Background: Benign thymic cysts are a rare clinical entity, representing approximately 3% of all mediastinal masses and can be congenital or acquired. We report a case of a 58-year-old female with a giant benign thymic cyst which entered in differential diagnosis with pericardial cyst and pericardial effusion.
Case presentation: A 58-year-old female was brought to our hospital due to new onset dyspnoea and fatigue. She had a history of a mediastinal liquid mass incidentally discovered 9 years before which was interpreted at that time as a loculated pericardial effusion (Fig. 1). An echocardiogram was performed and it revealed increased size of the known lesion without any signs of hemodynamic compromise (Fig. 2). A magnetic resonance showed a voluminous liquid lesion that originated from the anterior mediastinum, extending caudally to the diaphragm with maximum dimension on the right hemithorax of 16 x 12 x 9 cm and on the left hemithorax of 10 x 5 x 13 cm (antero-posteriorly, transversely and cranio-caudally respectively) (Fig. 3). It was initially interpreted as a pericardial cyst. All the findings were then collegially reviewed and it was suspected of being a giant thymic cyst especially due to its rapid growth and spatial evolution over time. The cyst was surgically removed using a thoracoscopic approach. Histological examination confirmed the thymic origin of the cyst and excluded malignancy. At three month follow up, the patient presented no symptoms, and the echocardiogram was normal.
Discussion: The echocardiographic features of mediastinal cysts are not specific for the type of cystic lesion, so the origin and location of the cyst can be used for differential diagnosis. Computed tomography has the best spatial resolution, while the magnetic resonance has better capacity to differentiate the content of the cyst, especially when it is not a homogeneous liquid lesion. In our case, the growth over nine years was a crucial hint to suspect the thymic origin of the lesion. On the other hand, the sole evaluation of the last imaging exam would have made the diagnosis more challenging because of its huge dimensions, which made the exact site of origin almost impossible to identify.
Conclusion: Our case is a rare example of a giant thymic cyst which was extremely difficult to differentiate from other paracardiac cystic lesions and required integrated imaging and discussion by a team of experts of different specialties to make the correct diagnosis.