Aperture-collimated proton LATTICE radiotherapy with a GRID-like entrance pattern: a compact and robust delivery strategy.

Abstract

ObjectiveProton LATTICE radiotherapy(LRT), a form of spatially fractionated radiotherapy(SFRT), delivers high doses to intratumoral vertices while maintaining low valley dose. Single-field optimization(SFO) preserves peak-to-valley structure robustly but often causes high entrance dose. Since smaller, more closely spaced high-dose regions may improve the therapeutic ratio, we developed AC-LATTICE, which reshapes SFO proton LATTICE fields into narrow GRID-like entrance beamlets to reduce entrance dose while preserving vertex coverage, spatial modulation, and robustness.
Approach: A multi-field, geometry-constrained semi-FCC vertex optimization framework was developed to generate non-overlapping intratumoral vertices (4.4 mm diameter; minimum 3D spacing 1.4 ± 0.2 cm) and map each vertex to a deliverable GRID beamlet under spacing and clearance constraints. A fixed GRID entrance pattern was imposed in beam's-eye view (1.0 cm pitch; 5 mm openings). AC-LATTICE plans were retrospectively generated in RayStation v2023B for eight DIBH primary liver cancer patients (mean GTV 89.7 cc; range 57.6-160.6 cc). Prescription was 18 Gy per vertex and 3 Gy to the GTV using SFO. Metrics included GTV D95%, Dmean, gEUD (a = -10), PVDR (D2%/D50%), skin D1%, and liver-GTV Dmean. Robustness was evaluated using 3.5% range and 5 mm setup uncertainties (second-worst case), with CBCT-based assessment throughout treatment.
Main results: AC-LATTICE maintained spatial modulation(PVDR ≥ 3.5) while preserving target and normal-tissue sparing. Nominal plans achieved PVDR 3.6 ± 0.4, GTV D95% 3.1 ± 0.08 Gy, GTV Dmean 5.8 ± 0.6 Gy, gEUD 3.5 ± 0.1 Gy, liver-GTV Dmean 0.9 ± 0.3 Gy, and skin D1% 1.4 ± 0.6 Gy. In the robust second-worst case, PVDR decreased modestly to 3.3 ± 0.4, while CBCT evaluation showed no meaningful degradation. End-to-end phantom testing achieved a 99% gamma passing rate(2%/2 mm).
Significance: AC-LATTICE is a compact, robust, and clinically feasible proton LRT strategy that may broaden SFRT applicability when entrance dose is a concern.