Pigmented PRRX1::NCOA1‐rearranged fibroblastic tumor: A rare morphologic variant of an emerging mesenchymal tumor

Abstract PRRX::NCOAx ‐rearranged fibroblastic tumor is a recently described, morphologically distinctive subcutaneous fibroblastic tumor with benign behavior. To date, 12 cases have been reported. Here, we report a new case of PRRX::NCOAx ‐rearranged fibroblastic tumor showing a prominent pigmented component. The lesion occurred on the shoulder of a 23‐year‐old male. It was an at least 2.5 cm subcutaneous tumor with a multinodular and plexiform appearance. Morphologically, the tumor was characterized by a variably cellular proliferation of uniform oval to spindle cells arranged in fascicles and cords within a myxocollagenous stroma. Irregular, elongated, dilated vessels were prominent at the periphery of tumor nodules. In addition, nests and clusters of pigment‐laden epithelioid and dendritic cells were present. Immunohistochemically, the non‐pigmented tumor cells showed patchy positivity for factor XIIIa and focal positivity for S100 protein. The pigmented cells were positive for S100 protein, SOX10, MITF, and a pan‐melanocytic cocktail (Melan‐A, HMB‐45, and tyrosinase). Next‐generation RNA sequencing identified an in‐frame PRRX1::NCOA1 fusion. In summary, this case highlights a rare pigmented variant of PRRX::NCOAx ‐rearranged fibroblastic tumor, expanding the morphologic spectrum of this newly described mesenchymal tumor.