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An Overview of the Commissioning of a Two-Gantry Proton Pencil Beam Scanning System

Y Zhang

Y Zhang*, E Lee , Z Xiao , M Lamba , A Mascia , University of Cincinnati Medical Center, Cincinnati, OH


SU-I-GPD-T-188 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: To describe an overview of the commissioning of a two-gantry proton pencil beam scanning.

Methods: The center is equipped with Varian’s ProBeam system integrated with Eclipse planning system and ARIA information system. The ProBeam system includes two clinical rooms with 360-degree rotating gantries, and consists of a cyclotron, an energy selection system, a beam transportation system and scanning nozzles, which is capable of delivering proton spots ranging from 4 to 36.3g/cm² to arbitrary shaped targets over a scanning area of 30x40cm at iso-center. As the most important factors for accurate beam delivery, proton ranges and beam positions were measured and verified independently at two rooms to ensure the agreement with beam specifications. All of required dosimetric parameters, including depth dose curves, in-air spot size and dose per monitor unit vs. energy, were then employed to create a calculation model in the treatment planning system. Treatment plans with the variable field sizes and depths were created, and measured for the validation of beam model accuracy. In addition, validations of the beam matching were performed between two rooms.

Results: The measured ranges were identical in two rooms within -0.5±0.1mm deviation from the specification. The beam position accuracy at isocenter was within 0.5mm for each of the eight gantry angles. For the validation plans, the average dose difference was -0.7%±1.6% for 58 fields within target regions. In particular, for small fields, the measurements were ~2% lower than calculations; but for fields with 5.7 cm WET range shifters, the measurement were ~2% higher than calculations. With up to 5% measured in-air spot size variations between two rooms, the output difference was -0.5±1.3% for 34 fields measured in both rooms.

Conclusion: The important parameters for beam delivery and dosimetry have been measured and validated, and the center is well-prepared to safely treat patients.

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