Answer: Yolk sac tumor

Histology: The tumor consists of glandular structures situated within a loose fibrous background. The glands are remarkable for areas where nuclei are lined up midway in the cytoplasm with prominent sub and supranuclear clear vacuoles. The nuclei are moderately atypical yet do not show marked pleomorphism.

Discussion: Although this lesion is composed of glands, it is not overtly pleomorphic as one would expect with an adenocarcinoma arising in a teratoma or a metastatic adenocarcinoma to the testis. The spaces surrounding many of the tumor nests lack endothelial lining and appear to be retraction artifact. The major differential diagnosis is between a malignant teratoma and glandular variant of yolk sac tumor. Typically, glands of teratoma are surrounded by smooth muscle trying to recapitulate normal structures such as respiratory mucosa or intestinal glands. In contrast, glands of yolk sac tumor are present within fibrous tissue, lacking such an investment by smooth muscle. The other typical feature seen in this case of glandular variant of yolk sac tumor is the presence of prominent subnuclear vacuoles in some of the glands reminiscent of day 16-17 secretory endometrium.

Immunonistochemistry was performed in this case demonstrating the glands were strongly positive for alpha-feto-protein and negative for epithelial membrane antigen consistent with the diagnosis of yolk sac tumor.