Phenotypical and molecular distinctness of sinonasal haemangiopericytoma compared to solitary fibrous tumour of the sinonasal tract

Aims Sinonasal haemangiopericytoma ( SN ‐ HPC ) is a rare sinonasal mesenchymal neoplasm of perivascular myoid cell origin. Solitary fibrous tumour ( SFT ) occurs only very rarely in the sinonasal tract. SFT and soft tissue HPC have been considered a single entity. Recently, recurrent gene fusions involving NAB 2‐ STAT 6 resulting in differential expression of STAT 6 were characterized as central molecular events in SFT . However, no data exist for NAB 2 – STAT 6 status or STAT 6 expression in SN ‐ HPC . Methods and results We examined six SN ‐ HPC s and two sinonasal SFT s by immunohistochemistry and RT – PCR for NAB 2– STAT 6 fusions. SN ‐ HPC affected three females and three males (mean age: 72 years). They expressed smooth muscle actin, lacked strong CD 34 reactivity and were negative for nuclear STAT 6 expression. RT – PCR analysis confirmed the absence of NAB 2– STAT 6 fusions in all cases. Conversely, both sinonasal SFT s (in males aged 39 and 52 years) displayed classical features of pleuropulmonary and soft‐tissue SFT s (uniformly CD 34‐positive with strong nuclear expression of STAT 6). RT – PCR revealed NAB 2 – STAT 6 fusions in both cases. Conclusions These findings confirm the molecular and phenotypical distinctness of these two entities. While SN ‐ HPC is a site‐specific sinonasal neoplasm of as yet unknown molecular pathogenesis, sinonasal SFT s show phenotypical and molecular identity to their pleural/extrapleural counterparts.