Study of a plastic scintillating plate-based quality assurance system for pencil beam scanning proton beams

Introduction: The purpose of this study was to evaluate a plastic scintillating plate-based beam monitoring system to perform quality assurance (QA) measurements in pencil beam scanning proton beam. Methods: Single spots and scanned fields were measured with the high-resolution dosimetry system, consisting of a plastic scintillation plate coupled to a camera in a dark box at the isocenter. The measurements were taken at 110-190 MeV beam energies with 30 degrees gantry angle intervals at each energy. Spot positions were determined using the plastic scintillating plate-based dosimetry system at the isocenter for 70-230 MeV beam energies with 30 degrees gantry angle intervals. The effect of gantry angle on dose distribution was also assessed by determining the scanning pattern for daily QA and 25 fields treated with intensity-modulated proton therapy. Results: Spot size, field flatness, and field symmetry of plastic scintillating plate-based dosimetry system were consistent with EBT3 at all investigated energies and angles. In all investigated energies and angles, the spot size measured was +/- 10% of the average size of each energy, the spot position measured was within +/- 2 mm, field flatness was within +/- 2%, and field symmetry was within +/- 1%. The mean gamma passing rates with the 3%/3 mm gamma criterion of the scanning pattern and 25 fields were 99.2% and 99.8%, respectively. Conclusions: This system can be effective for QA determinations of spot size, spot position, field flatness, and field symmetry over 360 degrees of gantry rotation in a time- and cost-effective manner, with spatial resolution comparable to that of EBT3 film.