Background: With advances in modern diagnostic and therapeutic modalities, the main goal of intrathoracic radiation treatment is to reduce treatment-related toxicities. Here, we describe a series of cases that involved positron emission tomography/computed tomography (PET/CT) and magnetic resonance imaging (MRI)-based radiation therapy (RT) for the treatment of primary cardiac angiosarcoma (PCA). Patients and Methods: The medical records of 3 patients who underwent image-guided RT at our department between 2012 and 2015 were analyzed. Results: 3 patients with PCA underwent pre-therapeutic imaging with fluorodeoxyglucose (FDG)-PET/CT (n = 3), as well as PET/MRI (n = 2) or MRI alone (n = 1). 2 patients underwent primary tumor resection (cases 1 and 2). The tumor in case 3 was unresectable and the patient underwent definitive chemoradiation. Intensity-modulated RT was applied with a median RT dose of 50.4 Gy. At the end of the study, 2 of the patients had survived for 35 and 16 months post-treatment (cases 1 and 3, respectively), and no evidence of tumor progression has since been detected. Conclusion: For the cases examined, RT was a feasible and tolerable treatment for PCA, and FDG-PET-MRI successfully characterized each PCA for therapy planning.