The emerging PRRX1-NCOA fibroblastic neoplasm: a combined reappraisal of published tumors and two new cases

Nuclear receptor coactivator (NCOA) family gene fusions have been increasingly discovered in diverse mesenchymal neoplasms, while PRRX1-NCOA-fused fibroblastic tumors still remain insufficiently characterized. We herein present two additional PRRX1-NOCA1-positive cases sharing lobulated hypocellular growth of innocuous spindle-to-stellate cells in a fibromyxoid stroma enriched with polymorphous vessels. A constellation of low cellularity, alternating myxocollagenous matrix, bland cytomorphology, and, especially, unusual collagenous rosettes in one case were morphologically reminiscent of low-grade fibromyxoid sarcoma. In both cases, immunoprofiles were similarly nondescript and negative for all diagnostic markers, including MUC4, emphasizing the diagnostic value of molecular testing. Review of published and current cases highlights a striking predominance of PRRX1-NCOA1, unusual collagenous rosettes, and favorable behavior in this emerging fibroblastic tumor type.