Room: Exhibit Hall | Forum 7
Purpose: When designing a collimation system for pencil-beam scanning proton therapy, a decision must be made whether or not to rotate, or focus, the collimator to match beamlet deflection as a function of off-axis distance. If the collimator is not focused, the beamlet position and fluence will vary as a function of off-axis distance due to partial transmission through the collimator. This study evaluated the viability of implementing an offset table, without focusing, to account for the beamlet changes.
Methods: Simulations were done using TOPAS along with a model of the IBA Proteus Plus. The collimator selected for this study was the dynamic collimation system (DCS), which utilizes rectangular nickel trimmers that move in synchrony with the pencil-beam to provide collimation at the edge of the target. A simplified model of the DCS, consisting of a single nickel trimmer, was initially aligned 1.5 mm from the central-axis of a 100 MeV beamlet which represented a realistic clinical offset. The nickel trimmer was then rotated from 0 to 2.5 degrees in 0.25 degree increments, mimicking the incident angles expected for representative unfocused, off-axis trimmer locations. Simulations were repeated for trimmer positions of 1.75 and 2 mm from the beamlet central-axis. A voxelized water phantom scored dose and was placed 4 cm downstream from the collimator.
Results: The trimmer rotation resulted in beamlet shifts that ranged from 0.05 to 0.6 mm with negligible impact on the lateral penumbra. However, partial transmission through the rotated trimmer created a tail in the dose profile, 4%- 10% in magnitude depending on rotation, reducing the benefit of collimation.
Conclusion: Although the dose profile shape was generally unchanged from trimmer rotation and an offset table could match the beamlet shifts, the partial transmission eliminates the possibility of implementing an offset table and collimator focusing is recommended.
Funding Support, Disclosures, and Conflict of Interest: Research reported in this abstract was supported by the National Cancer Institute of the National Institutes of Health under award number R37CA226518. Hyer, Flynn, and Wang are co-inventors on a patent that has been licensed to IBA.