Answer: Paraganglioma

Histology: The tumor consists of lobules or nodules of fairly uniform. The tumor cells grow in small clusters separated by thin fibrovascular stroma. Mitotic figures are rare. On high power the cells have abundant cytoplasm, which is somewhat clear in appearance, and some cells also show vacuolization. The nuclei are round to oval. Immunohistochemical stains show that the tumor cells are positive for chromogranin and S-100 stain highlights the sustentacular cells which surround the small nests of tumor cells. The pattern of small clusters of tumor cells is often referred to as Zellballen. The vessels show thrombus consistent with embolization prior to surgery.

Discussion: The tumor cells have the classic appearance of a paraganglioma. This particular case occurred in the carotid body region, which is one of the most common locations for an extra-adrenal paraganglioma. Paragangliomas, occurring in this location, may be referred to by clinicians as glomus jugulare or jugulotympanic paraganglioma which arise laterally in the temporal bones and present as a mass in the middle ear or external auditory canal. They may be referred to as glomas vagale when they arise adjacent to the vagus nerve. It is best to avoid the use of the term glomus tumor when referring to paragangliomas in this region to prevent confusion with the glomus tumors that are composed of smooth muscle and glomus cells that occur in the subungual region.

Although most extra-adrenal paragangliomas are benign, about ten percent of carotid body tumors behave in a malignant fashion and the most reliable means for distinguishing benign from malignant carotid body tumors is based on evidence of metastasis or distant spread. There are no reliable morphologic criteria by which to separate benign from malignant tumors, although a high mitotic activity and decreased immunohistochemical reactivity for neuropeptides are associated with more aggressive behavior. Immunohistochemical staining of the tumor for neuropeptides such as chromogranin and synaptophysin, as well as a stain for S-100 to highlight the sustentacular cells surrounding the small nests of tumor cells are extremely helpful in establishing a diagnosis of paraganglioma. It is common to embolize these tumors prior to surgery to reduce bleeding and other complications.