Operational Feasibility of Adaptive Lattice SBRT on Ethos Platform
Abstract
Purpose
Lattice stereotactic body radiation therapy (Lattice SBRT) delivers spatially fractionated high-dose vertices embedded within a lower-dose tumor base, producing steep dose gradients potentially sensitive to interfractional anatomical change. This study evaluated the feasibility of delivering 5-fraction Lattice SBRT on the Varian Ethos adaptive platform, assessing both dosimetric integrity under substantial anatomical changes and online adaptive workflow efficiency.
Methods
Five previously treated Lattice SBRT cases (lung, abdomen, head and neck) were retrospectively analyzed using a protocol delivering 66.7 Gy to lattice spheres and 20 Gy to the base in 5 fractions. For each case, the fraction with greatest anatomical change was emulated using patient CBCT, with up to approximately 50% reduction in target volume. To avoid manual sphere contouring during online adaptation, oversampled lattice structures (1.5 cm diameter spheres, 3 cm spacing) were pre-generated and propagated. Candidate spheres were identified via Boolean intersection with an inner margin structure, followed by manual exclusion of partial spheres. Plan quality and workflow timing were compared between reference and emulated adaptive plans.
Results
All emulated adaptive plans met institutional OAR constraints with target coverage consistent with prescription goals (V66.7 Gy ≈ 95%). Despite reduced sphere counts during adaptation (median reduction: 2 spheres), lattice dosimetric characteristics were preserved with peak-to-valley dose ratios of 3.5–4.3. Critically, these results demonstrate not only feasibility but also the necessity of adaptive delivery—non-adaptive plans would deliver high-dose vertices outside the regressed tumor boundary. Plan optimization times ranged from 7.1–23.4 minutes (median: 10 minutes), well within typical online adaptive session constraints.
Conclusion
Adaptive Lattice SBRT is feasible on Varian Ethos even with substantial anatomical changes. Preplanned oversampled lattice structures combined with Boolean sphere selection enable robust online adaptation while preserving lattice dosimetry within clinically acceptable treatment times, supporting Ethos as a practical platform for adaptive Lattice SBRT.