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Session: Therapy General ePoster Viewing [Return to Session]

Towards Conformal FLASH PBS Proton Therapy: Beam Model Verification and Plan Characteristics

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


PO-GePV-T-159 (Sunday, 7/10/2022)   [Eastern Time (GMT-4)]

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Purpose: The purpose of this work is to develop and validate conformal FLASH PBS proton planning model using single-layer transmission field. This development will allow for better exploitation of PBS advantages, and potentially support next-stage in-human FLASH-RT.

Methods: For this study, a cohort of ten extremity bone metastasis patients was selected for evaluation. A beam model with absolute dose calibration was created containing a library of single-layer rectangular-shaped transmission fields. Beam data including depth dose and in-air spot profiles, was acquired to calculate a beam model in a commercial TPS. For each patient, two transmission PBS plans were evaluated – a rectangular plan from a predefined library and a beams’ eye view conformal plan calculated in TPS, by projecting single-layer spots to an arbitrary-shape target. These plans were delivered in ultra-high dose rate (>40Gy/s) on a FLASH-enabled PBS gantry. Absolute doses were measured at a fixed depth using a parallel plate chamber, and were compared with dose calculations. Gamma index analysis was also performed on two-dimensional dose distributions acquired by radiochromic films. In addition, the conformal plans were compared to those rectangular-shaped fields in terms of spot numbers, normal tissue doses and voxel dose rates.

Results: For these ten patients, the point dose deviation was 0.93%±0.95% at 5cm WED, and gamma index achieved 91.24%±2.4% using 3%/3mm criteria. Compared to rectangular-shaped fields, conformal plans achieved a reduction of number of spots by 34.6±14.1%, and a reduction of normal tissue mean dose by 28.24±12.3%. Higher or comparable voxel dose rate was also obtained. Particularly, for three cases with most amount of spot reductions, voxel dose rate range increased from 50-60Gy/s to 90-100Gy/s.

Conclusion: For FLASH-RT, conformal planning can potentially reduce normal tissue dose with better delivery efficiency. The validation results demonstrated good clinical accuracy of the beam model calculated in arbitrary-shaped targets.


Not Applicable / None Entered.


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