CBCT dose reduction with HyperSight on an Ethos linear accelerator: A phantom-based TLD measurement study.

Abstract

The novel HyperSight cone-beam CT (CBCT) system (Varian Medical Systems, Siemens Healthineers, Palo Alto, USA) provides image quality comparable to native CT and offers advantages for contouring in online-adaptive radiotherapy. However, daily imaging increases organ dose. This study evaluates and compares organ doses across HyperSight protocols on an O-ring Linac and calculates the effective dose and secondary cancer risk associated with daily imaging. Thermoluminescent dosimeter (TLD) measurements were performed on an anthropomorphic phantom for all pre-configured CBCT protocols in HyperSight, with TLDs placed according to each entity (head, head and neck, thorax, chest, and pelvis). The resulting imaging doses were compared across the various protocols and TLD positions, followed by the calculation of effective doses and the associated secondary cancer risk. HyperSight CBCT demonstrated lower organ doses compared with previous systems. Mean bladder and rectum dose was 10mGy versus 27mGy for OBI Version 1.4.13, while mean head dose was 2mGy compared with 1.2mGy for Halcyon 3.0. A left-right dose asymmetry was observed due to partial gantry rotation, resulting in higher exposure in beam-facing organs. Imaging dose varied considerably between protocols. Low Dose protocols reduced exposure by up to 55%, whereas slow and large protocols increased dose by 40-60%. HyperSight provides high-quality imaging with lower dose levels than previous CBCT generations. The observed beam geometry-related asymmetry should be considered in clinical use, and individualized protocol selection is essential to minimize unnecessary exposure in image guided and adaptive radiotherapy under ALARA principles.