High-grade angiosarcomas (HGAS) of bone are rare and represent less than 1% of the primary malignant bone tumours. Because of their rareness little is known. Clinically, it is accepted that they are extremely aggressive. Due to the lack of uniform terminology and accepted histological criteria, terminology and classification of primary malignant vascular tumours of bone has been highly controversial. Today, angiosarcoma is the most accepted term for high-grade primary vascular tumour of bone, recognized by the 2002 WHO Classification. However, distinct histological hallmarks to define a HGAS of bone are not clear.

We collected 64 HGAS of bone diagnosed between 1964 and 2007 from the files of the departments of pathology, Leiden University Medical Center (Leiden), Rizzoli Institute (Bologna) and University Hospitals (Leuven). All clinical, radiological, and pathological data were reviewed and different histological criteria were scored. A tissue micro-array was constructed containing 57 HGAS of bone. To confirm the vascular origin of all lesions and to investigate the diagnostic value of commonly used markers, immunohistochemistry was performed for CD31, CD34, Factor VIII, and keratin AE1/AE3. Staining was evaluated positive or negative.

Among 64 patients with HGAS of bone, there are 41 males and 23 females. There is a wide age distribution, with a nearly equal distribution from the second to the sixth decade. The solitary cases are mostly located in the extremities (66%) followed by trunk (12.8%), axial/central location (10.6%) and pelvis (10.6%). 17 cases (73%) have multifocal bone lesions. HGAS of bone show variable histological patterns. Association with clinical outcome (chi-square test) reveals that there is a significant poor survival when the tumour has tree or more mitoses (p=0.001), a macronucleoli (p=0.011) or there is an absence of an eosinophilic infiltrate (p=0.023). The HGAS of bone are positive for CD31 in 53/55 (96%), CD34 in 33/57 (58%), Factor VIII in 47/55 (86%), and keratin in 40/57 (70%). Only 15 out of 40 (38%) keratin positive angiosarcomas, showed an epithelioid phenotype at classical morphology. All tumours with an epithelioid phenotype are keratin positive.

Although HGAS of bone in general have a poor outcome, histological criteria such as three or more mitoses, the presence of a macronucleolus and the absence of an eosinophilic infiltrate can be useful to predict a more aggressive course, consistent with the clinical behaviour of a high-grade angiosarcoma. CD31 and Factor VIII are the best diagnostic markers for HGAS of bone. It is striking that keratin positivity is seen in the majority of cases, and is independent of epithelioid morphology. Pathologists should be aware of this to avoid misinterpretation as metastatic carcinoma.

Haemangiopericytoma (HPC) was first described by Murray and Stout as a soft tissue neoplasm with distinct morphologic features, presumably composed of pericytes. Over the years, it became clear that many tumours could mimic a HPC-like pattern. These days, it is accepted that in soft tissue most lesions diagnosed as HPC in the past are actually solitary fibrous tumours (SFT), synovial sarcomas (SS) or myofibromatoses. It has been unclear whether the very rare HPC of bone is atrue entity, or that the HPC-like vessels are non-specific and part of other, different entities.

We collected 10 primary HPC of bone from four institutions diagnosed between 1952 and 2002. All data were reviewed. Immunohistochemistry was performed for CD31, CD34, factor VIII, SMA, keratin AE1/AE3 and EMA. Staining was evaluated as focal positive, diffuse positive or negative.

There were five female and five male patients between 21 and 73 years of age (mean 45.3 y). All tumors were located within bone. The primary site of the tumour was the femur in two patients, humerus in one, fibula in one, sacrum in two and vertebra in three. All tumours showed the presence of prominent thin-walled branching vessels surrounded by more undifferentiated spindle or round cells. However these cells showed some variation in their morphologic pattern: five tumours showed a patternless architecture and varying cellularity, consistent with SFT. Three tumours showed more densely packed sheets of poorly differentiated cells, similar to SS, and one case each represented paraganglioma and PEComa, possibly metastatic. Tumours resembling SFT showed usually focal to diffuse staining for CD34. All tumours were negative for SMA. Two tumours more similar to SS showed focal positive staining for keratin AE1/AE3 or EMA (66%). Some tumours showed severe decalcification artefact. None of the 10 tumours show CD31 and factor VIII expression. FISH is performed to study SYT rearrangements.

Our retrospective review of tumours diagnosed as HPC of bone in the past revealed the absence of true pericytic differentiation and the existence of both SFT of bone and SS of bone. Therefore, as in soft tissue tumours, HPC-like features are non specific. Diffuse CD34 staining is helpful to diagnose SFT of bone, whereas keratin/EMA staining is suggestive for SS of bone.