Hepatoid yolk sac tumors of the mediastinum: a clinicopathologic and immunohistochemical study of four cases.

Four cases of primary hepatoid yolk sac tumors of the anterior mediastinum are described. The patients were all men between the ages of 26 and 40 years (median 33). Clinically, they all presented with a history of shortness of breath and chest pain of several weeks' duration. None of the patients had a history of germ cell tumor elsewhere or evidence of any hepatic abnormality. Grossly, all the tumors were described as large mediastinal masses that impinged on adjacent structures. Histologically, they were characterized by sheets of medium-sized, round to polygonal neoplastic cells with moderate amounts of eosinophilic cytoplasm and round to oval nuclei with prominent nucleoli. The cellular proliferation was homogeneous and displayed moderate cellular atypia and scattered mitotic activity. All the tumors showed focally the presence of more conventional areas of yolk sac tumor, with islands of tumor cells showing a reticular pattern of growth admixed with scattered intra- and extracellular hyaline globules and occasional Schiller-Duval bodies. Immunohistochemical studies showed strong positivity of the tumor cells for alpha-fetoprotein in both components of the lesions. Follow-up information was available in three patients, all of whom developed lung metastases within a year after initial diagnosis. Two of these patients died of tumor within the same period, whereas a third patient has been lost to follow-up. The present cases illustrate an unusual histologic pattern of yolk sac tumor in the mediastinum and highlight the importance of considering this tumor in the differential diagnosis of lesions showing a hepatoid pattern of growth in the mediastinal area.