59. Dedifferentiated solitary fibrous tumour arising from the bladder: A case report

Abstract

Background: Solitary fibrous tumour (SFT) is a rare mesenchymal tumour of fibroblastic differentiation. They most commonly arise in the pleura, but have a wide anatomical distribution. Conventional SFTs are circumscribed, spindle cell neoplasms with characteristic histomorphology including patternless architecture, dense collagen and prominent haemangiopericytoma-like ‘staghorn’ vessels. Conventional SFTs express CD34, Bcl2 and STAT6, but are negative for S100, keratin and desmin. Molecular profiling has shown consistent 12q chromosomal abnormality resulting in NAB2-STAT6 fusion. The NAB2-STAT6 gene fusion product is reliably identified on STAT6 immunohistochemistry, which has proven both a highly sensitive and specific marker for SFTs. Most SFTs are clinically benign, however up to 15% may show features of malignancy including increased mitoses (>4 per 10 HPF), necrosis and infiltrative margins. In addition, areas of dedifferentiation may occur, characterised by sharply demarcated areas of hypercellularity and pleomorphism with loss of morphological features of conventional SFT. Dedifferentiated areas may lose the typical immunohistochemical profile of conventional SFT, including loss of CD34 and STAT6. Aim: To report a case of dedifferentiated SFT in the bladder and review the current literature. Method: A 39-year-old male presented with one month history of urinary frequency and abdominal pain. CT and MRI showed a large heterogenous necrotic mass in the posterior bladder wall. An en bloc tumour resection and cystoprostatectomy was performed. Results: Macroscopic examination showed a 15 cm circumscribed tumour arising from the superior bladder. Microscopy revealed a spindle cell tumour arising from the bladder muscularis propria with areas of conventional SFT adjacent to vast areas of dedifferentiation with malignant features including hypercellularity, pleomorphism, increased mitoses (40 per 10 HPF) and necrosis. Immunohistochemistry showed positive staining for STAT6, CD34 and Bcl2 in both conventional and dedifferentiated areas. Conclusion: This is a unique case of a dedifferentiated SFT of the bladder in a young patient.