Room: AAPM ePoster Library
Purpose: To evaluate the dosimetric effect of the residual breathing motion during gating for the first two patients treated with MRI-guided cardiac radiosurgery.
Methods: Two patients suffering from ventricular tachycardia were treated in one fraction of 25 Gy to the 85%-isodose line on the MRIdian system (Viewray). The target volume was defined according to the cardiologists interpretation of the electrophysiological mapping of the arrhythmic substrate and expanded by 2 mm axially and 3 mm craniocaudally. Respiratory gating at end-of-exhale was performed by tracking a structure close to the diaphragm in sagittal 2D cine-MR images. The target offset from the geometric center of the gate with and without gating was evaluated in the two imaged directions. Based on these 2D position-probability maps, the dose to the patient with target motion was reconstructed with and without gating. Mean doses (Dmean) of target volume and organs (bronchial tree, esophagus and lungs) were evaluated.
Results: The mean (±std) target offset in SI direction could be reduced with gating from -4.8±4.4 to -1.5±2.0 mm and from -1.6±4.5 to 0.3±1.5 mm for patient 1 and 2, respectively. The reconstructed dose distribution showed a target volume Dmean of 27.0/27.0 Gy (patient1/2) without gating and 27.7/27.6 Gy with gating (planned: 27.8/27.9 Gy). Organs at risk did not show any increase in Dmean, expect for the bronchial tree with a Dmean of 2.4/2.3 Gy without gating and 1.7/1.5 Gy with gating (planned: 1.6/1.6 Gy).
Conclusion: MRI-guided cardiac radiosurgery was successfully applied to two patients. With the motion tracking and gating, the offset of the target to the beam could be reduced, allowing for small safety margins. The dose reconstructions showed that with gating, the patients could be treated robustly with a high dose to the target volume and minimal dose to organs at risk.