R Liu*, S Charyyev, N Wahl , W Liu, J Zhou, X Yang, W Dynan, M Kang, K Higgins, J Bradley, L Lin, Emory University, Atlanta, GA

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  R Liu

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TU-F115-IePD-F5-6 (Tuesday, 7/12/2022) 1:15 PM - 1:45 PM [Eastern Time (GMT-4)]

Exhibit Hall | Forum 5

Purpose: Patient specific 3D ridge can cover SBRT target with IMPT-like dose even in one single field. By removing proton energy switching time, this approach can potentially achieve FLASH timing. We investigate the feasibility of proton SBRT FLASH treatment with dose, dose rate, and LET optimization using 3D ridge filters.

Methods: An in-house inverse planning software has been developed for design of patient-specific ridge filters, which are used to spread a fixed-energy, 250 MeV beam over a proximal beam-specific planning target volume (BSPTV) to ensure IMPT plan robustness. Ridge pin positions are defined in beam’s eye view of BSPTV. Geant4 was used to provide dose and LET influence matrices. We modified matRad optimization to accommodate integrated biological optimization IMPT (IBO-IMPT) objective function, considering dose, dose rate, and LET terms. A biological extra dose (XBD) model was used to evaluate the effectiveness of organ-at-risk (OAR) sparing. A minimum MU constraint was implemented. A lung case was studied in this work.

Results: For single beam, patient specific ridge filter enables FLASH delivery to achieve dose coverage while meeting constraints. The IBO-IMPT optimization further spares heart by increasing volume of FLASH dose rate (40 Gy/s) about 3% with slight risk of inducing hotspot and under-coverage on target. The lung is spared by reducing its dose. On the contrary, with three beams, the proposed workflow enables further sparing OARs by increasing DADR while maintaining the target coverage. For heart, the volume of FLASH dose rate (40 Gy/s) increases about 4%, and for lung it increases about 3%. Also, the volume of high LET (>4 keV/µ) of heart can be decreased through IBO-IMPT.

Conclusion: Our preliminary data shows that it is feasible to perform FLASH SBRT, with reduced LET and increased DADR for OARs, using IBO-IMPT planning in combination with a patient-specific 3D ridge filter.

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