Answer: 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. The tumor is seen in association with a scarred area, atrophic seminiferous tubules, and leydig cell hyperplasia. Tumor cells can be seen infiltrating in between tubules and in the leydig cell hyperplasia.

Discussion: The histologic appearance in this lesion with round polygonal cells having clear to lightly eosinophilic to clear cytoplasm associated with striking lymphoid infiltrate 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 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 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 embryonal carcinomas. CKIT is also positive in seminomas.

The background testis shows scarring and atrophy. The tumor was small, and the area of scarring possibly represents regressed tumor. The most common germ cell tumor to undergo regression is choriocarcinoma, but it can be seen in all germ cell tumors.

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, or lungs.