A novel EWSR1-HOXB13 rearrangement in a fibroblastic tumor from the abdomen of a young woman.

We describe a novel EWSR1-HOXB13-fusion in a fibroblastic tumor from the abdominal wall of a 29-year-old woman. This tumor caused intermittent intense pain and had grown to approximately 5 cm in size over two years. The tumor was located beneath subfascial section of the abdominal wall and was invading the abdominal cavity and pressing on the liver. The tumor was well-circumscribed and consisted of intersected fascicles of monomorphic spindle-shaped cells with uniform ovoid nuclei lacking nuclear pleomorphism or mitotic activity. This tumor was immunohistochemically negative for pan-cytokeratin AE1/AE3, desmin, SMA, S100, myogenin, MyoD1, CD34, melanosome, SOX10, STAT6, SS18-SSX, and ERG. H3K27me3 was retained. RNA sequencing revealed a unique EWSR1-HOXB13-fusion, and strong, diffuse nuclear immunostaining for HOXB13 was observed. No local recurrence or evident distant metastasis were observed over eight months without chemotherapy, implying that the behavior of this tumor is not yet known.