3136 - Free-Breathing Tumor Motion at Simulation Predicts Intrafraction Management Burden in Lung SBRT

Purpose/Objective(s): Free-breathing tumor motion assessed under fluoroscopy at simulation guides motion management decisions in lung SBRT, including the use of abdominal compression and respiratory gating. We hypothesized that greater free-breathing motion at simulation predicts increased intrafraction management burden during treatment delivery, as measured by total fraction time, intrafraction couch shifts, gate-window adjustments, and imaging utilization.

Materials/Methods: We retrospectively reviewed 89 patients (100 plans, 245 delivered fractions) treated with lung SBRT at a single institution (2014-2024). Initial free-breathing tumor motion was assessed under fluoroscopy at simulation prior to any compression or gating and categorized as low (<=5 mm), moderate (6-10 mm), or high (>10 mm). Each motion assessment was linked to the corresponding treatment plan. Plan-level gating status and fraction-level delivery metrics were extracted from treatment records: total fraction time (first setup event to last beam delivered), setup time, beam delivery time, CBCT series, gate-window adjustments, intrafraction couch shifts, and triggered imaging. Endpoints were compared across motion categories and by fraction number using median and IQR. Within-gated correlations between motion magnitude and delivery metrics were assessed with Pearson r.

Results: Median free-breathing motion was 7 mm (IQR 4-12; range 2-50). Gating was used in 86.1% of high-motion plans vs 62.5% of low-motion plans. High-motion fractions had the longest total fraction time (median 1162 s, IQR 818-1491) compared with moderate (879 s) and low (876 s), driven by setup time (627 vs 556 vs 524 s) rather than beam delivery time (390 vs 323 vs 343 s). High-motion fractions also had the most couch shifts (median 2, IQR 1-6) and triggered imaging (52.2% vs 40.2%). Among 161 gated fractions, baseline motion correlated with delivery time (r=0.31), couch shifts (r=0.277), and gate adjustments (r=0.266). First gated fractions were longest (median 1139 s) and stabilized by fraction 2 (890 s).

Conclusion: Free-breathing tumor motion assessed at simulation predicts intrafraction management burden in lung SBRT. Fractions planned for tumors with >10 mm motion required approximately 33% longer total treatment time than those with <=10 mm, primarily through extended setup. Within gated treatments, motion magnitude correlated with delivery time, couch shifts, and gate-window adjustments. Simulation motion assessment may inform not only the decision to gate but also anticipatory scheduling and staffing for treatment sessions.