Analytical Modeling of the Dynamic Collimation System Using Dual Trimmer Planes
L Bennett1*, D Hyer1, K Erhart2, N Nelson3, P Hill3, K Patwardhan1, T Geoghegan1, B Smith1, R Flynn1, (1) University of Iowa, Iowa City, IA, (2) .decimal LLC, Sanford, FL, (3) University of Wisconsin Medical Radiation Research Center, Madison, WI
Presentations
WE-A-TRACK 6-1 (Wednesday, 7/28/2021) 10:30 AM - 11:30 AM [Eastern Time (GMT-4)]
Purpose: Previous implementations of the Dynamic Collimation System (DCS) in the Astroid Treatment Planning System (TPS) modeled the trimmers as a single infinitesimally thin rectangular aperture. While this method is appropriate for a single brass aperture, it is insufficient to model DCS-based pencil beam scanning (PBS), which has two trimmer banks at different distances from the isocenter. This work introduces an analytical model that utilizes dual trimmer planes to enable more accurate dose calculation, with and without a range shifter present upstream of the trimmers.
Methods: The lateral dose distribution from a single proton beamlet was modeled as the weighted summation of sub-beamlets (bixels). The interaction of protons with the apertures was accounted for by modifying bixel weights (fluences). Weights were modified according to the overlap of apertures with each sub-beamlet and were dependent on beam energy, range shifter interaction, and aperture location. Our model introduces multiple trimmer planes 6.5 cm and 10.65 cm from isocenter in the extended nozzle position and 19.65 cm and 23.80 cm from isocenter in the retracted nozzle position. The analytical dose calculation model was validated against a commissioned Monte Carlo model in TOPAS.
Results: Without a range shifter present, 2D gamma passage rates at 1.0%/1.0mm were 100% between analytical and Monte Carlo for all energies considered (75 MeV, 100 MeV, and 150 MeV) and all trimmer positions relative to the central beamlet axis (1 mm, 5 mm) for both the single and dual trimmer plane models. With the range shifter present, the 2D gamma pass rates for all energies considered (80 MeV, 125 MeV, and 170 MeV) and all trimmer positions were >82% for the single plane model and >92% for the dual plane model.
Conclusion: Using dual trimmer planes is necessary to accurately calculate dose for DCS plans using analytical dose algorithms.
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 Hill are co-inventors on a patent that has been licensed to IBA.
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