Mini-ridge filter designs for conformal FLASH proton therapy

The present study designed a mini-ridge filter capable of generating a 30 mm spread-out Bragg peak (SOBP) specific to the proton FLASH beamline for use in proton FLASH mouse studies. Proton FLASH therapy delivers ultra-high dose rates exceeding 40 Gy/s, necessitating instantaneous SOBP generation via static method. The TOol for PArticle Simulation (TOPAS) was used to model the FLASH beamline, which enabled accurate replication of beam characteristics identified from measurements, and ensured accurate dose calculations. Comparisons between simulated and measured peak widths and R_80d values, accurate to within 1 mm, verified the accuracy and reliability of Monte Carlo code. Two mini-ridge filters of different materials were designed and optimized using MATLAB, simulated with TOPAS for SOBP verification, and fabricated using 3D-printing or computer numerical control manufacturing. The results indicated strong agreement between the simulated and measured SOBP widths for the polylactic acid mini-ridge filter but with discrepancies observed in the range and flatness measurements, which may be due to the use of 3D printing. By contrast, the aluminium alloy mini-ridge filter achieved excellent results, generating a 31.6 mm SOBP width with 0.89% flatness. Therefore, the present study validated the accuracy of our Monte Carlo code in simulating the FLASH beamline and the feasibility of our mini-ridge filters.