Characterisation of in-room leakage and scattered radiation for the Varian Halcyon linear accelerator

Unintended sources of secondary radiation resulting from photon beams in linear accelerators are patient scatter, collimator scatter, scatter from surfaces within the bunker, and head leakage. This work characterises the in-room leakage and scattered radiation for the Varian Halcyon linear accelerator. Scattered and leakage radiation for static gantry angles 0°, 45°, and 90° and largest field size 28 × 28 cm² were measured with an ionisation chamber survey meter at a radial distance of 1.5 m from the isocentre. The scatter within the treatment room was characterised with isocontour maps from measurements of 360° arc deliveries with the largest field size 28 × 28 cm² at various heights and distances from the isocentre. The transmission through the primary beam stopper was measured with a Farmer ionisation chamber. For static gantry angles, instantaneous dose rate readings were typically around 70 mSv/hr at 1.5 m from the isocentre with lower dose rates at room angles adjacent to the gantry. The head leakage was measured as less than 0.03% of the useful beam. In a full 360° arc, the radiation dose around the Halcyon was coldest in lateral areas, with hotter areas behind and in front of the gantry. The primary beam stopper transmission was measured as 0.019%, reducing the requirement of primary barriers in the shielding design by a factor of 1/500. The results presented in this study can be used to determine the out-of-field dose to patients and to inform bunker shielding designs for Halcyon linear accelerators.