Streamlined pin‐ridge‐filter design for single‐energy proton FLASH planning

Abstract

BackgroundFLASH radiotherapy (FLASH‐RT) with ultra‐high dose rate has yielded promising results in reducing normal tissue toxicity while maintaining tumor control. Planning with single‐energy proton beams modulated by ridge filters (RFs) has been demonstrated feasible for FLASH‐RT.PurposeThis study explored the feasibility of a streamlined pin‐shaped RF (pin‐RF) design, characterized by coarse resolution and sparsely distributed ridge pins, for single‐energy proton FLASH planning.MethodsAn inverse planning framework integrated within a treatment planning system was established to design streamlined pin RFs for single‐energy FLASH planning. The framework involves generating a multi‐energy proton beam plan using intensity‐modulated proton therapy (IMPT) planning based on downstream energy modulation strategy (IMPT‐DS), followed by a nested pencil‐beam‐direction‐based (PBD‐based) spot reduction process to iteratively reduce the total number of PBDs and energy layers along each PBD for the IMPT‐DS plan. The IMPT‐DS plan is then translated into the pin‐RFs and the single‐energy beam configurations for IMPT planning with pin‐RFs (IMPT‐RF). This framework was validated on three lung cases, quantifying the FLASH dose of the IMPT‐RF plan using the FLASH effectiveness model. The FLASH dose was then compared to the reference dose of a conventional IMPT plan to measure the clinical benefit of the FLASH planning technique.ResultsThe IMPT‐RF plans closely matched the corresponding IMPT‐DS plans in high dose conformity (conformity index of <1.2), with minimal changes in V7Gy and V7.4 Gy for the lung (<3%) and small increases in maximum doses (Dmax) for other normal structures (<3.4 Gy). Comparing the FLASH doses to the doses of corresponding IMPT‐RF plans, drastic reductions of up to nearly 33% were observed in Dmax for the normal structures situated in the high‐to‐moderate‐dose regions, while negligible changes were found in Dmax for normal structures in low‐dose regions. Positive clinical benefits were seen in comparing the FLASH doses to the reference doses, with notable reductions of 21.4%–33.0% in Dmax for healthy tissues in the high‐dose regions. However, in the moderate‐to‐low‐dose regions, only marginal positive or even negative clinical benefit for normal tissues were observed, such as increased lung V7Gy and V7.4 Gy (up to 17.6%).ConclusionsA streamlined pin‐RF design was developed and its effectiveness for single‐energy proton FLASH planning was validated, revealing positive clinical benefits for the normal tissues in the high dose regions. The coarsened design of the pin‐RF demonstrates potential advantages, including cost efficiency and ease of adjustability, making it a promising option for efficient production.

Navigating the straits: realizing the potential of proton FLASH through physics advances and further pre-clinical characterization

“…Current Bragg peak FLASH research lies at the interface between plan optimization and ridge filter design. One optimization approach seeks to achieve range modulation using steps coarse enough to lie within the tolerance of 3D printers or milling machines used to build ridge filters, but fine enough to generate close-to-optimal dose-distributions ( 78 ).…”

“…A further option would be to construct ridge filter pins from individual pre-manufactured columns ( 78 ). Each filter might comprise several thousand columns, and so a rapid-assembly system would be required for online adaptation.…”