[A case of mediastinal lymphatic cyst possibly originating from the right thoracic duct]

A 33-year-old male was admitted to our hospital because of an abnormal shadow in the right superior mediastinum on chest X-ray, which was first noted about half a year before his admission. Several diagnostic procedures, including fiber bronchoscopy, CT scan and MRI; however, no final diagnosis could be made. Thoracotomy was performed with the presumed diagnosis of benign mediastinal cystic tumor. Entry of the right thoracic duct into the cyst was observed at operation. Histological examination revealed that the cyst was lined by mono-layered cuboidal epithelial cells with lymphocytic infiltration in the wall of the cyst. The mono-layered cuboidal epithelium of the thoracic duct gradually changed to the epithelium of the cyst. Twenty ml of colorless transparent fluid was aspirated from the cyst. Analysis of aspirated fluid revealed low concentration of several substances except LDH compared to the values of the previously reported normal standards and other reported cases. From these findings, the cyst was diagnosed as mediastinal lymphatic cyst. Entry of the thoracic lymphatic duct into the mediastinal lymphatic cyst is a rare occurrence, and it is of interest to speculate on its relationship to the origin of the lymphatic cyst.