Primary solitary fibrous tumour (SFT) of bone is extremely rare with few cases reported in the literature. The incidence of the lesion is 0.08% of all primary bone tumours (0.1% of primary malignant bone neoplasms). Previously, such lesions may have been reported as haemangiopericytoma (HP).

Despite being previously considered as separate entities, the two types of tumour (SFT and HP) are now generally accepted as related, sharing similar morphological and immunohistochemical features. Cytogenetic and molecular analysis has, so far, been unable to unite or divide the two. Although frequently having a histologically benign appearance or being labelled as intermediate grade, these tumours may exhibit an unpredictable clinical course and behave in an aggressive manner. We present two cases of osseous solitary fibrous tumour (cellular haemangiopericytoma).

Using the histopathology and bone tumour databases at our institution, we identified two patients (one female aged 21 and one male aged 40) with a histopathological diagnosis of osseous SFT. The site of primary tumour in both patients was the sacrum. In the female patient, the lesion was confined to the sacrum and she underwent curettage. In the male patient, the tumour extended beyond the sacrum to the sacro-iliac joint, ilium and gluteal mass, therefore, total sacrectomy was performed. At presentation neither patient had evidence of metastatic spread.

The female patient was disease free at four years with no evidence of recurrence of metastases. The male patient developed metastases in both lung fields and bone (ribs, vertebrae) three years post-operatively and died four years post-operatively.

Orthopaedic surgeons and histopathologists should remain aware of SFT due to its erratic behaviour and the recent move towards unifying it with HP in a continuous spectrum. We recommend early staging and treatment of these tumours, even for histologically benign/low grade lesions, due to their potentially aggressive behaviour.