BRAF‐rearranged spindle cell mesenchymal neoplasm with a predominant lipofibromatosis‐like neural tumor pattern and co‐expression of CD34, S100 protein, and markers associated with perineurial differentiation: A rare case with potential diagnostic pitfall

Abstract Recently, a distinctive group of S100 protein/CD34‐positive spindle cell mesenchymal neoplasms characterized by a predominant lipofibromatosis‐like neural pattern harboring recurrent gene rearrangements involving NTRK1‐3 , RAF1 , RET , ROS1 , ALK , and MET has been identified. BRAF rearrangements have been rarely documented in this group of neoplasms. Herein, we report a 54‐year‐old man with a 1.3‐cm painless mass located in the subcutis of left back. The tumor was composed of mildly atypical, short‐spindle shaped to ovoid cells with fascicles and whorls intervening between and admixed with the subcutaneous adipose tissues and nerve bundles. Focally abundant thick, band‐like stromal hyalinization was also noted. The neoplastic cells showed diffuse reactivity for S100 protein and CD34 and multifocal immunopositivity for markers associated with perineurial differentiation including epithelial membrane antigen, GLUT1, and claudin‐1. Fluorescence in situ hybridization analyses showed positive for BRAF rearrangement and negative for rearrangements involving NTRK1 , RET , and ROS1 . The tumor was narrowly excised and recurred after 24 months of follow‐up. To our knowledge, we report the second case of BRAF ‐rearranged spindle cell mesenchymal tumor with predominant lipofibromatosis‐like neural tumor pattern. Expression of markers associated with perineurial differentiation is exceptional and represents a potential diagnostic pitfall, which may cause significant diagnostic confusion with a peripheral nerve sheath tumor.