A 50-year-old white male with a history of paroxysmal atrial fibrillation presented for transoesophageal echocardiogram prior to atrial fibrillation ablation. However, an echo lucent mass was noted (figure 1A). Colour Doppler and contrast administration showed no flow across the mass or the interatrial septum (see online supplementary videos 1 and 2). CT of the chest demonstrated a thin-walled, well-demarcated mass in the inferior border of the fossa ovalis protruding into the left atrium (figure 1B).Question
Which of the following is the most likely diagnosis?