The efficacy of stereotactic body radiation therapy (SBRT) cardiac radiosurgery in resolving cardiac arrhythmias was recently reported from a small clinical trial (NCT02919618). However, real-time tracking of the cardiac lesion is challenging using conventional cone-beam CT guided radiotherapy. MRI-guided radiotherapy (MRIgRT) systems integrate real-time MRI for lesion tracking with radiation therapy and can provide excellent cardiac tissue image quality at high frame rates. Real-time cardiac MRI using sparsely-sampled radial acquisitions is demonstrated with iterative reconstruction methods at low-field (0.35 T). The performance goal is to image the heart and track the lesion at 30 Hz with 2.5 mm in-plane resolution.