External beam ablation therapy has the potential to treat cardiac arrhythmias non-invasively by targeting arrhythmogenic myocardial tissue; however, a challenge of treating cardiac tissue with beam ablation therapy is cardiac motion. Currently, cardiac motion is typically compensated by expansion of the target volume which can potentially lead to collateral damage of surrounding healthy tissue. This collateral damage could be minimized by gating the beam delivery to a portion of the cardiac cycle. In prior work, we evaluated cardiac motion using anatomic landmarks in multi-phase cardiac computed tomography volumes of swine hearts across the left atria and ventricles. Other work evaluated left atrial motion using implanted fiducial clips. In the current work, we extend this prior work by quantifying cardiac motion using gold standard implanted fiducial clips across all four chambers of the heart. Cardiac motion varied by chamber, ranging from 2.1 to 7.2 mm in the x direction, 7.2 to 8.1 mm in the y direction, and 3.1 to 9.7 mm in the z direction. In addition, we quantify the reduction in motion if delivery were gated to phases 40% to 90% of the cardiac cycle, which corresponds to treating across 50% of the cardiac cycle. Cardiac motion across 50% of the cardiac cycle ranged from 1.1 to 5.3 mm in the x direction, 4.5 to 5.2 mm in the y direction, and 1.2 to 7.8 mm in the z direction. Percentage reduction in motion for treating during 50% of the cardiac cycle ranged from 18% to 47% in the x direction, 31% to 43% in the y direction, and 11% to 61% in the z direction. These results demonstrate that a substantial improvement in target localization could be achieved by gating the beam to 50% of the cardiac cycle.

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.

External beam therapy has recently been proposed for treatment of ventricular tachycardia (VT). One of the goals of VT ablation is to eliminate myocardial channels by homogenizing infarct scar, thereby inhibiting reentry and eliminating VT. It has been previously shown that proton beam therapy can create lesions in healthy myocardial tissue, thereby suggesting a potential for treatment of VT. To date; however, the effects of proton beam energy in infarcted myocardial tissue is unknown. In the current work, we track proton beam dose maps in a series of delayed contrast-enhanced magnetic resonance imaging swine datasets across 16 weeks to assess the effects of proton beam ablation on infarcted myocardial tissue.

Proton beam therapy has recently been proposed as a noninvasive approach for treating ventricular tachycardia (VT), where target regions are identified in the myocardium and treated using external beam therapy. Effective treatment requires that lesions develop at target sites of myocardial tissue in order to stop arrhythmic pathways. Precise characterization of the dose required for lesion creation is required for determining appropriate dose levels in future clinical treatment of VT patients. In this work, we use a deformable registration algorithm to align proton beam delivery isodose lines planned from baseline computed-tomography scans to follow-up delayed contrast-enhanced magnetic resonance imaging scans in three swine studies. The relationship between myocardial lesion formation and delivered dose from external proton beam ablation therapy is then quantitatively assessed. The current study demonstrates that myocardial tissue receiving a dose of 20Gy or higher tends to develop into lesion, while tissue exposed to less than 10Gy of dose tends to remain healthy. Overall, this study quantifies the relationship between external proton beam therapy dose and myocardial lesion formation which is important for determining dose levels in future clinical treatment of VT patients.

Cardiac arrhythmias, a condition in which the heart beats irregularly, are typically treated with drug or cardiac ablation therapy. More recently, external beam ablation therapy has been proposed as a potential approach for treating cardiac arrhythmias. Currently, a significant challenge regarding external beam ablation therapy in the heart is compensation for cardiac motion to ensure precise targeting. Porcine animal models are often used for evaluating image-guided intervention systems for cardiac applications; however, to date there have been relatively few studies evaluating motion in the swine heart. In this study, we model and quantify cardiac motion in the left atrium and left ventricle of three beating porcine hearts by tracking anatomic landmarks across twenty phases of the cardiac cycle from multi-phase computed tomography images. 10 landmarks are tracked for each porcine heart, 5 in the left atrium and 5 in the left ventricle. The mean (std) displacement for the 5 left atrial landmarks is 5.5(3.5) mm in x, 5.0(2.9) mm in y, and 5.6(3.3) mm in z. The mean (std) displacement for the 5 left ventricular landmarks is 7.1(3.8) mm in x, 9.9(5.2) mm in y, and 7.7(3.1) mm in z.

ESTRO 37 S533 0.7%(-12.9% to 20.2%) and 16.3% (-33.8% to 66.4%), respectively.The amplitude deviation was in average within 2 mm (figure 1).However, underestimations up to 6.3mm were observed.No correlations were found between acquisition parameters and volume or amplitude deviations. ConclusionOur data suggest that the expiration phase is the most accurate phase to define the tumour volume and should therefore be preferred for GTV delineation when using a mid-position, gating or tracking strategy.The large variation found among the institutions indicated that further improvements in 4DCT imaging are possible and local 4DCT QA could be improved.

Ventricular tachycardia is increasingly treated with ablation therapy, a technique in which catheters are guided into the ventricle and radiofrequency energy is delivered into the myocardial tissue to interrupt arrhythmic electrical pathways. Recent efforts have investigated the use of noninvasive external beam therapy for treatment of ventricular tachycardia where target regions are identified in the myocardium and treated using external beams. The relationship between the planned dose map and myocardial tissue change, however, has not yet been quantified. In this work, we use a deformable registration algorithm to align dose maps planned from baseline computed-tomography scans to delayed contrast-enhanced magnetic resonance imaging scans taken at 4 week intervals following proton beam therapy. From this data, the relationship between the planned dose and image enhancement, which serves as a surrogate for tissue change, can be quantified.