Room: Exhibit Hall | Forum 7
Purpose: Quantify changes in spot shape and position in proton pencil beam scanning system using a commercially available scintillator detector and camera device.
Methods: A plan was created with 49 spots placed in a 7x7 matrix like arrangement with each spot 4 cm apart. At this distance the spots will have little to no overlap depending on the energy. This arrangement covers the useful area of the scintillator-camera device of our choice, the IBA Lynx PT. We selected 9 energies approximately equally distributed ranging from 70 MeV to 220 MeV. The spot position for all energies is kept the same. The data collection was initially done using still frames but recently a movie-mode data analysis has been implemented, which allows for quick data acquisition of multiple energies. The data is analyzed in an in-house developed software written in R. The code automatically detects the spot position and fit a Gaussian profile from which we take the central position as its centroid and its sigma for both X and Y axis. A scatter plot of spot sigma is used to evaluate the spot shape. Another scatter plot is used to evaluate overall spot position for all energies when compared to the planned positions. To troubleshoot individual energies we resort to vector maps to represent the distortion between planned and measured spot positions.
Results: We observed that most of the spots are well within 1 mm from the planned position. Spots more likely to reach or pass the 1 mm mark when located on the edges of field for high energies. We believe that this is due to the higher magnetic strength needed to steer the high energy proton beams outside central axis.
Conclusion: This methodology was found to be useful when troubleshoot for position and shape interlocks during proton treatment.