Current delivery limitations of proton PBS for FLASH

Wei Zou, Eric S. Diffenderfer, Keith A. Cengel, Michele M. Kim, Steve Avery, Joshua Konzer, Yongliang Cai, Paul Boisseu, Kan Ota, Lingshu Yin, Rodney Wiersma, David J. Carlson, Yi Fan, Theresa M. Busch, Costas Koumenis, Alexander Lin, James M. Metz, Boon Keng K. Teo, Lei Dong

Research output: Contribution to journalArticlepeer-review


Purpose: Proton Pencil Beam Scanning (PBS) is an attractive solution to realize the advantageous normal tissue sparing elucidated from FLASH high dose rates. The mechanics of PBS spot delivery will impose limitations on the effective field dose rate for PBS. Methods: This study incorporates measurements from clinical and FLASH research beams on uniform single energy and the spread-out Bragg Peak PBS fields to extrapolate the PBS dose rate to high cyclotron beam currents 350, 500, and 800 nA. The impact of the effective field dose rate from cyclotron current, spot spacing, slew time and field size were studied. Results: When scanning magnet slew time and energy switching time are not considered, single energy effective field FLASH dose rate (≥40 Gy/s) can only be achieved with less than 4 × 4 cm2 fields when the cyclotron output current is above 500 nA. Slew time and energy switching time remain the limiting factors for achieving high effective dose rate of the field. The dose rate-time structures were obtained. The amount of the total dose delivered at the FLASH dose rate in single energy layer and volumetric field was also studied. Conclusion: It is demonstrated that while it is difficult to achieve FLASH dose rate for a large field or in a volume, local FLASH delivery to certain percentage of the total dose is possible. With further understanding of the FLASH radiobiological mechanism, this study could provide guidance to adapt current clinical multi-field proton PBS delivery practice for FLASH proton radiotherapy.

Original languageEnglish (US)
Pages (from-to)212-218
Number of pages7
JournalRadiotherapy and Oncology
StatePublished - Feb 2021
Externally publishedYes


  • Dose rate
  • Proton PBS

ASJC Scopus subject areas

  • Hematology
  • Oncology
  • Radiology Nuclear Medicine and imaging


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