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We examined the role of spatiotemporal visualization of stereotactic body radiotherapy (SBRT) for cardiac radioablation to the cavotricuspid isthmus (CTI) based on both 13C labeled pyruvate metabolic imaging by Dynamic Nuclear Polarized (DNP) and redox reaction by in vivo DNP MRI compared with routine MRI. First, four mini pigs underwent electrophysiology assessment using electroanatomical mapping (EAM) before and 3 months after SBRT with single-fraction doses of 25Gy. Routine MR examination, including diffusion-weighted, dynamic Gd-enhanced, spectroscopy, and T2-weighted images were performed before and 3 months after SBRT. Second, the dynamics of 13C compound pyruvate for metabolic imaging by DNP MRI (SpinAligner), free radical imaging by low filed type of in vivo DNP MRI and routine MR (1.5T) were performed on the four mice before and 3 days and 7 days after single-fraction doses of 25Gy and 10Gy to whole heart. EAM visualized the irradiated site and confirmed clockwise conduction block across the CTI after SBRT. Both Abnormal signals which reflect the damaged myocardium on T2WI and DWI images and late gadolinium enhancement which reflects fibrosis could not be detected by routine MRI in four mini pigs. Pyruvate metabolic imaging by DNP MRI showed a significant increase in 13C lactate/pyruvate ratio after SBRT at all time points compared to before SBRT. In vivo DNP MRI signals were decreased depending on prescribed irradiation dose and days after SBRT. The mice treated by 10Gy irradiation showed a decrease on day 3 in whole heart. The mice treated by 25Gy irradiation showed a marked decrease on day 3 and a further decrease on day 7 in whole heart. Pyruvate metabolic imaging by DNP and redox reaction by in vivo DNP MRI could be useful for the evaluation of early response to the cardiac damaged after SBRT for cardiac radioablation to CTI. [ABSTRACT FROM AUTHOR]

Cardiac arrhythmias are usually treated with invasive, time consuming catheter ablation techniques. While recently stereotactic body radiotherapy (SBRT) is an emerging non-invasive treatment in the management of cardiac arrhythmias. Although we used the motion management by compression techniques for the reduction of the internal target volume, we did not use the gating and real-time tracking techniques. We assessed the necessity of the precise target planning and monitoring the complex respiratory and cardiac motion to cardiac radioablation to the cavotricuspid isthmus (CTI) by pathological assessment in mini pigs. All animal procedures were approved by institutional animal care and use committee and performed in full compliance with its guidelines. Four mini pigs underwent magnetic resonance (MR) and electrophysiology assessment using electroanatomic mapping (EAM) before and 3 months after SBRT with single-fraction doses of 25Gy. The target of CTI was defined by cardiologist. We defined the internal margin (IM), respiratory: SI 7 mm, AP 3 mm, LR 3 mm, cardiac motion: 3 mm in all direction and set up margin (SM): 5 mm in all direction. Then, planning target volume (PTV) margin: IM + SM = SI 15 mm, AP 10mm, LR 10 mm were added to the target, defining PTV.MR examination, including diffusion-weighted, dynamic Gd-enhanced, spectroscopy, and T2-weighted images were performed before and 3 months after SBRT, followed by histopathologic examination. Radiation oncologist and radiologist evaluated the detectability and image quality for the irradiated site. MRI findings of the CTI radioablation lesion was compared to electrophysiology and histopathology findings. A total dose of 25 Gy was successfully delivered to PTV in a single procedure in all mini pigs. EAM visualized the irradiated site and confirmed clockwise conduction block across the CTI. MRI could not detect the radiation injury clearly. Pathological examinations were analyzed after 3 months of follow-up, fibrous hyperplasia and capillary hyperplasia were observed around the nerve bundle beyond CTI in all pigs. The atrophy and necrosis of the nerve bundle itself is scarce. These data demonstrated the feasibility of SBRT for creating conduction block across the CTI in mini pigs. Fibrosis or scarring around the nerve bundle beyond CTI may affect the radiation injury. Then, precise margins of cardiac radioablation to the cavotricuspid isthmus will reduce the radiation injury. Not only gating and breath-hold, but also including the use of real-time tracking such as MRI-guidance cardiac radiosurgery will open the possibility of treating cardiac arrhythmias patients completely safety and noninvasively. [ABSTRACT FROM AUTHOR]