Proton beam spot size and position measurements using a multi-strip ionization chamber

Purpose: To develop and characterize a large-area multi-strip ionization chamber (MSIC) for efficient measurement of proton beam spot size and position at a synchrotron-based proton therapy facility. Methods and Materials: A 420 mm x 320 mm MSIC was designed with 240 vertical strips and 180 horizontal strips at 1.75 mm pitch. The MSIC was characterized by irradiating a grid of proton spots across 17 energies from 73.5 MeV to 235 MeV and comparing to simultaneous measurements made with a reference Gafchromic EBT3 film. Beam profiles, spot sizes, and positions were analyzed. Short term measurement stability and sensitivity were evaluated. Results: Excellent agreement was demonstrated between the MSIC and EBT3 film for both spot size and position measurements. Spot sizes agreed within +/- 0.18 mm for all energies tested. Measured beam spot positions agreed within +/- 0.17 mm. The detector showed good short term measurement stability and low noise performance. Conclusion: The large-area MSIC enables efficient and accurate proton beam spot characterization across the clinical energy range. The results indicate the MSIC is suitable for pencil beam scanning proton therapy commissioning and quality assurance applications requiring fast spot size and position quantification.