In-Silico Trial of Same-Day Simulation-Free Spatially Fractionated Adaptive Radiotherapy (SF²-ART)

Here we present an in-silico trial of the feasibility and deliverability of a same-day, simulation-free workflow for spatially fractionated adaptive radiotherapy (SF²-ART) using the Varian Ethos platform.

Ten patients (five thoracic and five extremity), previously treated with spatially fractionated radiotherapy (SFRT), were selected for this in silico trial. A two-phase regimen was simulated: Phase 1 delivered 12 Gy SBRT to a uniform, predefined high-dose lattice and 4 Gy to the gross tumor volume (GTV); Phase 2 consisted of 4 additional fractions of 4 Gy to the GTV. The planning workflow was performed entirely in silico, without the need for CT simulation. High-dose sphere matrices were generated with a custom script and aligned to physician-defined GTVs. Adaptive plans were created based on CBCT anatomy and evaluated for dosimetric quality, deliverability, and workflow timing. Comparisons were made to the original clinically delivered plans. Mobius3D was used for secondary dose verification.

All 10 cases were successfully replanned using the SF²-ART workflow without simulation-based imaging. The entire planning and treatment process, including consultation, contouring, adaptive plan generation, and QA, was performed within a single session. Dosimetric comparisons showed minimal differences between simulation-free adaptive and clinically delivered plans, with target coverage and normal tissue constraints maintained across all cases. Median total estimated workflow time was 166.4 min.

This in-silico study demonstrates the feasibility of delivering spatially fractionated SBRT without simulation CT using a same-day adaptive workflow. The SF²-ART approach may enable rapid, high-quality treatment for patients with urgent or symptomatic presentations and supports future prospective clinical implementation.