FOSL1 immunohistochemistry clarifies the distinction between desmoplastic fibroblastoma and fibroma of tendon sheath

Aims Although desmoplastic fibroblastoma ( DFB ) and fibroma of tendon sheath ( FTS ) are well‐established entities, they may show overlapping clinicopathological features. In addition, cytogenetic data showing a shared 11q12 rearrangement in a small number of cases suggest a close link between these entities. A recent microarray study revealed up‐regulation of FOSL 1 m RNA in DFB s with 11q12 rearrangement. The aim of this study was to clarify the relationship between DFB and FTS . Methods and results We tested 42 cases diagnosed originally as either DFB s or FTS s for interobserver concordance based on the existing histological criteria and correlated the diagnosis with FOSL 1 immunohistochemistry. In addition, FOSL 1 gene status was determined by chromogenic in‐situ hybridization ( CISH ). Using joint histological evaluation, 41 of 42 tumours were classified unanimously by three pathologists into 25 DFB s and 16 FTS s, whereas only one case received discordant opinions. Immunohistochemically, all DFB s showed diffuse, strong FOSL 1 nuclear immunoreactivity (25 of 25, 100%), while none of the FTS s showed such overexpression. None of the selected 42 DFB mimics overexpressed FOSL 1. FOSL 1 was not rearranged in seven DFB s tested by CISH . Conclusions We confirm here that DFB and FTS are two distinct entities that can be distinguished using the existing histological criteria. This distinction corresponds perfectly with FOSL 1 immunohistochemical expression status, and diffuse strong FOSL 1 expression specific to DFB s sharpens the border between the two categories. FOSL 1 overexpression in DFB may not be caused directly by FOSL 1 gene rearrangement. FOSL 1 may also be a diagnostic aid for differentiating DFB from other histological mimics.