Optimisation of Radiotherapy Bunker Design with the use of Advanced Materials and Monte Carlo Modelling

Abstract

The maze of a radiotherapy treatment room is designed to attenuate the photon dose-rate of the outside environment to the level required by international radiation protection organisations such as the ICRP. The important factors that are the main contribution to dose at the maze entrance are leakage and backscattering of X-ray photons, which have differing photon spectra and therefore differences. Measuring the spectra near the maze entrance is challenging due to the pulsed nature of the LINAC source which generates a form of pulse pile-up. In this study, experimental measurements using NaI (sodium iodide) and plastic scintillation detectors have been carried out to determine the energy spectra at different points outside the radiotherapy room. Mathematical methods to account for pulse pile-up have been examined, incorporating the highly periodic pulsing structure of the LINAC, differing from the pulse pile-up effects of high-intensity radioactive sources. The proposed algorithm uses the FLUKA Monte Carlo code to calculate the energy spectrum at the maze entrance and to determine a response function for both detectors. The agreement between the mathematically modelled and measured spectra indicates that the Monte Carlo modelling can accurately determine the spectrum of a LINAC X-ray machine in the environment outside of the maze. Additionally, there is an increased interest in the determination of photon reflection coefficient for composite materials of various atomic number. Therefore, a part of this study aims to calculate the reflection coefficients of photons for various thickness of high and medium Z materials covering the concrete. The generation of accurate reflection coefficient data has implications many fields especially radiation protection and radiotherapy room design. Finally, modelling of improvement in existing maze designs showed that the maze walls covered with a few millimetres of lead would reduce the dose by up to 80%, while a few centimetres of baryte would reduce the dose by up to 70%. However, extending the maze by 150 cm gave a better dose reduction of up to 95%, but with implication for increased costs associated with additional space required.