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Practical Method for Periodic Quality Assurance Monitoring of Spot Position and Spot Size for a Spot-Scanned Proton System

E Tryggestad

E Tryggestad*, C Beltran , A Deisher , K Furutani , G Gilson , M Hernandez , S Ito , J Johnson , J Kruse , D Mundy , N Remmes , A Tasson , T Whitaker , Y Zhang , J Gustafson , M Lowe , K Long , M Herman , Mayo Clinic, Rochester, MN


SU-I-GPD-T-152 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: Spot-scanning particle therapy systems demand complex quality-assurance testing for independent monitoring of beam-line magnet calibration and performance. Spot-position accuracy (scanning/steering magnets) over the deliverable field and stability of spot size (tuning magnets) are critical. We describe a rigorous but efficient solution for periodic testing.

Methods: Two 8”×10” EBT3 films (Ashland Inc., Bridgewater NJ) are indexed to our 2.5cm range shifter (“RS”, upstream face), providing full coverage of the 30cm×40cm field (defined at isocenter) and overlapping sufficiently at nozzle axis y=0. A multi-energetic pattern comprised of 26 spot locations (5cm grid, 1MU/spot) and 18 different beam energies is delivered. Films are scanned with a commercial flatbed scanner and dose-converted using FilmQAPro (Ashland, Inc.). Post-processing is performed using in-house software to fit observed spots with a 2D-Gaussian function and combine data from the films. Measured spot locations are projected to the isocenter plane. Data for each gantry room and gantry angle (0°,90°,180°) are monitored relative to a prior baseline measurement. Projection to the isocenter plane requires a one-time gantry-room- and gantry-angle-specific commissioning step involving simultaneous irradiation of films at the RS and isocenter planes to derive effective points of divergence (upstream) due to the x,y scanning magnets.

Results: Each measurement requires ~5 minutes of gantry room time; analysis (including scanning and processing) requires <10 minutes. Spot position tolerances of baseline±1.2mm (x,y) and spot size (sigma) tolerances of baseline±0.4mm (major, minor axes) were established based on system acceptance criteria and plan quality implications. Since clinical operation began in June of 2015, no failing deviations have been recorded; results are generally well inside these thresholds.

Conclusion: This method is efficient, cost-effective, accurate (due to the high spatial fidelity of Gafchromic film and a robust 2D fitting method) and can be applied to any spot/energy combination (provided adequate spot separation is maintained).

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