Objective To investigate the impact of different iterative Cone Beam CT (CBCT) scanning beam currents from a ring-mounted linac on synthetic CT image quality for lung adaptive radiotherapy under lung scanning protocol.

Methods The CIRS lung motion phantom was configured to simulate conventional respiratory motion pattern, followed by 4D-CT simulation. After transferring the radiotherapy plan to the ring-mounted Halcyon 3.0 linac, three groups of typical kV-CBCT scans with different beam currents (I_low (160 mA), I_middle (282 mA), and I_high (491 mA)) were performed and corresponding image reconstructions were completed. Synthetic CT images were subsequently obtained based on the deformable registration algorithm.

Results Compared to the corresponding CBCT images, the sCT images exhibited a significant reduction in artifacts. The fine structure of the planning CT image was preserved for sCT images corresponding to different scanning beam currents, with Dice similarity coefficients exceeding 0.90 for all cases.

Conclusion The image quality of sCT corresponding to different iCBCTs is comparable to that of pCT and changes in iCBCT beam parameters have a negligible impact on sCT image quality. Taking into account both image quality and imaging dose factors associated with the beam currents, iCBCT with a lower beam current on the ring-mounted Halcyon linac offers greater clinical value in lung adaptive radiotherapy.