Dynamic Individual Spot Collimation In Proton Beam Scanning with a Robotic Arm–Driven Pinhole Aperture
Abstract
Purpose
Static apertures in pencil beam scanning (PBS) proton therapy improve lateral dose falloff but are optimal only for a single energy layer and perform poorly for shallow targets where range shifters increase spot size. This work proposes dynamic individual spot collimation (DISC) using a robotic arm–mounted pinhole aperture to enable spot-by-spot collimation across energy layers and evaluates its positioning accuracy and dosimetric benefit.
Methods
A custom tungsten pinhole aperture (6-mm diameter) was mounted on a robotic arm and calibrated for precise movement along the proton gantry coordinate system using an in-house kV imaging–based protocol. Positioning accuracy was assessed by commanding the robotic arm to predefined locations and verifying aperture positions with kV imaging. Dosimetric performance of DISC was evaluated using radiochromic film and Monte Carlo simulations. Experiments included single-spot measurements for quantitative comparison between collimated and uncollimated beams; multi-spot array deliveries to assess robotic arm motion consistency and spot-to-spot alignment; and line-beam profile optimization to evaluate lateral penumbra reduction while maintaining dose uniformity.
Results
The accuracy of robotic arm-driven aperture movement is within 0.1 mm. Collimated spot profiles exhibited reduced spot size and sharper lateral dose falloff compared with uncollimated delivery. Film and simulation results were in close agreement, with a gamma passing rate of 97.8% using 2 mm/3% criteria. Film measurements of multi-spot arrays demonstrated consistent spot separations, confirming reliable dynamic aperture tracking of individual beam positions. Line-beam experiments showed that DISC reduced lateral penumbra for low-energy proton beams while preserving a uniform dose across the target region, consistent with Monte Carlo simulations.
Conclusion
Robotic arm–driven DISC with a pinhole aperture is feasible and accurate for PBS proton therapy. The demonstrated sub-millimeter positioning accuracy, consistent spot tracking, and improved lateral dose resolution support DISC as a promising approach for small-field proton beam collimation and enhanced dose conformality.