Endodermal sinus tumor (yolk sac tumor) of the orbit.

The endodermal sinus tumor is a malignant germ cell neoplasm that usually arises in gonads, but on rare occasion occurs in extragonadal locations. Five tumors of the orbit, which by light microscopy displayed features characteristic of gonadal endodermal sinus tumor, were tested for the presence of alpha fetoprotein using paraffin-embedded tissue and an immunoperoxidase technique. Each tumor contained intracytoplasmic alpha fetoprotein. Review of the clinical histories revealed that orbital endodermal sinus tumors differ from other extragonadal endodermal sinus tumors in that they occur at a younger age and when treated aggressively can result in long-term survival. In children with rapidly progressive proptosis the distinction between endodermal sinus tumor and rhabdomyosarcoma poses diagnostic difficulties for both the clinician and the pathologist. When considering the diagnosis of endodermal sinus tumor, the use of immunohistochemical testing for alpha fetoprotein can be helpful.