PRRX1-Rearranged Mesenchymal Tumor in a Core Needle Biopsy

PRRX1-rearranged mesenchymal tumors are a recently described subset of soft tissue tumors, characterised by a PRRX1::NCOA1/2 or PRRX1::KMT2D gene fusion and by distinctive morphological features.¹ They show a circumscribed, multi-nodular growth pattern with bland spindle cells in a myxo-collagenous stroma, surrounded by staghorn-like vessels with perivascular hyalinization. First described by Lacambra et al in 2019,² they were originally named PRRX1-NCOAx-rearranged fibroblastic tumors. A total of 22 tumors have been reported since then.^{1- 7} Later, a subsequent study expanded the molecular spectrum of these tumors by detecting an alternative PRRX1::KMT2D fusion and variable degrees of S100 and SOX10 expression in a subset of tumors, which led to a proposed revision of the original nomenclature to PRRX1-rearranged mesenchymal tumors, a term, that comprises also tumors with non-NOCA1/2 fusion partners and allows for the possibility of partial neural or neuroectodermal differentiation. Loss of RB1 has been recently detected in two PRRX1-rearranged mesenchymal tumors on immunohistochemistry and FISH analysis,⁷ suggesting a potential overlap with RB1-deficient soft tissue tumors. As an emerging entity, PRRX1-rearranged mesenchymal tumors have not been included (yet) in the publication of the 2020 World Health Organization classification of soft tissue and bone tumors.⁸.