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Experimentally determined proton discrete spot scanning time structures for an improved synchrotron-based PBS spot
James Kuan Huei Lee1,2, Clifford Ghee Ann Chua2,3, Kah Seng Lew2,3
1Department of Physics, National University Singapore, Singapore, Singapore.
Background:
An accurate discrete spot scanning (DSS) beam delivery time (BDT) model is essential, particularly as 4D dynamic dose accumulation calculations rely on machine-specific time models for reliable interplay evaluation.
Purpose:
To derive a BDT model for a synchrotron-based Hitachi Probeat PBS proton system from oscilloscope measurements of key BDT structures, including multi-energy extraction (MEE) characteristics.
Methods:
Key BDT structures were characterized using oscilloscope measurements, supplemented by delivery log files where appropriate. Spot-to-spot scanning trajectories were visualized using a CROSSmini 2D strip ionization chamber (Liverage Biomedical Inc, Taiwan) via the dose-driven continuous scanning (DDCS) mode. The MEE design and MEE charge recapture efficiencies were also characterized. In addition, the impact of the number of delivered spots on both the extractable fraction of charge and MEE recapture efficiency was investigated.
Results:
An improved synchrotron-based BDT model was derived that incorporates dead times between radiofrequency knockout (RFK) and high-speed steering magnet (HSST) signals. Spot-to-spot movement follows a "hockey-stick"-like behavior where the effective spot-to-spot move time is governed by the slower axis. An energy-dependent variation in the number of MEE layers deliverable per spill was also observed, with fewer MEE layers per spill (1-3) was observed in low and high energy ranges compared to the mid-energy range (4-5), deviating from the nominal vendor value of five MEE layers per spill per block. A reduction in the extractable fraction of charge was observed with an increasing number of spots per spill, while increasing the number of delivered spots in the preceding layer significantly reduced the MEE recapture efficiency.
Conclusion:
MEE characteristics should be incorporated into synchrotron-based time models for accurate beam delivery modeling. The methodology presented here also provides a framework for centers seeking to derive machine-specific delivery time models for their proton systems.
