Does Copper Beam-Hardening Filtration Reduce Imaging Dose Without Compromising Targeting Accuracy in Robotic Radiosurgery?

<span>Accurate target localization in frameless stereotactic radiosurgery (SRS) is commonly achieved using intrafraction kV X-ray imaging, which contributes to patient imaging dose. This study investigated the feasibility of reducing the imaging dose in CyberKnife^TM SRS by implementing copper (Cu) beam-hardening filtration while preserving image quality and image-guided targeting accuracy. Entrance Skin Dose (ESD) at isocenter was measured for standard aluminum (Al) filtration and additional Cu filtration thicknesses of 0.12mm and 0.24mm. Monte Carlo (MC) simulations were used to characterize Cu-induced X-ray spectral changes and to estimate imaging doses using patient-specific voxelized anatomical models over a range of tube potentials and filtration conditions. The effect of Cu filtration on targeting accuracy was assessed using end-to-end (E2E) tests for the 6Dskull^TM- and Xsight^TM spine-tracking algorithms, supplemented by patient image data. Copper filtration progressively removed low-energy photons, increasing the mean photon energy from 56keV with standard Al filtration to 60keV and 64keV for 0.1mm and 0.2mm Cu filtration at 120kVp. At 10mAs, ESD increased from 0.248mGy at 80kVp to 0.822mGy at 150kVp with Al filtration, while addition of 0.12mm and 0.24mm Cu reduced ESD by 46% and 64% at 80kVp and by 25% and 36% at 150kVp, respectively. Patient model calculations confirmed meaningful organ dose sparing for the eye lenses, thyroid gland, and skin. For 6Dskull tracking, implementation of Cu filters induced modest image quality changes which were readily compensated by small kVp increase without mAs adjustment. For Xsight spine tracking, image quality was unaffected by Cu filtration. E2E testing confirmed that the total system targeting error was maintained for both tracking algorithms. Additional Cu filtration substantially reduced CK imaging dose without compromising image-guided tracking accuracy. These findings support the clinical feasibility of Cu-filtered imaging as an effective strategy for optimizing CK imaging protocols, particularly in workflows involving frequent image acquisition.</span>