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

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.