Evaluation of lead shielding against head leakage from a 6 MV agility linac using FLUKA Monte Carlo simulation

Abstract

Most studies in the literature focus on in-field dosimetry, with relatively few addressing the accuracy of linear accelerator (linac) models in leakage or out-of-field regions. This study investigates leakage radiation from a linac using Monte Carlo (MC) simulations. An Elekta Synergy linac equipped with an Agility head was modelled using the FLUKA code. Head leakage radiation measurements were performed using a 6 MV photon beam directed both horizontally and vertically. A PTW Spherical Chamber (TK-30) was used to acquire measurements at 13 different locations, each at a fixed source-to-chamber distance of 100 cm. The ion chamber was also explicitly modelled in the simulations. Based on the mean photon energy at each position, the required lead shielding thickness was initially calculated using narrow-beam linear attenuation coefficients. To account for broad-beam geometry, buildup factors—dependent on photon energy and the number of mean free paths—were incorporated. An iterative method was applied until the calculated shielding thicknesses converged. All measured leakage values were within the recommended limit of 0.1% of the maximum absorbed dose at the isocentre, ranging from 0.01% to 0.06%, except at the isocentre itself, which exceeded the limit. The MC simulation results showed good agreement with the measurements. The calculated lead thicknesses required to match the measured leakage ranged from 3.6 cm to 13 cm, depending on the location. This would help improve shielding accuracy and reduce secondary cancer risk from leakage or out-of-field exposure.