Answer: Malignant germ cell tumor, seminoma

Histology: The tumor consists of sheets and cords of large pleomorphic tumor cells. The cells have a prominent nucleoli and pale eosinophilic cytoplasm. Mitotic figures are easily identified. Associated with the neoplastic proliferation are admixed lymphocytes and numerous epithelioid histiocytes that in areas have a granulomatous appearance.

Discussion: The large cell proliferation in this mass is clearly neoplastic and appears to be associated with an epithelioid histiocytic reaction. Although epithelioid histiocytic reactions can be seen in a large number of different tumors, they are more commonly seen in certain neoplasms such as lymphoma and seminoma. They are less often seen in metastatic carcinoma. In this case, immunocytochemical stains can be helpful in establishing the correct diagnosis. The tumor cells in this case fail to stain for common leukocyte antigen or pan B and T cell antibodies excluding the diagnosis of a large cell lymphoma. The tumor cells are also negative for keratin, which would help exclude the diagnosis of an undifferentiated metastatic carcinoma. The tumor cells are positive for PLAP (placental alkaline phosphatase). This immunohistochemical staining pattern is consistent with a retroperitoneal seminoma.

Retroperitoneal germ cell tumors in adults may represent metastases from primaries, or they may be primary in this location. In general, primary retroperitoneal neoplasms tend to form a single mass in contrast to a metastasis from the testis that may involve several lymph nodes. Also, seminomas are more likely to be primary than non-seminomatous germ cell tumors. In patients with retroperitoneal germ cell tumors, thorough clinical and radiographic evaluation should be employed to rule out the possibility of an occult testicular tumor.