The differential diagnosis of yolk sac tumor and seminoma. Usefulness of cytokeratin, alpha-fetoprotein, and alpha-1-antitrypsin immunoperoxidase reactions.

Yolk sac tumor and seminoma may have a similar appearance in focal areas. Small biopsies may therefore be difficult to interpret. The authors studied 20 yolk sac tumors and 21 seminomas to investigate the utility of immunohistochemical markers in this differential diagnosis. All yolk sac tumors stained positively for cytokeratin (CK) but so did 43% of seminomas. The CK positivity of yolk sac tumors was generally more diffuse and intense, however, there was an overlap in the spectrum of intensity of CK positivity in yolk sac tumor and seminoma. Alpha-fetoprotein (AFP) was a less sensitive (55%) marker for yolk sac tumor than CK, but AFP was quite specific in this differential diagnosis because no seminoma stained for AFP. Alpha-1-antitrypsin was not a very useful marker because of poor sensitivity and specificity. The interpretation of light microscopic patterns remains of paramount importance in the differentiation of solid yolk sac tumor from seminoma.