Diffuse embryoma of the testis. An immunohistochemical study of two cases.

The authors report the histologic and immunohistochemical findings of two cases of diffuse embryoma of the testis, a distinct form of mixed-germ-cell tumor characterized by diffuse, orderly arrangement of embryonal carcinoma (EC) and yolk sac tumor (YST) with scattered trophoblastic components. The patients were 37 and 38 years old when they presented with a right testicular tumor, which was confined to the testis (stage I) in both cases. Histologically, the tumor was composed predominantly of intimately intermingled EC and YST components in almost equal proportion. The tumor cells were arranged in necklacelike fashion; the EC cells formed glandular structures rimmed by a single cell layer of YST cells. The YST component was highlighted by positive staining for alpha-fetoprotein and strong staining for cytokeratin, whereas the EC component was positive for Ki-1 (BerH2, CD30) antigen, was negative for alpha-fetoprotein, and stained more weakly for cytokeratin. The randomly distributed few trophoblastic elements stained for human chorionic gonadotropin. The patients are alive with no evidence of disease, 11 years and 9 months after surgery, respectively. This newly described but distinct variant of mixed-germ-cell tumor should be differentiated from polyembryoma, which is composed of multiple discrete embryoid bodies.