At low power, DT appears similar to a cavernous lymphangioma. A unique pattern is its papillary structure lined with atypical columnar endothelial cells. Some papillary plugs have a central hyalinized core and project into a lumen lined by atypical columnar endothelial cells. These outpouchings sometimes appear in a glomeruluslike pattern. Individual endothelial cells are cuboidal to tall cylindrical with vacuolated cytoplasm and hyperchromatic eccentric nuclei on their luminal border, thus resembling epithelium.
Mitotic figures are uncommon.
Many intraluminal lymphocytes may be evident, often attached to the endothelial cells. Focal changes characteristic of retiform hemangioendothelioma occasionally may be observed. Intravascular proliferations may stain actin-positive. Ultrastructural studies demonstrate tumor cells with irregular nuclei, abundant perinuclear cytoplasmic filaments, and many pinocytotic vesicles. Weibel-Palade bodies may be evident.