Mediastinal yolk sac tumors associated with prominent multilocular cystic changes of thymic epithelium: a clinicopathologic and immunohistochemical study of five cases.

Five cases of yolk sac tumor of the anterior mediastinum associated with prominent thymic cystic changes are presented. The patients were five men between the ages of 17 and 22 years (mean, 19.5 yr). Clinically, four patients presented with symptoms of chest pain and shortness of breath, whereas the fifth was asymptomatic: his tumor was discovered during a routine physical examination. None of the patients had a history of previous tumor elsewhere. Grossly, the tumors were described as cystic lesions measuring between 8 and 12 cm in greatest dimension. Histologically, the tumors showed prominent cystic changes reminiscent of those observed in acquired multilocular thymic cysts, i.e., the presence of cystic cavities lined by squamous epithelium with prominent inflammatory changes, lymphoid hyperplasia, cholesterol cleft granulomas, and remnants of thymic tissue in the walls of the cysts. In addition, there was a neoplastic cellular proliferation composed of small cells with scant cytoplasm, round nuclei, and inconspicuous nucleoli adopting a reticular pattern of growth and, in some areas, embedded in abundant myxoid stroma. Numerous intra- and extracellular hyaline globules, as well as Schiller-Duval bodies, could also be identified. Immunohistochemical studies showed positive reaction with alpha-fetoprotein antibodies in the tumor cells. Follow-up information obtained in these patients showed that all had died of tumor 1 to 3 years after initial diagnosis. The present cases document another unusual but distinctive growth pattern of yolk sac tumor in the mediastinum and highlight the importance of proper sampling of cystic lesions in this location.