Teratoma with naturally occurring malignant transformation in a child.

We present a 12-year-old girl with a teratoma with malignant transformation (TMT) of the mediastinum. Computed tomography showed a cystic mass (5.0 cm × 4.0 cm) with a thick solid portion, in the anterior mediastinum. Six months later, the solid portion of the mass had enlarged, and surgical resection was performed. The resected tumor was 7.0 × 5.0 × 4.0 cm in size. The cystic portion was a mature teratoma, and the solid portion predominantly comprised a viable embryonal rhabdomyosarcoma. There were no immature teratomatous elements or other germ-cell components. The histopathologic diagnosis was a mature teratoma with embryonal rhabdomyosarcoma, a so-called TMT. The tumor recurred, despite adjuvant chemotherapy. The patient died of progressive disease 16 months postoperatively. To the best of our knowledge, no naturally occurring TMT of the mediastinum has previously been reported in a child. Surgical resection at an early stage is necessary.