Real‐time monitoring and appropriate margin selection in linac‐based prostate SBRT: Analysis of two different intrafraction motion monitoring systems

Intrafraction motion poses a major challenge in ultrahypofractionated prostate SBRT, where high doses and fewer sessions amplify the impact of positional shifts. Its management varies across centers due to differences in technology, protocols, and resources, and standardized clinical guidelines are still lacking despite widespread recognition of its importance. To provide insights into real-time monitoring and margin selection by quantifying intrafraction prostate motion using two distinct monitoring systems-system A (electromagnetic transmitter-based) and system B (4D transperineal ultrasound-based)-and evaluate their impact on motion patterns in Linac-based SBRT. Intrafraction motion data were collected from 92 treatment fractions using system A (47 patients) and 100 fractions using system B (30 patients). All patients underwent SBRT (1-5 fractions) with strict rectum and bladder preparation. Motion was recorded at 1 Hz (system A) and 2 Hz (system B) from CBCT acquisition to treatment completion, independent of any patient repositioning. Motion trend was analyzed using a mixed-effects regression model, and Kaplan-Meier failure analysis estimated the time to exceed 2, 3, 4, and 5 mm motion thresholds. Median monitoring time was 13 min for system A (range: 5.9-35.6) and 6.4 min for system B (range: 4.9-29.3). Motion increased over time in all directions (p < 0.001), predominantly in the posterior direction (time coefficient = -0.20, p < 0.001). A significant longitudinal (LNG) motion divergence was observed between systems (p < 0.001). Kaplan-Meier failure curves were comparable, except for a significant difference in the LNG axis at the 2 mm threshold (log-rank test, p = 0.006). The failure rates at 3-10 min for this threshold ranged from 7.3% to 52.4% (system A) and 4.2% to 22.4% (system B). A substantial percentage of fractions (> 5%) exceeded the 2 mm motion threshold at 1.8 min, the 3 mm threshold at 3.8 min, the 4 mm threshold at 6.6 min, and the 5 mm threshold at 8.1 min. Both systems exhibited predominant posterior motion, with system B reducing small LNG prostate fluctuations, likely due to probe pressure against the perineum. For fast Linac-based SBRT, intrafraction motion can be managed with 4-5 mm margins without continuous monitoring. However, if 2-3 mm margins are used, real-time monitoring becomes essential.