Germ cell tumors of the mediastinum are uncommon. Less than 5-7% of germ cells occur outside the gonads, but of the extragonadal sites, the mediastinum is the most common location for germ cell tumors. Other areas where germ cell tumors can occur are the retroperitoneum, the intra-abdominal cavity, and the chest. The exact mechanism whereby germ cell tumors originate in the mediastinum remains unknown.

Germ cell tumors in the mediastinum were first reported nearly 50 years ago. Pathological studies from autopsy data revealed that the tumors were usually large and frequently locally invasive.

The first theory on extragonadal malignant germ cell tumors postulated that the tumors developed from primitive germ cells in the endoderm of the yolk sac or from the urogenital ridge. These cells normally move into the scrotum during development, but when this migration fails, the cells may remain localized to either the mediastinum or the retroperitoneum. Other researchers hypothesized that these totipotential cells become detached during embryogenesis and result in primitive masses, which may develop into germ cell tumors. So far, however, no theory is proven.

No research documents that these cells metastasize from gonadal tissue. Mediastinal germ cells tumors are now postulated to be autonomous oncologic entities.

For excellent patient education resources, visit eMedicine's Procedures Center and Cancer and Tumors Center. Also, see eMedicine's patient education articles Bronchoscopy and Cancer: What You Need to Know.