Implementation of a novel pencil beam scanning Bragg peak FLASH technique to a commercial treatment planning system

Ultra-high dose rate, or FLASH, radiotherapy has shown promise in preclinical experiments of sparing healthy tissue without compromising tumor control. This "FLASH effect" can compound with dosimetric sparing of the proton Bragg peak (BP) using a method called Single Energy Pristine Bragg Peak (SEPBP) FLASH. However, this and other proposed FLASH techniques are constrained by lack of familiar treatment planning systems (TPSs). Creating modules to implement SEPBP FLASH into a commercial TPS opens up the possibility of more widespread investigation of FLASH and lays the groundwork for future clinical translation.

To implement, investigate, and benchmark the capacity of a commercial TPS research extension for BP FLASH SBRT treatment planning by studying the dosimetric properties and FLASH ratio for critical organs-at-risk (OARs) at several sites.

A 250 MeV clinical proton beam model was commissioned in the Eclipse TPS (Varian Medical Systems, Palo Alto, USA). BP FLASH fields were single-layer maximum-energy beams with a universal range shifter (URS) and field-specific range compensators (RCs). RCs for each beam angle were included as contours within the structure set, while the URS was modeled in the PBS beamline. Spotmaps were created using Lloyd's algorithm with minimum monitor units (MU)-based spacing to ensure plan quality and preserve FLASH coverage for critical OARs. Inverse optimization while preserving minimum MU constraints was done with scorecard-based optimization. Fifteen SBRT cases from three anatomical sites (liver, lung, base-of-skull [BOS]) previously treated at the New York Proton Center were re-optimized using this method, and dosimetric characteristics of BP plans were compared to clinically treated plans. FLASH ratios for critical OARs were evaluated for BP FLASH plans.

The dose distributions, including target uniformity, conformity index (CI), and DVHs, showed no significant difference in clinically-used metrics between BP FLASH and clinically delivered plans across all anatomical sites. Mean 40 Gy/s FLASH ratios for critical OARs were above 84% for all but one OAR with 2 Gy threshold and above 98% for all OARs with 5 Gy threshold. D_max for liver and BOS cases was 111.3 ± 2.68 and 112.88 ± 1.29, respectively, and D_2% for lung cases was 112.04 ± 1.09. All D_max remained below 115%.

Inverse planning using a single-energy BP FLASH technique based on sparse spots and ultra-high minimum MU/spot can achieve intensity-modulated proton therapy (IMPT)-equivalent quality and sufficient FLASH coverage. This successful prototype brings us closer to commercial implementation and may increase the availability of proton FLASH dosimetry studies.