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.
