Before mediastinal surgery, identification of thoracic duct by image diagnosis is important for reducing the risk of chylothorax. A 40s woman had an abnormal findings in the left upper lung field on a chest X-ray film. CT and MRI revealed a 25 mm sized oval cyst which located on the top of aortic arch and dorsal esophagus in the posterior mediastinum. Using EUS, thoracic duct was visualized as a 3 mm of tubular structure without Doppler flow between the cyst and esophagus. Thoracoscopic resection was performed. Because we could confirm that thoracic duct existed in contact with the cyst by EUS before surgery, we paid close attention to thoracic duct during surgery. The surgery was completed without complication of chylothorax. EUS is useful for understanding anatomical relation between thoracic duct and the posterior mediastinal cyst, and avoiding thoracic duct injury during surgery.
