A practical framework for routine verification of Spot Position Monitor coordinate consistency in scanned carbon-ion radiotherapy.

BACKGROUND

Log file-based patient specific quality assurance (PSQA) in scanned particle therapy relies on the consistency of spot position information recorded by the spot position monitor (SPM), but practical methods for routine independent verification of these coordinates remain limited.

PURPOSE: This study describes a practical framework for routine verification of SPM coordinate consistency using independent scintillator-based measurements in raster-scanning carbon-ion radiotherapy.

METHODS

Reference spot positions were measured at the isocenter plane using a scintillator-based detector. Corresponding SPM-recorded spot coordinates were extracted from delivery log files. The two coordinate sets were aligned using a similarity transformation accounting for translation, rotation, and isotropic scaling. Post-transformation root-mean-square (RMS) residuals were used as a practical consistency metric reflecting the agreement between SPM-reported and independently measured spot positions under routine QA conditions.

RESULTS

After similarity transformation, the median RMS residual across all investigated accelerator energies and irradiation ports was 1.45 mm, with an interquartile range of 1.36-1.63 mm. No statistically significant difference was detected across accelerator energies, whereas a significant difference was observed among irradiation ports.

CONCLUSIONS

The proposed framework provides a practical approach for routine verification of SPM coordinate consistency and for establishing facility-specific baselines through longitudinal monitoring. The RMS residual should not be interpreted as the intrinsic positional error of the SPM alone, but to provide a framework for establishing a facility-specific baseline for monitoring the agreement between SPM-reported and independently measured spot coordinates under routine QA conditions.