Purpose: To develop an independent dosimetric imaging system consisting of an optical scanner and reusable storage phosphor dosimeters that can obtain two-dimensional (2D) dosimetric images of proton spots at sub-millimeter resolution.
Methods: An in-house scanner was constructed using stepper motors that can achieve translations to sub-millimeter precision. BaFBrI:Eu2+ storage phosphor detectors were then irradiated using proton pencil beams at maximum energy of 227.1MeV. During optical readout, the detector panels were mounted vertically and actively translated relative to a static, focused stimulating laser light produced by a 594nm helium-neon laser. The intensities of the photostimulated luminescence (PSL) produced will correlate to the proton dose delivered at specific locations of the latent dosimetric image. Proton spots were delivered while varying the proton doses delivered from 1-150 monitor units (MUs), where 1MU corresponded to 6cGy peak proton dose. Typical dose delivery errors such as lateral displacements and beam divergence were simulated too.
Results: The combined system managed to attain a sub-millimeter precision and close agreement with 1MU proton dose images collected with a commercial scintillator. Dosimeters showed good linearity and no saturation of up to 150MU proton dose. A signal-to-background ratios of at least 2400:1 was attainable at 150MU demonstrating the ability for proton halo measurements down to at least 0.04% of peak proton doses. The system was sensitive enough to detect ±1mm lateral displacements inline and crossline and ±0.30mm divergence in the Gaussian profile.
Conclusion: We have developed an independent dosimetric platform that is able to perform profile and dosimetric checks of proton pencil beams at sub-millimeter precision and at high dosimetric accuracy. It was sensitive enough to detect beam delivery errors including lateral shifts and beam divergence. The demonstrated high dynamic range shows potential utility for versatile measurement applications such as proton beam halo measurements.