MR-Simulation Motion Emulation Versus Measured Gating Performance for Ultra-Central Lung Adaptive Treatments on 1.5T MR-Linac
Abstract
Purpose
To compare MR-simulation motion emulation with measured respiratory gating performance for ultra-central lung treatments on 1.5T MR-linac and to identify the dominant factors affecting gating efficiency. The study focuses on the role of tracking surrogate selection and evaluates whether motion amplitude alone explains observed duty cycle differences between simulation and treatment.
Methods
Five ultra-central lung PULSAR patients treated on a 1.5T MR-linac were analyzed. Respiratory motion was characterized at MR-simulation using cine imaging and used to emulate expected gating duty cycle with carefully selected case-specific tracking surrogates. Surrogates were defined to maximize tumor visibility while explicitly cropping adjacent structures that introduce competing motion signals. During treatment, real-time MR tracking recorded delivered duty cycle, tracking success, and 95th-percentile motion amplitudes in the left-right (LR), superior-inferior (SI), and anterior-posterior (AP) directions. Emulator predictions were compared with patient-averaged treatment metrics.
Results
Emulated duty cycle agreed with delivery within 2 percentage points in two patients (P1: 82.6% vs 82.95%; P5: 55.7% vs 54.3%). In the remaining three patients, delivered duty cycles exceeded emulation by 10–32 percentage points, with treatment duty cycles consistently above 90 percent despite one patient with values as low as 56%. Motion amplitudes varied across patients, with SI or AP excursions exceeding 4–6mm in several cases. Larger motion did not correspond to lower duty cycle. Tracking success remained high across all treatments, indicating stable tracking once an appropriate surrogate was defined.
Conclusion
The results show that respiratory gating performance for ultra-central lung is not primarily limited by motion magnitude. Gating efficiency depends on selecting a tracking surrogate that isolates respiratory motion and excludes nearby anatomy with independent motion. MR-simulation motion emulation reflects initial breathing behavior but does not capture subsequent improvements in respiratory regularity during treatment. With optimal surrogate selection, high gating efficiency can be achieved even for large-amplitude motion