Giant mature mediastinal teratoma mimicking a pericardial cyst.

BACKGROUND: Mature mediastinal teratomas are uncommon benign germ cell tumors. When the cystic component predominates and typical features such as fat or calcification are absent, diagnosis can be misleading. A 17-year-old male had a brief syncopal episode two months prior. At presentation, his primary complaint was mild chest tightness. Chest computed tomography showed a 13.6 × 10.3 × 7.9 cm homogeneous cystic mass in the anterior mediastinum, closely attached to the pericardium. Attenuation measured 17 ± 15 Hounsfield units on non-contrast computed tomography, consistent with fluid density and showing no enhancement. Transesophageal echocardiography confirmed that the lesion compressed but did not invade the heart, indicating an extrapericardial mass. It was initially diagnosed as a pericardial cyst. Because of its large size and dense pericardial attachment, the tumor was resected via median sternotomy. Histology demonstrated mature cystic teratoma. Recovery was uneventful. CONCLUSION: Cyst-dominant teratoma can closely resemble a pericardial cyst on computed tomography. Using complementary imaging such as transesophageal echocardiography or magnetic resonance imaging helps avoid misinterpretation. Early complete removal ensures accurate diagnosis and excellent prognosis.