Room: ePoster Forums
Purpose: A debated issue in pencil-beam scanning (PBS) proton therapy is the optimal sequence of the collimator and range-shifter (RS) in order to minimize lateral penumbra. It has previously been shown that the combination of materials and thicknesses of the collimator and RS are the main factors that must be explored to determine the optimal sequence in the beamline. The purpose of this work was to evaluate the optimal sequence of these components for the dynamic collimation system (DCS).
Methods: Simulations were done using TOPAS along with a model of the IBA Proteus Plus. The DCS utilizes rectangular nickel trimmers that move in synchrony with the pencil-beam to provide collimation at the edge of the target. Simulations for this work included a simplistic model of the DCS which was comprised of a single 3 cm thick nickel trimmer, either upstream or downstream of the RS, which was modeled as a 4 cm thick polyethylene slab. Air-gaps of 4 and 8 cm were used between the collimator and RS to represent the two levels of trimmers (X and Y-direction) as used in the DCS. A voxelized water phantom was placed 4 cm downstream of the last beamline component. For all simulations, the trimmer edge was aligned 1.5 mm from the beamlet central axis which represents a realistic clinical offset. The penumbra (80%-20%) was evaluated at the surface and at depth for both configurations using beam energies of 85, 100, 130, and 160 MeV.
Results: The penumbra was always smaller with the collimator downstream of the RS. At the surface, the penumbra was reduced by 15-30%, for both airgaps, across the range of energies studied. Higher energies were impacted less and the effect diminished with depth.
Conclusion: Placing the collimator downstream of the RS yielded the smallest lateral penumbra for the DCS.
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.