Surface-Guided Radiotherapy for Lung SBRT: A Large-Cohort Analysis of Intrafraction Motion and Setup Accuracy
Abstract
Purpose
This study aims to (1) quantify time-resolved intrafraction motion in six degrees of freedom during lung SBRT delivered under optical surface monitoring system (OSMS) guidance and (2) derive clinically relevant motion metrics for tolerance selection.
Methods
OSMS log files from over 1,100 minimally immobilized lung SBRT patients (~6,000 fractions) were retrospectively analyzed. Beam-time–stamped surface positions were grouped into consecutive 5s intervals from beam-on to beam-off. For each fraction and time bin, the mean displacement and standard deviation (representing “jitter”) were calculated for all six degrees of freedom. Across all fractions and bins, distributions of means and jitter were summarized using 90th and 95th percentiles.
Results
Fractions with irradiation durations ranging from 30 to 480 s were included in analysis with a mean irradiation time was 150 s, 95% of fractions were completed within 270 s. The 90th percentiles of mean translational displacement were 1.0 mm (vertical), 1.4 mm (longitudinal), and 1.0 mm (lateral), with 95th percentiles at 1.0, 2.0, and 1.2 mm, respectively. The 90th percentiles of mean rotational motions were 0.4° (yaw), 0.3° (roll), and 0.4° (pitch), with 95th percentiles of 0.5°, 0.4°, and 0.6°, respectively. Longitudinal translation and pitch showed the largest variability, whereas lateral translation and roll were the most stable.
Conclusion
In this large lung SBRT cohort, OSMS-derived intrafraction motion remained predominantly sub-millimeter and sub-degree over typical 2–4 minute beam-on times. These results support the adequacy of our current SGRT-guided immobilization strategy and provide quantitative benchmarks for center-specific motion limits and margin design.