Establishing CT-number-to-electron/density curves for hypersight CBCT with a standard cirs 062 M phantom: a methodological comparison of three practical approaches

HyperSight cone-beam computed tomography (CBCT) with Acuros reconstruction is reported to enable direct dose calculation in adaptive radiotherapy, but its accuracy depends on a reliable CT-number-to-electron/material-density (CT-ED/MD) calibration curve.The CT-number-to-material-density phantom setup may affect HU calibration. This study compared three practical CT-ED/MD calibration strategies for HyperSight CBCT using a thin CIRS Model 062 M phantom and to evaluate their dosimetric agreement with simulation CT. A CIRS Model 062 M phantom was scanned with simulation CT to generate the reference CT-ED/MD curve and with an Ethos HyperSight system using three protocols: 4 cm y-direction blade collimation (Method A), 24.2 cm y-direction blade collimation (Method B), and 24.2 cm y-direction blade collimation with 10 cm solid-water slabs added to both superior-inferior ends of the phantom (Method C). Head, thorax, and pelvis anthropomorphic phantoms were scanned on both systems. Patient-derived volumetric modulated arc therapy (VMAT)/intensity modulated radiotherapy (IMRT) plans representing three anatomical sites were transferred to the corresponding phantom image sets. HyperSight phantoms’ dose calculated based on each CT-ED/MD curve were compared with referenced simulation CT using dose-volume metrics and 3D global gamma analysis at 1 mm/1%, 1 mm/2%, 2 mm/2%, 2 mm/3%, and 3 mm/3%, with a 10% low-dose threshold. Direct 24.2 cm blade scanning produced prominent streak artifacts, whereas 4 cm blade scanning and 24.2 cm blade scanning with solid water produced image quality comparable to simulation CT. At 1 mm/1%, Method A achieved body pass rates of 96.6%, 99.5%, and 99.9% for the head, thorax, and pelvis, respectively; the corresponding PTV pass rates were 99.0%, 85.7%, and 99.9%. Method B produced marked PTV degradation (40.4%, 58.9%, and 7.0%). Method C restored agreement, with body pass rates of 99.8%-100.0% and PTV pass rates of 98.2%-99.9%. At 2 mm/2%, all methods except direct 24.2 cm blade scanning exceeded 99% pass rates. For HyperSight CBCT calibration with a thin CIRS 062 M phantom, full-field scanning without solid-water extension reduced dosimetric agreement. Solid-water extension provided the best agreement with simulation CT, while 4-cm blade collimation offered a practical alternative when solid water was unavailable. Not applicable.