Adenocarcinoma of small intestinal type in retroperitoneal mature teratoma

We report a case of small intestinal type adenocarcinoma arising in retroperitoneal mature cystic teratoma in a young male. The patient initially experienced intermittent abdominal pain, and radiographic workup revealed an encapsulated retroperitoneal mass. A laparotomy was performed and the resected specimen represented a cystic lesion, the inner surface of which was mostly covered by sebaceous debris with hairs. Microscopically, mature derivatives of all three germ layers as well as completely developed gastrointestinal tract were identified. Adenocarcinoma without stromal invasion was observed adjacent to the small intestinal mucosa. Immunohistochemistry of the adenocarcinoma tissue revealed p53 overexpression and high Ki-67 labeling index as well as positive staining for CD10, cytokeratin 7, and cytokeratin 20. Therefore, the diagnosis of small intestinal adenocarcinoma was made. To our knowledge, this is the first case of small intestinal adenocarcinoma arising in retroperitoneal mature cystic teratoma. A unique feature of this case is that malignant transformation in retroperitoneal mature teratoma arose even in the fully developed intestine. Favorable prognosis due to detection in the 'early stage' is also discussed.