Unique mitral valve mass: Think beyond vegetation.

Osteosarcoma is a rare cardiac malignant tumor. This case of cardiac osteosarcoma presented with atrial fibrillation. Initial echocardiogram demonstrated mitral valve echodensity and mitral valve regurgitation. Surgery and histopathological examination identified the tumor as an osteosarcoma. Tumor grade appeared to be prognostically important in cardiac sarcoma, with poor prognosis in high-grade tumors.