Sub-centimeter adenocarcinoma arising from mature mediastinal teratoma during 11 years of follow-up.

A 54-year-old male presented with exertional dyspnea, right massive pleural effusion, and an anterior mediastinal tumor (6.5 × 6.2 × 7.6 cm) which continued to grow during the 11-year follow-up period. Fluorodeoxyglucose positron emission tomography demonstrated a focal but remarkable uptake within the tumor mass. After chest tube drainage, the patient underwent surgical resection via a right thoracotomy. The final pathology was determined to be a mature cystic teratoma with malignant transformation to adenocarcinoma. Teratomas with somatic-type malignancy are challenging to diagnose using only preoperative evaluations. Intra-operative findings, histopathological findings, and postoperative management techniques are discussed in this report and emphasize the importance of early resection for treatment of mature teratomas, as well as the recommendation of careful inspection of pathological specimens of long-standing teratomas.