Solitary Fibrous Tumor of the Vulva: Report of 2 Cases, Including a De Novo Dedifferentiated Solitary Fibrous Tumor Diagnosed After Molecular Demonstration of NAB2-STAT6 Gene Fusion.

Solitary fibrous tumor (SFT) is a neoplasm of fibroblastic lineage that has been documented in almost every anatomic location. Vulval SFT is very rare with only 10 cases reported to date. We present 2 additional SFTs located in the vulva, in adult women of 59 and 25 yr of age. The first showed a classic morphology and immunophenotype with uniform and strong STAT6 nuclear expression. The other one was a spindle-cell de novo dedifferentiated SFT with heterogeneous nuclear and cytoplasmic STAT6 staining, which could only be correctly diagnosed after molecular analysis with demonstration of a NAB2-STAT6 gene fusion. This genetic aberration is considered to represent the major pathogenic driver in SFT and is highly specific for this neoplasm. The differential diagnosis of vulval SFT is wide and varies depending on the histologic SFT subtype. Molecular analysis is mandatory for a correct diagnosis in cases without the characteristic histopathologic and immunophenotypical features.