1129 - Longitudinal Analysis of Log Files from Gated Delivery of Stereotactic Body Radiotherapy Using Motion Management in a 1.5 T MR-Linac

Purpose/Objective(s): A recent upgrade in the 1.5 T MR-Linac enables gated treatments triggered by respiratory and non-respiratory motion. We present initial results from the longitudinal analysis of patients treated with SBRT during the first four months of the clinical implementation of comprehensive motion management (CMM).

Materials/Methods: Delivery and CMM log files were used to extract and estimate the target’s position during cine MRI measurements and when the treatment beam was on. The total motion range was defined as the 95th percentile of the target’s positional distribution. To separate beam-on motion into drift and respiratory motion, a 20-second moving average low-pass filter was applied along with a high-pass filter at 0.05 Hz. Power spectral analysis of the position vector was used to estimate the main frequencies in the measured signal. The duty cycle was categorized into CMM and non-CMM events and quantified as the percentage of time each event did not interrupt treatment. Data from patients undergoing SBRT in five or fewer fractions were pooled for analysis. Patients whose treatments were not completed in the MR-Linac were not included in the analysis.

Results: A total of 113 non-respiratory gated and 50 respiratory gated treatment sessions were analyzed. The median (IQR) delivery times were 13.3 (3.9) minutes and 17.5 (7.4) minutes, respectively (p < 0.001). Table 1 presents the median (IQR) motion range during delivery and the duty cycle for each event. Under both gating strategies, the median 95th percentile of motion along each cardinal axis differed significantly between monitoring and beam-on periods. A significant negative correlation was observed between the measured motion range and the CMM duty cycle. The median (IQR) frequency of CMM beam-hold events attributed to low prediction accuracy and low tracking quality were 0.0 (6.25)% and 1.4 (13.7)%. Dominant frequencies were primarily associated with respiratory motion and noise.

Conclusion: The initial clinical experience with comprehensive motion management demonstrates the feasibility and effectiveness of both respiratory and non-respiratory gating strategies for SBRT. Significant differences in target motion between monitoring and beam-on underscore the importance of real-time gating for maintaining accurate target coverage.