345 - Commissioning, Quality Assurance, and First Canine Treatment with a Conformal Proton Flash Delivery System

E. S. Diffenderfer¹, F. Vander Stappen², M. M. Kim¹, S. A. O. Motlagh¹, W. Zou¹, R. Labarbe², L. Hotoiu², A. Pin², R. Nilsson³, E. Traneus³, A. Lin¹, K. A. Cengel¹, L. Dong¹, and B. K. K. Teo¹; ¹Department of Radiation Oncology, Perelman School of Medicine, University of Pennsylvania, Philadelphia, PA, ²Ion Beam Applications, Louvain-la-Neuve, Belgium, ³RaySearch Laboratories AB, Stockholm, Sweden

Purpose/Objective(s): A pencil beam scanned (PBS) proton FLASH system delivering spread out Bragg peaks (SOBP) was commissioned and tested to exploit enhanced normal tissue sparing effects of ultra-high dose rate radiotherapy. A new gantry nozzle accessory holder accommodates a 3D printed conformal energy modulator (CEM), interchangeable range shifting plates, and a brass field collimator. The new accessories are integrated with beam delivery and monitoring enhancements which allow increased beam intensity to nozzle currents of 500 nA at 230 MeV. Hardware and beam delivery commissioning was performed prior to QA validation of the first conformal Bragg peak proton FLASH treatment of a canine patient. Materials/

Methods: Treatment nozzle accessories were tested following published guidelines and procedures for proton therapy device commissioning and QA. PBS in-air spot size at multiple planes and gantry angles, spot positioning, and range were measured. Field accessory measurements included water equivalent thickness of range shifting plates, collimator and CEM shape and alignment. The measured data was used for building a beam model in a FLASH treatment planning system (TPS). The TPS was used to plan a PBS spot map, CEM, and collimator for the canine treatment. After the CEM and collimator were manufactured, QA measurements were performed to validate SOBP range and modulation, 2D dose distributions, dose rate and absorbed dose at several depths.
Results: A new proton FLASH delivery system and TPS were commissioned, and FLASH PBS spot map delivery was validated. A conformal SOBP treatment field was planned, and field specific devices were manufactured. QA measurements validated absorbed dose at several depths in the SOBP to within 2% of the prescribed 12Gy at field averaged dose rate of 42.3 Gy/s. 2D dose distributions were confirmed and analyzed using gamma analysis with 95% of points passing within 3%/3 mm. Beam range was validated to within 1 mm.
Conclusion: Proton FLASH can be delivered using a single layer conformal SOBP technique with dosimetric accuracy comparable to intensity modulated proton therapy.