Primary vascular tumors of bone: a spectrum of entities?

Vascular tumors of bone are a heterogeneous group. Numerous terms have been introduced as well as different classification systems. None of the classification schemes have been accepted due to lack of consistent terminology, accepted histologic criteria, and limited correlation with clinical outcome. It is acknowledged that vascular tumors of bone originate from endothelial cells, resulting in variable expression of endothelial markers. None of these markers are useful to discriminate between benign and malignant lesions. Although radiologic appearance is not specific, radiologic multifocality should trigger to include a vascular neoplasm in the differential diagnosis. This review gives an overview of current literature by describing all different histologic subtypes in correspondence with clinical, radiologic and genetic data. We propose the classification of vascular tumors of bone according to the three-tiered World Health Organization classification scheme for soft tissue tumors dividing entities into a benign, intermediate and malignant category. Hemangioma is the most often and commonly recognized benign lesion. Epithelioid hemangioma has been better defined over the past few years. Based on its locally aggressive behavior and occurrence of lymph node metastases, classification within the intermediate category could be considered. Angiosarcoma is the only accepted term for high-grade malignant vascular tumor of bone and so far, epithelioid hemangioendothelioma is the only accepted low-grade malignant vascular tumor of bone. It is still unclear whether other low-grade malignant vascular tumors of bone (e.g. hemangioendothelioma) truly exist. Unfortunately, molecular / genetic studies of vascular tumors of bone which might support the proposed classification are very sparse.