Purpose: Stereotactic arrhythmia radioablation (STAR) is a promising treatment option for ventricular tachycardia (VT) and atrial fibrillation (AF). The target is, however, subject to complex cardiorespiratory motion and is situated close to dose-sensitive structures. Therefore, small PTV margins achieved with active motion mitigation are preferred. We investigate the technical feasibility of using MRI-guided MLC-tracking on the 1.5T Unity MR-linac (Elekta AB, Stockholm, Sweden) to simultaneously compensate for respiratory and cardiac motion.
Methods: MRI-guided MLC-tracking was enabled by coupling in-house software to a research interface on the MR-linac. First, the leaf tracking performance was evaluated by tracking sinusoidal trajectories with maximum speeds ranging from 6-12cm/s and estimating the RMSE of the actual leaf positions with respect to the requested ones. Second, dosimetry measurements were performed using a Quasarᵀᴹ MRI⁴ᴰ (ModusQA, Ontario, Canada) motion phantom with a radiochromic film insert, which was programmed to perform cardiac-only (sin⁴, 10mm peak-to-peak, 70bpm) and composite cardiorespiratory (card: sin⁴, 10mm peak-to-peak, 70bpm; resp: sin, 20mm peak-to-peak, 12bpm) trajectories. A 15-beam IMRT plan (1x25Gy) was created and scaled by a factor of ¼ to fit the film’s dynamic range. 11.6Hz 2D Cine MRI (FOV=400x207x15mm³, resolution=3x3x15mm³) was used to estimate the positions, after which a prediction filter (ridge regression) was applied to compensate for the end-to-end system latency (160-246ms). For both trajectories tracked- and non-tracked deliveries were compared to a static delivery by means of a local 2%/2mm gamma analysis.
Results: The RMSE of the actual leaf positions ranged from 0.46mm (6cm/s) to 0.57mm (12cm/s). The gamma-pass-rate of the cardiac-only trajectory increased from 72% without MLC-tracking to 96% with MLC-tracking. Similarly, the gamma-pass-rate of the cardiorespiratory trajectory increased from 33% to 92%.
Conclusion: The Unity MR-linac is technically capable of performing very fast MLC-tracking, enabling real-time motion-compensated deliveries for targets subject to complex cardiorespiratory motion.
Funding Support, Disclosures, and Conflict of Interest: Partially funded by NWO, BREATH EASY (#17515)
MRI, Linear Accelerator, Heart
IM/TH- MRI in Radiation Therapy: MRI/Linear accelerator combined- IGRT and tracking