Two-Dimensional Scintillation Imaging Dosimetry for Validation of Pencil-Beam Proton Delivery from Two Matched Gantry-Mounted Synchrocyclotrons
Abstract
Purpose
Conventional methods for testing proton pencil-beams require multiple quality assurance tools that lack efficiency and spatiotemporal resolutions. Here, we used our efficient ultra-high-resolution scintillation imaging system to verify that two gantry-mounted synchrocyclotrons are dosimetrically equivalent.
Methods
A high-speed CMOS-camera and two plastic scintillators (BC-408) were used to acquire ultra-high-resolution scintillation images (0.08 mm/pixel) and characterize proton beams. Longitudinal dose-profiles (Bragg-Peak) obtained using a 30×30×5cm3 plastic-scintillator provided proton range (distal R90%), distal fall-off (80%-20%), and Bragg-peak widths at different peak-heights. In addition, pencil beam widths at multiple depths were extracted. Six ranges (5-28 cm) were evaluated at four airgaps (3.6, 13.6, 23.6 and 33.6 cm snout-extension). Five vertically positioned spots at Y=-9.0, -4.5, 0, +4.5 and +9.0 cm were tested. Spots’ positions and widths were measured utilizing a 30×30×0.5 cm3 plastic-scintillator perpendicular to the beam direction while a 45°-mirror reflected the scintillation-images toward the camera. A plan consisting of 25 spots (5×5 array, 3.5 cm spot-spacing) using Rmax (R=32.2cm) were tested. All measurements were performed on both machines, and the results were compared.
Results
Thousands of images were analyzed to verify the equivalency of the two machines. Range measurements compared to those using a multi-layer-ionization chamber as well as relative comparison between the two synchrocyclotrons revealed that the measured ranges are within ±0.5 mm. Bragg-peak widths were within ±0.4 mm, distal 80%-20% penumbra widths were within ±0.3 mm and the pencil beam widths were within ± 0.1 mm. Spot sizes were within ±0.4 mm, and the spots’ positions were within ±0.8 mm indicating a close agreement between both machines.
Conclusion
Scintillation imaging is highly efficient for simultaneously extracting multiple beam parameters of proton therapy pencil-beams. Traditional methods only measure one metric at a time while our scintillation-dosimetry-system allows acquisition of several beam parameters simultaneously, minimizing beam-on time on the machine.