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Initial Daily Quality Assurance (QA) Experiences of the Adaptive Aperture in Mevion Hyperscan Proton Pencil Beam Scanning System

X Zhu*, A Rashid , M Kang , Y Chen , J Lischalk , D Pang , Medstar Georgetown University Hospital, Washington, DC


(Sunday, 7/14/2019)  

Room: ePoster Forums

Purpose: The Mevion Hyperscan is the first single vault proton pencil beam scanning system in the world, with a synchrocyclotron mounted on the treatment gantry. This system utilizes the Adaptive Aperture mounted in the nozzle, a multileaf collimator (MLC) unit, to reduce beam penumbra without focusing magnets. Here we report the daily QA implemented to verify its readiness and consistency for patient treatment.

Methods: : A 2-dimensional ion-chamber detector array was utilized for daily QA and includes: (1)dose outputs and beam profiles for a field size of 10x10 cm with an energy(E)of 227 MeV, (2)single spot beam size with E=110 and 202 MeV, (3)spot positions of a 9-spot pattern with E=90 and 196 MeV,(4)energy check at 50% distal range of passing through 8.4cm solid water with E=110 MeV. Beam characteristics were simulated with TOPAS Monte-Carlo simulation running on Amazon Web Server.

Results: From June 2018 to January 2019, the output at E=227 MeV was 100.3±1.4 cGy ; the penumbra was 8.80±0.21 mm (cross plane) and 8.79±0.21 mm ( in-plane); the flatness was 2.50±0.28% (cross-plane) and 2.77±0.27% (in-plane); the symmetry was 1.73±0.42% (cross-plane) and 1.80±0.53% (in-plane), respectively; Using adaptive aperture, the outputs was 99.7±1.5 cGy; the penumbra was 6.85±0.16 mm (cross plane) , and 6.97±0.32 mm ( in-plane); the flatness was 1.79±0.38% (cross-plane) and 1.29±0.29% (in-plane); the symmetry was 2.11±0.65% (cross-plane) and 1.89±0.54% (in-plane), respectively. The MLC reduced the output by 0.6%, and reduced the beam penumbra by 10%. Finally, the distal 50% range was measured at 51.1 ± 5.8% for the beam energy check.

Conclusion: Stable performance of Mevion Hyperscan system was verified in clinic delivery. The adaptive aperture ensures consistent penumbra reduction, while the MLC cross-plane motion position variation is related to the larger in-plane penumbra than the cross-plane, and further study is necessary for beam quality improvements.


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