Answer: Yolk sac tumor

Histology: Yolk sac tumor displays a wide range of histologic patterns. Several patterns are often seen in combination in the same tumor. This case demonstrates predominantly microcystic, glandular-alveolar, and myxomatous patterns. Some areas contain small aggregates and microcysts within myxomatous stroma. In other foci, there are numerous cavities, channels and gland-like spaces lined by epithelial like cells. The degree of cytologic atypia is out of proportion relative to rather well-differentiated architecture. Numerous intracytoplasmic and extracellular hyaline globules are present in some areas. Perivascular formation recapitulating endodermal sinuses (aka Sciller-Duval bodies) that are virtually diagnostic of yolk sac tumor, are present in only 30% of cases (not identified in this case).

Discussion: Yolk sac tumor (YST) can mimic surface epithelial tumors of the ovary. Although some areas of YST can closely resemble secretory variant of endometrioid carcinoma, the latter does not display the other histologic patterns seen in YST. Clear cell carcinoma shows more regular tubular pattern that is associated with dense fibromatous stroma, or papillary pattern with hobnail cells lining papillae with hyalinized stroma. YST will express AFP and pancytokeratin, but will not label with EMA and CK7 in contrast to surface epithelial tumors of the ovary. Embryonal carcinoma lacks specific histologic patterns observed in YST. It is composed of primitive embryonal cells that are larger than cells of YST and have a greater degree of nuclear pleomorphism and prominent nucleoli.