KEY POINTS
- This technical study characterized discrete spot-scanning delivery on a synchrotron-based Hitachi ProBeat proton system using oscilloscope measurements, delivery log files, and a two-dimensional strip ionization chamber.
- The model incorporated previously uncharacterized dead times between radiofrequency knockout and high-speed steering magnet signals: 0.176 ± 0.004 ms before steering activation and 0.112 ± 0.004 ms before beam reactivation.
- Single-energy extraction layer switching required 1.91 ± 0.05 seconds, compared with 0.254 ± 0.015 seconds for multi-energy extraction. Deliverable multi-energy layers varied by energy, with 4-5 layers in the mid-energy range but only 1-3 layers at low and high energies.
- Increasing the number of spots reduced the extractable charge fraction from 96.36% with 400 spots to 92.89% with 600 spots. Increasing spots in the preceding layer from 1 to 500 reduced mean multi-energy extraction recapture efficiency from approximately 53.4% to 29.5%.
- Validation across 605 clinical treatment fields showed strong agreement between predicted and recorded beam delivery times: R² = 0.988, with a mean difference of 3.77 ± 2.63 seconds. Single-energy spill changes accounted for 56% of mean delivery time.
CLINICAL TAKEAWAY
Proton centers using synchrotron-based pencil beam scanning should not rely solely on nominal vendor timing parameters when modelling respiratory interplay or treatment throughput. Machine-specific multi-energy extraction behavior, charge losses, and spot-transition timing materially affect delivery-time predictions, although the measurements reflect one Hitachi system and require local characterization before implementation elsewhere.