Development of a storage phosphor imaging system for proton pencil beam spot profile determination.

Abstract

Accurate two-dimensional (2D) profile measurements at submillimeter precision are necessary for proton beam commissioning and periodic quality assurance (QA) purposes and are currently performed at our institution with a commercial scintillation detector (Lynx PT) with limited means for independent checks. The purpose of this work was to create an independent dosimetry system consisting of an in-house optical scanner and a BaFBrI:Eu2+ storage phosphor dosimeter by: (a) determining the optimal settings for the optical scanner, (b) measuring 2D proton spot profiles with the storage phosphors, and (c) comparing them to similar measurements using a commercial scintillation detector. An in-house 2D laboratory optical scanner was constructed and spatially calibrated for accurate 2D photostimulated luminescence (PSL) dosimetry. Square 5×5cm2 BaFBrI:Eu2+ dosimeter samples were uniformly irradiated with line scans performed to determine the physical and electronic scanner settings resulting in the highest signal-to-noise ratios (SNR) at a sub-millimeter spatial resolution. The resultant spatial resolution of the scanner was then quantitatively assessed by measuring (a) line pairs on a standard X-ray lead bar phantom and (b) modulation transfer functions. Following this, 2D proton spot profiles from a Mevion S250i Hyperscan proton unit were obtained at 1, 10, 20, 30, 40, and 50 monitor unit (MU) settings at maximum energy (E0 =227.1MeV) and compared to baseline profiles from a commercial scintillation detector, where 1MU is calibrated to deliver 1Gy absolute proton dose-to-water under reference conditions, that is, 41×41 proton spots uniformly spaced by 0.25cm within a 10×10cm2 square field size at maximum energy (227.1MeV) in water at depth of 5cm at isocenter. The dosimetric system's sensitivities to (a) ±1mm positional shifts and (b) ±0.3mm beam lateral spread changes were quantitatively evaluated through a Gaussian fitting of the crossline and inline plots of the respective artificially shifted beam profiles. The physical scanner settings of (a) Δτ=27ms time interval between data samples, (b) vx =1.235cm/s scanning speed, (c) 1% laser transmission (0.02mW power) and (d) (Δx, Δy)=(0.33, 0.50mm) pixel sizes with electronic settings of (a) 300 microseconds time constant, (b) normal dynamic reserve, (c) 24dB/oct low pass filter slope, and (d) 160Hz chopping frequency resulted in the highest SNR while maintaining sub-millimeter spatial resolution. The BaFBr0.85 I0.15 :Eu2+ storage phosphor dosimeters were linear from 1 to 50MU and their profiles did not saturate up to 150MU. The scanner was able to detect lateral displacements of ±1mm in both the crossline and inline directions and ±0.3mm beam spread changes that were artificially introduced by varying the incident proton energy. Specific to our proton unit, proton energy changes of ±1MeV can also be detected indirectly via beam spread measurements. Our combined dosimetric system including an in-house laboratory optical scanner and reusable BaFBr0.85 I0.15 :Eu2+ storage phosphors demonstrated a sufficient spatial resolution and dosimetric accuracy to support its use as an independent proton spot measurement dosimeter system. Its wide dynamic range allows for other versatile applications such as proton halo measurements.