An adaptive proton FLASH therapy using modularized pin ridge filter

In our previous study, we developed a modular pin ridge filter (pRF) design framework to streamline assembly, enabling the fast manufacture of custom filters optimized for single-energy proton FLASH planning.

In this paper, we propose a method to optimize adaptive proton FLASH therapy (ADP-FLASH) using modularized pRFs by recycling module pins from the initial plan while reducing pRF adjustments in adaptive FLASH planning.

Initially, single energy (250 MeV) FLASH-pRF plans were created using pencil beam directions (PBDs) from initial IMPT plans on the planning CT (pCT). PBDs are classified as new/changed (ΔE > 5 MeV) or unchanged by comparing spot maps for targets between pCT and re-CT. We used an iterative least-square regression model to identify recyclable PBDs with minimal relative changes to spot MU weighting. Two PBDs with the least square error were retrieved per iteration and added to the background plan, and the remaining PBDs were reoptimized for the adaptive plan in subsequent iterations. The method was validated on three liver SBRT cases (50 Gy in five fractions) by comparing various dosimetric parameters across initial pRF plans on pCT, re-CT, and the ADP-FLASH-pRF plans on re-CT.

V₁₀₀ for initial-pRF plans on pCT, re-CT, and ADP-FLASH-pRF plans for the three cases were as follows: (93.7%, 89.2%, 91.4%), (93.5%, 60.2%, 91.7%), and (97.3%, 69.9%, 98.8%). We observe a decline in plan quality when applying the initial pRF to the re-CT, whereas the ADP-FLASH-pRF approach restores quality comparable to the initial pRF on the pCT. FLASH effect of the initial pRF and ADP pRF plans were evaluated with a dose and dose rate threshold of 1 and 40 Gy/s, respectively, using the FLASH effectiveness model. The proposed method recycled 91.2%, 71%, and 64.7% of PBDs from initial pRF plans for the three cases while maintaining all clinical goals and preserving FLASH effects.

This study validated a method for recycling the pRFs in single-energy proton FLASH planning for SBRT cases. This framework offers a scalable solution for adaptive proton therapy, balancing clinical effectiveness and practicality.