Mediastinal type B3 thymoma combined with germ cell tumor: cytologic diagnosis.

Primary mediastinal thymoma combined with germ cell tumor (GCT) is extremely rare, and is likely to be misdiagnosed. Here we report a case of mediastinal type B3 thymoma combined with seminoma in which the seminoma component was missed by histologic examination and initially diagnosed by using a pleural effusion sample. The patient was a 46 year old male with chest distress, cough, and supraclavicular lymph node enlargement. A large anterior mediastinal mass was revealed by diagnostic imaging. The tumor was completely removed by thoracotomy. Grossly, a solid mass about 10 cm × 8 cm × 5 cm with cystic degeneration was found. Histologic examination revealed Type B3 thymoma accompanying with multiple lymph node metastases. One year later, CT scan found an irregular mass on the right side of anterior-superior mediastinum with a large amount of effusion in the right side pleural cavity. Cytologic examination and immunostains of the pleural effusion sample revealed metastatic seminoma. Then the original surgical sample was reviewed and the seminoma component also was found besides the thymoma. To the best of our knowledge, this is the first description of type B3 thymoma combined with seminoma, diagnosed by histology and pleural effusion together. We also present a literature review.