Answer: Seminoma with syncytiotrophoblastic giant cells

Histology: This lesion shows loosely cohesive cells arranged in nests. The cells contain highly eosinophilic cytoplasm with central prominent nucleoli. The cells are loosely cohesive. Surrounding dilated capillaries and areas of recent hemorrhage are numerous syncytiotrophoblastic giant cells.

Discussion: In contrast to case #5, these syncytiotrophoblastic giant cells are not intimately admixed with the adjacent tumor cells but rather surround dilated capillaries and the areas of recent hemorrhage. Furthermore, the background tumor cells lack the pleomorphism and cohesiveness seen with cytotrophoblasts as would be seen within a choriocarcinoma. The background cells are typical of seminoma. Seminomas may have scattered syncytiotrophoblastic giant cells in about 20% of cases. This case is distinct in that the extent of syncytiotrophoblastic giant cells is greater than many cases, causing the potential confusion with choriocarcinoma. This patient had a serum HCG level in the hundreds, which is in concert with the extent of syncytiotrophoblastic giant cells yet is still much lower than what one would see with a choriocarcinoma. It is critical to make the distinction of seminoma with syncytiotrophoblastic giant cells versus choriocarcinoma as seminoma is treated differently and has a better prognosis than a non-seminomatous germ cell tumor. If one has difficulty on the H&E stained sections, one can employ immunohistochemistry for cytokeratin, which would be negative within seminoma cells as opposed to strong positivity in choriocarcinoma.