Effect of tumor size on motion tracking and dosimetric accuracy using Radixact Synchrony

This study aimed to evaluate how planning target volume (PTV) size influences treatment-planning parameters and their relationship to tracking and irradiation accuracy. A high-resolution two-dimensional diode array mounted on a motion phantom was irradiated under four respiratory waveforms using Fiducial Tracking with Respiratory Correlation mode. Spherical PTVs with diameters of 1-4 cm were defined. Treatment-planning parameters, including beam-on time, couch speed, gantry period, kilovoltage imaging interval and modulation factor, were analyzed to assess their relationship with tracking and irradiation accuracy. The root-mean-square error (${\delta}_{\mathrm{RMS}}$) between tracked and reference target positions, γ pass rate and potential difference as the control parameter of the correlation model were compared across PTV sizes. Correlation and partial correlation analyses were performed to evaluate the relationships between the ${\delta}_{\text{RMS}}$ and treatment-planning parameters. The ${\delta}_{\text{RMS}}$ was influenced by the inherent interdependence of beam-on time, gantry period and the kV imaging interval. The kV imaging interval for the PTV of φ1 cm was 4.2 s, longer than those for the other PTV sizes, which made the potential difference unstable during the initial tracking period and kept γ pass rates below 92% across all respiratory waveforms. For the PTVs of φ2-3 cm, potential difference stabilized within ~20 s after beam-on and was associated with an improvement in the γ pass rate. In contrast, for the PTV of φ4 cm, degradation of tracking accuracy was reflected in a reduction in the γ pass rate. Optimizing treatment-planning parameters for each PTV size was essential for precise motion-synchronized irradiation.