“ PRRX1 -rearranged mesenchymal tumors”: expanding the immunohistochemical profile and molecular spectrum of a recently described entity with the proposed revision of nomenclature

Since the publication of the 2020 World Health Organization classification of soft tissue and bone tumors, the classification of “fibroblastic” tumors has expanded to include a novel subset of tumors characterized by PRRX1 :: NCOA1/2 gene fusions. These tumors defy conventional classification and are morphologically distinct, characterized by a multi-nodular growth of bland spindle cells suspended in a myxo-collagenous stroma with mild cytologic atypia, “staghorn-like” vessels, and variable perivascular hyalinization. Mitotic activity is rare, and necrosis is not identified. Herein, we present six additional cases of PRRX1 -rearranged mesenchymal tumors, including five cases with PRRX1 :: NCOA1 fusion and one case with PRRX1 :: KMT2D fusion. Three cases (3/6, 50%) demonstrated focal co-expression of S100 protein and SOX10, thereby expanding the immunohistochemical profile of this emerging entity. Like prior reported cases, there was no evidence of malignant behavior on short-term follow-up. The novel fusion, PRRX1 :: KMT2D , further expands the molecular spectrum of this entity and leads to a proposed revision of the provisional nomenclature to “ PRRX1 -rearranged mesenchymal tumor” to both accommodate non- NCOA1/2 fusion partners and allow for the possibility of partial neural or neuroectodermal differentiation.