Reduced beam time and distal linear energy transfer with mini-ridge filters in pencil beam scanning proton therapy

Background and purpose: Previous studies have demonstrated that mini-ridge filters (MRF) can reduce beam delivery time in pencil beam scanning (PBS) proton therapy, but the effect on plan quality has not been analyzed in detail. Furthermore, the range modulation produced by the MRF may lead to an added benefit of reduced dose-averaged linear energy transfer (LETd) on the distal edge of the beam. The aim of this study is to demonstrate a statistically significant reduction in beam delivery time and distal LETd with a MRF without any degradation in plan quality. Materials and methods: Twenty patient plans from a range of treatment sites, including five deep inspiration breath hold (DIBH) patients, have been re-optimized with a Monte Carlo based MRF beam model and compared to the corresponding clinical plans in beam delivery time, clinical acceptability, and a range of dose and LETd metrics. Results: The average delivery time per beam decreased from (96.7 ± 4.9) s to (75.3 ± 4.0) s (p < 0.001). DIBH patients saw a reduction from (82.8 ± 15.5) s to (70.4 ± 12.7) s. The average reduction in maximum LETd to any organ-at-risk (OAR) within the 500 cGyRBE isodose line was (0.43 ± 0.06) keV/μm (p < 0.001). There was no significant difference in target dose homogeneity, conformality, robust target coverage, OAR dose metrics or LETd within the target. Conclusions: MRFs can be used in PBS proton therapy to achieve a clinically relevant reduction in beam delivery time, and distal LETd without sacrificing plan quality.