18MV steel laminate bunker designed for VMAT and IMRT

Introduction: The radiation shielding design for three additional linear accelerator bunkers has been completed and assessed at the Princess Alexandra Hospital. The bunkers are to hold dual modality Elekta linear accelerators with high energy photon beams of up to 18 MV. The shielding design included allowances for new technologies including IMRT and VMAT. Space limitations required the shielding to include laminated steel primary barriers. Methods: The workload for the new linear accelerators was calculated from past data on the existing linear accelerators. This patient workload was then increased to account for the Physics QA workload, and the additional leakage workload due to IMRT and VMAT. Dose calculations from NCRP 151 were used to assess the shielding design. Particular attention was paid to the possible increase of dose behind the primary barriers due to the production of photoneutrons and neutron capture gamma rays within the steel. Conclusion: Steel thicknesses of 250 mm encased within a 1350 mm thick concrete wall (1600 mm total thickness) were proved sufficient to keep the dose within the limits specified by the Queensland Radiation Safety Standard PRoo4:2004. A neutron door within the maze is required.