Kaposiform hemangioendothelioma is a locally aggressive, semi-malignant tumor which is often associated with a Kasabach-Merritt phenomenon and occurs almost exclusively in childhood (up to 90% of cases manifest themselves in the first year of life). These hemangioendotheliomas are most commonly found in the soft tissues of the extremities, while a small percentage can also develop in body cavities, mainly in the retroperitoneal space. As a rule, the tumors are ill defined, unevenly colored partly violet-blue and partly reddish, and hard on palpation. Surrounding tissue edema is common. Large lesions can be accompanied by marked thrombocytopenia and disseminated intravascular coagulation. This coagulation disorder can often be corrected by embolization, since surgical removal is usually not possible because of the extent, localization, strong vascularization and concurrent coagulation disorder. A treatment attempt can be made with the m-TOR inhibitor sirolimus as an alternative to traditional chemotherapy.
Histologically the tumors consist of irregular vascular nodules, which diffusely infiltrate the soft tissue (“cannon-ball” fashion) and are occasionally accompanied by stromal desmoplasia. Within the tumor, areas are found that resemble a hemangioma as well as a Kaposi’s sarcoma. A key characteristic is the formation of glomeruloid structures consisting of very dense, CD31-positive capillaries with actin-positive pericytes. Fibrin thrombi are frequently found here.