Answer: Sertoli cell tumor

Histology: This is a highly sclerotic lesion which is centered on the tunica albuginea at the periphery of the testis. The lesion is composed of cords of epithelioid, bland clear cells with focal tubular formations. The stroma is highly fibrous and paucicellular. The lesion labels for inhibin but not for cytokeratin, supporting the diagnosis of Sertoli cell tumor.

Discussion: Adenomatoid tumors should label for cytokeratins and mesothelial markers, and are typically centered at the periphery of testis. Carcinoid tumors may be difficult to distinguish from Sertoli cell tumors. Sertoli cell tumors can label for synaptophysin and CD56, furthering the confusion. Carcinoid tumors typically label for cytokeratins and infrequently label for inhibin. Their chromatin is also distinctive from that of Sertoli cell tumor, in that carcinoid tumors have finely granular, salt and pepper type chromatin. Seminoma may have a tubular pattern which simulates Sertoli cell tumor. However, seminoma demonstrates greater cytologic atypia, mitotic activity, and squared-off nuclei. Seminoma should not label for inhibin but would label for germ cell markers like C-kit or OCT4.