  Purpose: This study explored the feasibility of a streamlined pin-shaped
ridge filter (pin-RF) design for single-energy proton FLASH planning. Methods:
An inverse planning framework integrated within a TPS was established for FLASH
planning. The framework involves generating a IMPT plan based on downstream
energy modulation strategy (IMPT-DS), followed by a nested spot reduction
process to iteratively reduce the total number of pencil beam directions (PBDs)
and energy layers along each PBD for the IMPT-DS plan. The IMPT-DS plan is then
translated into the pin-RFs for a single-energy IMPT plan (IMPT-RF). The
framework was validated on three lung cases, quantifying the FLASH dose of the
IMPT-RF plan using the FLASH effectiveness model and comparing it with the
reference dose of a conventional IMPT plan to assess the clinical benefit of
the FLASH planning technique. Results: The IMPT-RF plans closely matched the
corresponding IMPT-DS plans in high dose conformity, with minimal changes in
V7Gy and V7.4Gy for the lung (< 5%) and small increases in Dmax for other OARs
(< 3.2 Gy). Comparing the FLASH doses to the doses of corresponding IMPT-RF
plans, drastic reductions of up to ~33% were observed in Dmax for OARs in the
high-to-moderate-dose regions with negligible changes in Dmax for OARs in
low-dose regions. Positive clinical benefits were observed with notable
reductions of 18.4-33.0% in Dmax for OARs in the high-dose regions. However, in
the moderate-to-low-dose regions, only marginal positive or even negative
clinical benefit for OARs were observed, such as increased lung V7Gy and V7.4Gy
(16.4-38.9%). Conclusions: A streamlined pin-RF design for single-energy proton
FLASH planning was validated, revealing positive clinical benefits for OARs in
the high dose regions. The coarsened design of the pin-RF demonstrates
potential cost efficiency and efficient production.
