Quality assurance of the inter-spot beam delivery of a dose driven continuous scanning proton therapy system

The article presents the development and validation of a dedicated quality assurance (QA) program for dose-driven continuous scanning (DDCS) in a synchrotron-based proton therapy system. Unlike conventional discrete spot scanning, DDCS allows continuous beam movement between spots, improving delivery efficiency but introducing complex temporal and dosimetric characteristics that require specific verification.

The study focuses on characterizing and monitoring inter-spot delivery parameters that directly influence dose accuracy. Key quantities evaluated include minimum break spot time (tb), beam current rise time (trise), delay between beam shutoff and magnet deflection (tdelay), and the resulting inter-spot or “flap” dose. Measurements were performed using high-resolution dosimetric tools and oscilloscope recordings, and were compared with machine log file data to assess reproducibility and consistency. The authors also implemented gamma analysis and dose comparisons to evaluate spatial and temporal agreement between planned and delivered dose distributions.

Results showed that inter-spot timing parameters were stable and reproducible across different energies and beam currents. The measured flap dose was minimal and clinically negligible, while rise time and break spot time remained within predefined tolerances. Strong correlation between direct measurements and log file data supported the reliability of log-based QA for routine monitoring.

In conclusion, the study demonstrates that a structured QA framework can effectively ensure accurate and safe inter-spot beam delivery in DDCS proton therapy. By establishing tolerance levels and validating measurement methodologies, the work supports the clinical implementation of continuous scanning while maintaining high dosimetric precision and treatment reliability.

Published by Medical Physics
DOI: 10.1002/mp.18050.