Abstract
Purpose or Objective Stereotactic arrhythmia radioablation (STAR) serves as a novel alternative approach for the treatment of refractory ventricular tachycardia (VT). Early evidence demonstrates impressive reductions (>90%) in VT burden with a single 25 Gy fraction. Clearly, toxicity is a major concern and should be proactively minimised by cardiorespiratory motion
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management. While respiratory motion mitigation is widely applied in stereotactic radiotherapy, cardiac motion presents a unique challenge due to its rapid periodicity. In this study, we provide the first experimental demonstration of real-time adaptive MRI-guided STAR. Materials and Methods An MRI-guided real-time adaptive STAR workflow was developed on the 1.5 T Unity MR-linac (Elekta AB, Stockholm, SE) in research mode (Fig. 1). The Quasar MRI4D phantom was used with a cardiorespiratory motion pattern consisting of a respiratory (sine, 12 bpm, 20 mm peak-to-peak) and a cardiac (cos4, 60 bpm, 10 mm peak-to-peak) component. Embedded in the water-filled body oval was a moveable film dosimetry insert with a spherical 3 cm target. An IMRT treatment plan for Unity (1x25 Gy) was created in Monaco v.5.40.01 (Elekta AB). MR-guidance was performed by using 2D cine-MRI (bSSFP, 13 Hz, TR/TE=3/1.48 ms, SENSE=1.5, FOV = 400x207 mm2, voxel size = 3x3x15 mm, PF factor = 0.65). The detected cardiorespiratory motion was deconvoluted by prospective linear Kalman filtering. The respiratory motion was then used as input for MLC tracking, while the cardiac motion was used as input for beam gating (“track+gate”). Alternative delivery scenarios were created either without any motion (“static”), or by ignoring cardiac motion (“track-only”), or by gating independently on both cardiac and respiratory motion (“dual-gate”). To fit the dynamic range of the Gafchromic EBT3 film, MUs were quartered before the delivery. For cardiac motion, a gating window of 3.8 mm was defined to simulate delivery in diastole. For dual-gating, the exhale gating window was set to 10 mm to limit excessive treatment times. System latencies were quantified by using the portal imager on Unity. Results The system latencies of 175 ms (MLC tracking) and 70 ms (gating) were effectively eliminated by using a linear regression predictor. The treatment delivery times were 5.7 mins (static, track-only), 8.8 mins (track+gate), and 20 mins (dual-gate). Dose difference maps confirm that the track+gate scenario best mimics the static dose distribution (Fig. 2). Dose profiles on the 2 (6.25) Gy isolines along the CC direction showed widening of 3.9 (-3.2) mm for track-only, 2.5 (-2.7) mm for dual-gate, and 0.5 (0) mm for track+gate deliveries with respect to the static reference. The gamma pass rates (2%/2mm) were 96.6% (track-only), 96.7% (dual-gate) and 99.4% (track+gate). Conclusion This is the first experimental demonstration of simultaneous cardiorespiratory motion management for MRI- guided STAR resulting in very high dosimetric accuracy.
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