Characteristic evaluation of photoneutron in radiotherapy room usingMCNPX

Abstract

Linear accelerators are now playing a pivotal role in radiotherapy and high energy photon beams of a strength exceeding 8 MV have recently been mainly used. However, when using high energy photons, neutron contamination due to photonuclear reaction develops. This study focused on the dose distribution of photoneutrons emitted from a linear accelerator using Monte Carlo MCNPX code. MCNPX was used to simulate transportation of photoneutrons in the linear accelerator and the entire space of the radiotherapy room and is useful for calculating the flux, spectrum and absorbed dose. As result of the simulation, we could know that the neutron absorbed dose was as less as negligible when comparing to the photon absorbed dose in radiotherapy room. And it was found that the photoneutron flux increased substantially starting from 10 MV while the absorbed dose rose sharply between 10 MV and 12 MV. It was observed that although the ratio of thermal neutrons to fast neutrons was not altered as the energy increased, it was found that as the distance from the source increased the ratio of thermal neutrons rose markedly.