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Investigating Aperture-Based Approximations to Model a Dynamic Collimation System for Pencil Beam Scanning Proton Therapy

N Nelson1*, B Smith1, W Culberson1, R Flynn2, D Hyer2, S Rana3, P Hill4, (1) University of Wisconsin - Madison, School of Medicine and Public Health, Department of Medical Physics, Madison, WI, (2) University of Iowa, Department of Radiation Oncology, Iowa City, IA, (3) Miami Cancer Institute, Baptist Health South Florida, Department of Radiation Oncology, Miami, FL, (4) University of Wisconsin - Madison, School of Medicine and Public Health, Department of Human Oncology, Madison, WI

Presentations

(Sunday, 7/12/2020)   [Eastern Time (GMT-4)]

Room: AAPM ePoster Library

Purpose: The Dynamic Collimation System (DCS) provides energy layer-specific collimation for pencil beam scanning proton therapy. The DCS has two nickel trimmer blade pairs located on upper and lower planes that intercept the scanned proton beam while also matching the beam divergence. Dose calculations of collimated pencil beams across multiple treatment planning systems use the assumption that collimators lie in an infinitesimally thin, beam-limiting plane. Inclusion of a range shifter creates additional beam divergence and scatter that is not effectively modeled with a single plane, and this work investigates potential aperture-based approximations for modeling range shifted, DCS-collimated dose distributions.


Methods: A TOPAS Monte Carlo model of the IBA dedicated nozzle at the Miami Cancer Institute was used to simulate three-dimensional dose distributions. The DCS was represented using three different aperture approximations: (1) a single aperture placed halfway between the two trimmer planes, (2) two apertures located at planes centered on the upper and lower trimmers, and (3) four apertures, located at both the upstream and downstream faces of each pair of trimmers. Spot weights for DCS-collimated fields were optimized to provide a uniform dose within a 30x30x20 mm³ target and applied to aperture-collimated fields for comparison.


Results: Dose differences between DCS- and aperture-collimated fields were mainly observed in the penumbra, leading to discrepancies up to 8% for the single and four-aperture models. Out of the aperture approximations, using two apertures had the best overall agreement with DCS. The four-aperture model performed the worst overall agreement due to a suspected overcompensation of trimmer transmission.


Conclusion: The double aperture approximation was most consistent with the DCS model for beamlets collimated by the lower trimmers, but for beamlets collimated by the upper trimmers the position of the upper aperture could be further optimized to better represent the DCS dose distribution.

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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. Flynn, Hyer, and Hill are co-inventors on a patent that has been licensed to IBA.

Keywords

Protons, Collimation, Monte Carlo

Taxonomy

TH- External Beam- Particle/high LET therapy: Proton therapy – computational dosimetry-Monte Carlo

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