Respiratory-Induced Translations and Rotations of the Heart and Their Impact on Internal Target Volume Margins In Cardiac Radioablation
Abstract
Purpose
To investigate characteristics of the heart’s respiratory-induced motion and its geometric impact for motion monitoring in cardiac radioablation.
Methods
Six patients were imaged with 5 Hz, bi-planar, kV fluoroscopy on the Vero4DRT linac for 15-20 seconds under both abdominal compression and free breathing conditions in preparation for cardiac radioablation. Each patient had cardiac leads in the (i) right ventricle, (ii) right atrium, and (iii) coronary sinus, on the left ventricle. An in-house MATLAB application was used to track, triangulate, and separate respiratory-induced cardiac lead motion using a low-pass filter. The lead tip’s average position was registered to the planning CT DICOM structure set and the 3D respiratory-induced motion trace used to model the translations and rotations of the heart using singular value decomposition. This motion model was applied to the GTV to model respiratory induced (i) translations and rotations or (ii) rotations alone. Accounting for the rotations alone assumes that the translational motion of the cardiac lead cluster is perfectly accounted for. Non-isotropic internal target volume (ITV) margins were iteratively applied with a resolution of 0.5 mm (right-left and anterior-posterior) and 1.25 mm (inferior-superior) by extending the static GTV mask until it encompassed the total volume swept out by the moving GTV.
Results
The translation and rotational motion model fit the respiratory-induced cardiac lead tip motion with an average (±STD) 3D error vector magnitude of (0.4±0.1) mm. The peak-to-peak extent of rotation ranged from 0.2-4.9° depending on the patient and rotation axis. Across all 12 datasets, the average (±STD) margin to account for respiratory-induced translations and rotations was [3.3±1.7 RL, 7.5±3.8 SI, 4.4±2.0 AP] mm. Perfect compensation of the cardiac lead cluster’s translations reduced the needed ITV margins to [2.1±1.1 RL, 2.8±0.8 SI, 2.0±0.6 AP]mm.
Conclusion
Respiratory-induced rotations have a geometric effect on ITV margins, warranting dosimetric investigation.