Intracardiac Neoplastic Extension of a Malignant Solitary Fibrous Tumor Mimicking a Giant Left Atrial Thrombus.

Intracardiac masses in patients with advanced malignancy pose a major diagnostic and therapeutic challenge, as neoplastic intravascular extension can closely mimic thrombus. We report a patient with metastatic malignant solitary fibrous tumor (SFT) who presented with a giant multilobulated left atrial mass (>5 cm) initially suspected to represent thrombus. Transthoracic and transesophageal echocardiography demonstrated a highly mobile mass prolapsing through the mitral valve into the left ventricle during diastole, with no right-sided involvement, no significant valvular pathology, and no history of atrial fibrillation or structural heart disease. Given the imminent risk of systemic embolization or mitral valve obstruction, therapeutic anticoagulation was started, and the mass was surgically resected after multidisciplinary discussion. Histopathology demonstrated an intravascular malignant neoplasm of epithelioid cells, with severe nuclear atypia, brisk mitotic activity, and a hemangiopericytoma-like vascular pattern; tumor cells were STAT6-positive, confirming metastatic malignant SFT. This case illustrates the diagnostic difficulty of distinguishing thrombus from neoplastic intravascular extension in cancer patients and underscores the complementary roles of echocardiography and histopathological confirmation in guiding urgent, multidisciplinary management of highly mobile left atrial masses.