A predictive algorithm for spot position corrections after fast energy switching in proton pencil beam scanning

Area of Science:

• Medical Physics
• Particle Therapy

Background:

• Fast energy switching is essential for motion mitigation in pencil beam scanning proton therapy.
• Eddy currents in bending magnets cause magnetic field settling time issues, leading to beam displacement and reduced precision at isocenter.

Purpose of the Study:

• To investigate transient magnetic field effects in PSI Gantry 2.
• To develop a correction strategy for beam position misplacement caused by energy changes.

Main Methods:

• Utilized position and proton range sensitive detectors to measure beam position and range over time.
• Analyzed magnetic field effects from gantry and upstream elements separately.
• Designed a spot position algorithm for gantry scanning magnets to correct temporal displacements.

Main Results:

• Observed exponentially decaying spot position displacement at isocenter, increasing with energy difference (ΔE).
• Initial position residuals exceeded 1mm, settling below 1mm within ~1s; no range time dependence was found.
• A double exponential model fitting the displacement was implemented, reducing residuals to <0.5mm post-correction.

Conclusions:

• A novel spot position correction for PSI Gantry 2 was developed.
• The correction significantly reduces the required magnetic field settling time after energy changes.
• Stable spot position within 100ms enables efficient implementation of motion mitigation techniques.