Expression of neuroendocrine markers in meningeal and extrameningeal solitary fibrous tumours - a potential diagnostic pitfall

Solitary fibrous tumours (SFTs) may show unusual morphologies and in such circumstances, unexpected immunoprofile can be misleading. Following an index case of myxoid meningeal SFT with neuroendocrine immunoprofile, we decided to assess a neuroendocrine profile in SFTs from various locations. The cohort of 9 meningeal and 28 extrameningeal SFTs was evaluated for CNS WHO grade (G1-G3) and four tiered Demicco risk stratification. Immunohistochemical detection of synaptophysin, chromogranin, INSM1, CD56, and CD57 was performed in each case and semiquantitatively assessed (0 - no expression; 1+ <10% positive; 2+ 11-50%; 3+ >51%); whole sections (meningeal SFTs) or tissue microarray (extrameningeal SFTs) were used for immunohistochemistry. The cohort included 13 men and 24 females. Meningeal SFTs included 5 WHO G1, 3 WHO G2 and 1 WHO G3 tumours. Extrameningeal SFTs included 21 low-risk, 4 intermediate risk and 2 high risk tumours. INSM1 immunoreactivity was observed in 12/37 cases (32%, 8 cases 1+, 3 cases 2+, 1 case 3+); synaptophysin was positive in 6/35 cases (19%, 5 cases 1+, 1 case 2+); CD56 was positive in 20/37 (54%, 16 cases 1+, 3 cases 2+ and 1 case 3+) and CD57 was expressed in 14/36 cases (39%, 5 cases 1+, 4 cases 2+ and 5 cases 3+). Chromogranin positivity was not observed. No significant association was observed between expression of neuroendocrine markers and tumour grade, Demicco risk group or meningeal and extrameningeal location. Extrapleural SFTs showed tendency for positivity of INSM1 (p=0.014, χ2) and CD57 (p=0.017, χ2), compared to pleural SFTs.