The "ring sign" of necrotic pheochromocytoma.

Pheochromocytomas are usually hypervascular tumors exhibiting dense stain during angiography. The cystic nature of the neoplasm or tumor necrosis can result in failure of opacification of the mass or a central lucent area. Four cases of pheochromocytoma are described where a central avascular zone was surrounded by a dense rim of contrast, giving a "ring sign." Since this appearance is best seen in the capillary and venous phases, delayed films are essential.