Answer: Kaposiform hemangiothelioma

Histology: In areas the tumor closely resembles Kaposi’s sarcoma composed of spindle cells with slit-like vessels containing erythrocytes. Other areas have a more capillary hemangioma-like morphology with well-formed rounded vessels. Focally the lesion has anastomosing vascular channels. Despite the lesion’s high cellularity, there is no pleomorphism and the lesion lacks appreciable mitotic activity.

Discussion: Although areas of this lesion closely resemble Kaposi’s sarcoma, the lesion lacks hyaline globules typically seen within Kaposi’s. In addition, Kaposi’s sarcoma does not show areas with well-formed capillary-like vessels as seen in the current case. Similarly, although there are areas within this lesion that resemble angiosarcoma given the appearance of anastomosing channels lined by endothelial cells. These areas are relatively focal within the lesion. More importantly, the endothelium lining these channels show no cytologic atypia. The finding of capillary-like areas elsewhere in the lesion also rules out a diagnosis of angiosarcoma. Although Kaposiform hemangiothelioma typically occurs in children, recently the lesion has been described in adults. The retroperitoneum is a good location for this tumor, and in children the lesion is often associated with consumption coagulopathy and thrombocytopenia (Kasabach-Merritt) syndrome. Although these tumors do not metastasize, they can involve regional lymph nodes and the associated complications due to the consumption coagulopathy can result in death.