 code was used to model a radiotherapy room of a medical linear accelerator operating at 18 MV and to calculate the neutron and the secondary gamma ray energy spectra and the dose equivalent, at various points along the centerline of the maze, in the absence and presence of the patient. Results and Conclusion: The obtained results show a significant change in the neutron energy spectra in the presence of the patient especially in the thermal neutron energy region. The results also indicate that the presence of a patient does not affect the simulated neutron and gamma rays dose equivalents at the maze entrance for mazes greater than 3 m long. A significant change in dose equivalent calculated values was observed when the length of the maze is less than 3 m