Ultrafast Proton Delivery with Pin Ridge Filters (pRFs): A Novel Approach for Motion Management in Proton Therapy

TL;DR

This paper introduces a novel use of pin ridge filters (pRFs) to create a single-energy proton therapy plan, significantly reducing treatment time and organ dose compared to conventional methods.

Contribution

The study develops a new beam modeling and planning approach using pRFs for ultrafast proton delivery, extending their application from FLASH radiotherapy to proton therapy motion management.

Findings

Dose to organs at risk was significantly reduced.

Treatment delivery time was decreased by over 80%.

Method validated on lung and liver SBRT cases.

Abstract

Active breath-hold techniques effectively mitigate respiratory motion but pose challenges for patients who are ineligible for the procedure. Conventional treatment planning relies on multiple energy layers, extending delivery time due to slow layer switching. We propose to use pin ridge filters (pRFs), initially developed for FLASH radiotherapy, to construct a single energy beam plan and minimize dose delivery time. The conventional ITV--based free--breathing treatment plan served as the reference. A GTV--based IMPT--DS plan with a downstream energy modulation strategy was developed based on a new beam model that was commissioned using the maximum energy of the IMPT plan. Consequently, a nested iterative pencil beam direction (PBD) spot reduction process removed low--weighted spots along each PBD, generating pRFs with coarser resolution. Finally, the IMPT--DS plan was then converted into an IMPT--pRF plan, using a monoenergetic beam with optimized spot positions and weights. This approach was validated on lung and liver SBRT cases (10 Gy RBE x 5). For the lung case, the mean lung--GTV dose decreased from 10.3 Gy to 6.9 Gy, with delivery time reduced from 188.79 to 36.16 seconds. The largest time reduction was at 150{\deg}, from 47.4 to 3.99 seconds. For the liver case, the mean liver--GTV dose decreased from 5.7 Gy to 3.8 Gy, with delivery time reduced from 111.13 to 30.54 seconds. The largest time reduction was at 180{\deg}, from 38.57 to 3.94 seconds. This method significantly reduces dose delivery time and organ at risk dose. Further analysis is needed to validate its clinical feasibility.