Unusual thymic seminomas.

Although most thymic seminomas have a histologic appearance similar to those arising in the testis, some seminomas undergo pronounced secondary changes that may obscure the diagnosis. We found three cases that showed a combination of reactive follicular hyperplasia (three cases), large cysts (three cases), epithelioid granulomas (two cases), and fibrosis (two cases), to such a degree that the seminoma component was indistinct. Immunoperoxidase staining for keratin in one case highlighted the intimate association of the positively staining thymic epithelial cells with the nonstaining seminoma cells. The seminoma cells in this case were reactive with placental alkaline phosphatase. No other group of thymic or mediastinal tumors, including those of Hodgkin's disease, exhibited this combination of secondary reactive changes to a similar degree. Because of the favorable prognosis of thymic seminomas, these variants should not be confused with other mediastinal tumors.