Current delivery limitations of proton PBS for FLASH

Area of Science:

• Medical Physics
• Radiation Oncology
• Particle Therapy

Background:

• FLASH radiotherapy offers potential normal tissue sparing.
• Proton Pencil Beam Scanning (PBS) is a promising technique for delivering FLASH doses.
• PBS mechanics may limit effective dose rates.

Purpose of the Study:

• To investigate the effective dose rate achievable with PBS under various conditions.
• To extrapolate PBS dose rates at high cyclotron beam currents.
• To identify limitations and possibilities for FLASH dose delivery using PBS.

Main Methods:

• Measurements from clinical and research beams (uniform single energy and spread-out Bragg Peak PBS fields).
• Extrapolation of PBS dose rates to high cyclotron currents (350, 500, 800 nA).
• Analysis of factors influencing effective field dose rate: cyclotron current, spot spacing, slew time, and field size.

Main Results:

• FLASH dose rates (≥40 Gy/s) are achievable for small fields (<4x4 cm²) with >500 nA cyclotron current if slew and switching times are ignored.
• Slew time and energy switching time are critical limiting factors for high effective dose rates.
• Dose rate-time structures were analyzed, and the proportion of dose delivered at FLASH rates was studied.

Conclusions:

• Achieving full FLASH dose rates in large fields or volumes with PBS is difficult.
• Local FLASH delivery, where a portion of the total dose is delivered at FLASH rates, is feasible.
• Findings can guide the adaptation of current clinical proton PBS for FLASH proton radiotherapy, pending further understanding of FLASH radiobiology.