Intense 18F-FDG uptake in an organizing right atrial thrombus mimicking malignancy.

We present a case of an intensely hypermetabolic intracavitary cardiac mass, standardized uptake values max 44.4, that was pathologically proved to be organizing and organized thrombus, negative for tumor. Our patient had previous right atrial mass resection 2 years prior that was pathologically described as either thrombus or infarcted atrial myxoma. She had since been on lifelong controlled anticoagulation; and on routine follow-up imaging, she had recurrent slow growth of a new right atrial mass. During a later hospital admission for chest pain, the mass was evaluated on both transthoracic and transesophageal echo cardiogram, which could not differentiate thrombus vs neoplasm. Cardiac magnetic resonance imaging was equivocal for mass enhancement. The patient underwent fluorodeoxyglucose positron emission tomography/computed tomography (18F-FDG PET/CT) evaluation, which revealed intensely hypermetabolic activity within the mass concerning for malignancy, potentially an aggressive tumor. Subsequently, the mass was surgically excised for pathological diagnosis.