Expression of neuroendocrine markers in meningeal and extrameningeal solitary fibrous tumors: a potential diagnostic pitfall*,**

Solitary fibrous tumors (SFTs) may show unusual morphologies, and in such circumstances, an unexpected immunoprofile can be misleading. Following an index case of myxoid meningeal SFT with a 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 4-tiered Demicco risk stratification. Immunohistochemical detection of sy-naptophysin, chromogranin, INSM1, CD56, and CD57 was performed in each case and semiquantita-tively assessed (0: no expression; 1+: <10% positive; 2+: 11e50%; and 3+: >51%); whole sections (meningeal SFTs) or tissue microarray (extrameningeal SFTs) were used for immunohistochemistry. The cohort included 13 men and 24 women. Meningeal SFTs included 5 WHO G1, 3 WHO G2, and 1 WHO G3 tumors. Extrameningeal SFTs included 21 low-risk, 4 intermediate-risk, and 2 high-risk tu-mors. INSM1 immunoreactivity was observed in 12 of 37 cases (32%; 8 cases 1+, 3 cases 2+, and 1 case 3+); synaptophysin was positive in 6 of 35 cases (19%; 5 cases 1+ and 1 case 2+); CD56 was positive in 20 of 37 cases (54%; 16 cases 1+, 3 cases 2+, and 1 case 3+); and CD57 was expressed in 14 of 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 tumor grade, Demicco risk group or meningeal and extrameningeal location. Extrapleural SFTs showed a tendency for positivity of INSM1 (P = .014, c2) and CD57 (P = .017, c2) compared to pleural SFTs.& COPY; 2023 Elsevier Inc. All rights reserved.