Mediastinal Mature Teratoma With Malignant Transformation to Colorectal-Type Adenocarcinoma.

Mature teratomas represent 5% to 10% of all mediastinal tumors, and malignant transformation is exceedingly rare. A 51-year-old man with newly diagnosed prostate cancer was incidentally found to have an anterior mediastinal mass suggestive of a teratoma on staging workup. Robotic thymectomy was performed, revealing a mature teratoma with malignant transformation to colorectal-type adenocarcinoma. Circulating tumor DNA was negative for measurable residual disease. The patient was managed with surveillance, including serial imaging and tumor markers.