Cardiac motion remains a challenge in the treatment of ventricular tachycardia with external beam ablation therapy. Current techniques involve expansion of the treatment area which can lead to unwanted collateral damage. Surrounding healthy tissue could be spared by gating the delivery of the beam to the cardiac cycle. In prior work, we assessed cardiac motion using in vivo fiducial markers and demonstrated that motion would be reduced if treatment were gated to half of the cardiac cycle, approximately corresponding to diastole. In the current work, we extend our prior analysis by quantitatively assessing the optimal gating window for motion reduction in the left ventricle. Motion was assessed in five porcine models with two fiducial clips per animal for a total of ten clips. The minimal cardiac motion occurred when the gating window started at 70% of the cardiac cycle. Without gating, three-dimensional cardiac motion was 7.0 ± 3.9 in x (left/right), 5.3 ± 2.5 in y (anterior/posterior), and 5.6 ± 2.3 in z (superior/inferior) mm. Using an optimal gating window, cardiac motion was 3.1 ± 1.8 in x (left/right), 2.5 ± 1.2 in y (anterior/posterior), and 3.1 ± 1.7 in z (superior/inferior) mm. The percentage reduction in motion with optimal gating was 51 ± 23 in x (left/right), 49 ± 21 in y (anterior/posterior), and 45 ± 24 % in z (superior/inferior). This work demonstrates that gating shows significant promise for reducing the effects of left ventricular motion when treating ventricular tachycardia with external beam ablation therapy.
Proton beam therapy has the potential to non-invasively treat ventricular tachycardia (VT) by homogenizing infarct scar. It has been previously demonstrated that proton beam therapy can create lesions in healthy myocardial tissue, thereby suggesting a potential for treatment of VT. In prior work, we quantified the relationship between dose delivered to myocardial tissue with lesion formation identified with in vivo, delayed contrast-enhanced magnetic resonance imaging (DCE-MRI) scans. In the current work, we evaluate the relationship of delivered dose with lesions identified in high resolution, post-mortem DCE-MRI scans. Deformable registration is used to align the dose maps from the baseline planning CT scans to ex vivo scans following proton beam therapy in swine. The current study demonstrates that nearly 100% of tissue exposed to a dose of 30 Gy or higher developed into lesion and approximately 85% of the tissue in the 20-30 Gy interval developed into lesion. On the other hand, tissue exposed to doses of 10 Gy or less tended to remain healthy myocardium, with less than 10% of tissue in the 5-10 Gy range and almost no tissue in the 0-5 Gy range developing into lesion.
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