KEY POINTS
- Longitudinal phantom-based characterization of a novel upright computed tomography system for proton therapy planning, benchmarked against conventional computed tomography.
- Consensus calibration phantom measurements were acquired 14 times over 7 months; a simplified quality assurance configuration was tested over 8 months with 15 imaging sessions.
- Upright computed tomography number stability was excellent: inter-session standard deviation was ≤4.9 HU with the consensus configuration and ≤3.2 HU with the simplified configuration.
- Body-versus-head phantom differences were significant, with maximum computed tomography number difference of 136 HU in cortical bone and stopping power ratio difference up to 0.079, supporting size-specific calibration.
- Proton dose calculations for spine and prostate plans showed target coverage differences <0.3% and gamma pass rates >99% at 1 mm/1% versus conventional computed tomography, although local distal-range dose differences were observed.
CLINICAL TAKEAWAY
This study supports upright computed tomography as technically stable enough for proton therapy planning and as a potential platform for future upright adaptive proton workflows. The simplified phantom setup looks useful for routine machine-stability quality assurance, but not for clinical Hounsfield look-up table specification. The evidence remains technical and preclinical: single system, phantom-based testing, evolving acquisition protocols, and no patient-level validation.