Rotationally Intensified Proton Lattice: A Novel Lattice Technique Using Spot-Scanning Proton Arc Therapy

Purpose: The aim of this study was to explore the feasibility and dosimetric advantage of using spot-scanning proton arc (SPArc) for lattice radiation therapy in comparison with volumetric-modulated arc therapy (VMAT) and intensity modulated proton therapy (IMPT) lattice techniques. Methods: Lattice plans were retrospectively generated for 14 large tumors across the abdomen, pelvis, lung, and head-and-neck sites using VMAT, IMPT, and SPArc techniques. Lattice geometries comprised vertices 1.5 cm in diameter that were arrayed in a body-centered cubic lattice with a 6-cm lattice constant. The prescription dose was 20 Gy (relative biological effectiveness [RBE]) in 5 fractions to the periphery of the tumor, with a simultaneous integrated boost of 66.7 Gy (RBE) as a minimum dose to the vertices. Organ-at-risk constraints per American Association of Physicists in Medicine Task Group 101were prioritized. Dose-volume histograms were extracted and used to identify maximum, minimum, and mean doses; equivalent uniform dose; D95%, D50%, D10%, D5%; V19Gy; peak-to-valley dose ratio (PVDR); and gradient index (GI). The treatment delivery time of IMPT and SPArc were simulated based on the published proton delivery sequence model. Results: Median tumor volume was 577 cc with a median of 4.5 high-dose vertices per plan. Low-dose coverage was maintained in all plans (median V19Gy: SPArc 96%, IMPT 96%, VMAT 92%). SPArc generated significantly greater dose gradients as measured by PVDR (SPArc 4.0, IMPT 3.6, VMAT 3.2; SPArc-IMPT P = .0001, SPArc-VMAT P < .001) and high-dose GI (SPArc 5.9, IMPT 11.7, VMAT 17.1; SPArc-IMPT P = .001, SPArc-VMAT P < .01). Organ-at-risk constraints were met in all plans. Simulated delivery time was significantly improved with SPArc compared with IMPT (510 seconds vs 637 seconds, P < .001). Conclusions SPArc therapy was able to achieve high-quality lattice plans for various sites with superior gradient metrics (PVDR and GI) when compared with VMAT and IMPT. Clinical implementation is warranted. (c) 2024 The Authors. Published by Elsevier Inc. on behalf of American Society for Radiation Oncology. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).