Answer: Metastatic seminoma

Histology: The lesion is composed of cells with centrally located round vesicular nuclei with clear cytoplasm. The cells have a homogeneous appearance. There is an associated lymphocytic infiltrate. Areas of necrosis are apparent in the tumor, and ghost outlines of the tumor cells can be seen in the necrotic region. There is also an extensive granulomatous reaction in the tumor. Immunohistochemical stains show that the tumor cells are positive for PLAP, and negative for EMA, cytokeratin and lymphoid markers.

Discussion: The histologic appearance in this lesion with round polygonal cells, having clear to lightly eosinophilic cytoplasm, associated with striking lymphoid infiltrate and a granulomatous reaction is characteristic of a seminoma. Seminomas can be confused with malignant lymphoma. However, high-power microscopic features of lymphoma cells tend to show twisted or angulated nuclei and a more polymorphous cell population with respect to cell size, compared to seminoma. The cytoplasm of lymphoma cells in general is less well-defined and not as clear as in seminoma cells. Immunohistochemistry or flow cytometry will also be helpful. Common leukocyte antigen (CLA) will mark a high percentage of lymphomas, but should be negative in seminomas. While PLAP (placental-like alkaline phosphatase) is positive in seminoma cells, it is negative in lymphomas and carcinomas. Cytokeratin staining will also be helpful since it is either absent or scant in most seminomas. It should be positive in carcinomas.

Most seminomas are extremely sensitive to radiation and chemotherapy and cure rates of 95% or better are usually achieved. Recurrences are unusual and most develop outside the radiated field such as the mediastinum, cervical lymph nodes, as in this case, or lungs.