Mediastinal and retroperitoneal teratoma with focal gastrointestinal adenocarcinoma.

We report an unusual case of gastrointestinal adenocarcinoma arising in a giant posterior mediastinal mature cystic teratoma extending into the retroperitoneum, which was treated by complete excision with a good outcome for more than 2 years. Teratomas with malignant transformation are rare non-germ cell malignant tumors arising from a preexisting mature teratoma. Histological examination revealed that the cyst wall was composed of mature ectodermal, mesodermal, and endodermal elements. Neoplastic glands with a cribriform pattern were found in a small, solid nodule. Strong cytokeratin 20 cytoplasmic immunostaining of the tumor cells supported the diagnosis of gastrointestinal adenocarcinoma. In this report, we describe the potential aggressiveness of a giant mature cystic teratoma with adenocarcinoma and suggest that complete surgical resection without adjuvant chemotherapy be considered as a therapy in the treatment of teratoma with focal malignant transformation.