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Validation of Mechanical Accuracy and Impact On Dose Sensitivity for a Proton Dynamic Collimation System

T Geoghegan1*, N Nelson2, K Patwardhan1, R Flynn1, P Hill2, D Hyer1, (1) University of Iowa Hospitals and Clinics, Iowa City, IA, (2) University of Wisconsin - Madison, Madison, WI,

Presentations

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

Room: AAPM ePoster Library

Purpose: The Dynamic Collimation System (DCS) is a proton collimator comprised of four nickel trimmers that move in synchrony with a scanned proton beam to sharpen the lateral penumbra. To enable focused collimation, a simple mechanism which rotates the trimmers as they translate was developed. Herein, the mechanical and dosimetric accuracy of a prototype focused DCS is evaluated.
Methods: The positional and angular accuracy of the trimmers were measured to within 25 µm using a FARO® Edge ScanArm. The goal is to achieve a trimmer rotation of 0.35º per centimeter of translation. Dosimetric sensitivity to trimmer position and rotation errors was evaluated with the TOPAS Monte Carlo calculation system. An IBA Proteus Plus was modeled along with the DCS at an energy of 100 MeV. Dose was scored by a voxelized water phantom with a 0.25 mm resolution and an uncertainty of 1.5% at the maximum dose point. Trimmer position errors of ±250, ±500, 750, ±1000 µm were also simulated to represent the combined mechanical accuracy of the trimmer positioning motors and potential misalignment between the DCS and the proton beam.
Results: For a range of clinically expected trimmer positions, the maximum positional and angular error was found to be ±50 µm and ±0.25º, respectively. This level of accuracy resulted in negligible changes to shape, position or fluence of the beamlet when compared to baseline. When the positional error increased to ±1000 µm or greater, the 1%/1 mm gamma pass rate degrades to 90%.


Conclusion: The mechanical accuracy of the DCS, which was validated to be within ±50 µm and ±0.25º, does not have a clinically relevant impact on the dose distributions of single proton beamlets. If inaccuracies compound and reach 1000 µm or greater, the beamlets are meaningfully altered and fail the gamma test.

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

Keywords

Protons, Mechanical Properties, Dose Response

Taxonomy

TH- External Beam- Particle/high LET therapy: Proton therapy – Development (new technology and techniques)

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