[A case of endodermal cyst in front of the pontomedullary junction].

We reported a rare case of an endodermal cyst on the posterior fossa. The patient was a 51-year-old woman with tinnitus, ear blockage and a mild swallowing disturbance. MR images revealed a cystic mass 5 cm in diameter in front of the pontomedullary junction. Gadolinium-enhanced images showed some spotty lesions in the cystic mass. The brain stem was compressed strongly by the mass. The left vertebral artery was involved in the mass. Total removal of the tumor was performed via the transcondylar approach. During the operation the lower cranial nerves, the bilateral vertebral artery and the vertebral union were recognized. The cystic mass consisted of a yellow colored thin wall and a watery fluid which contained some small hard lesions. Histopathological examination of the cyst wall revealed a single layer of ciliated columnar epithelium, and that of the small lesions showed histiocytes and granulation. Immunohistochemically, the cyst wall was stained by an epithelial membrane antigen and a carcinoembryonic antigen. From these histopathological findings, the final diagnosis of the cystic lesion was an endodermal cyst. Twenty such cases have been reported to date since Afshar reported a case of an endodermal cyst on the posterior fossa in 1981. There are many types of cystic lesions in the intracranial region, and immunohistochemical studies are necessary for diagnosis of these intracranial cysts.