Pediatric soft tissue tumor of the upper arm with LMNA-NTRK1 fusion.

A 6-year-old girl was admitted to our hospital because of the presence of a slow-growing tumor in her right elbow. Biopsy specimens showed a round to spindle cell neoplasm with uncertain malignant potential, leading to the decision of surgical resection. Histologically, the resected tumor was encapsulated by fibrous tissue but focally invaded the surrounding skeletal muscles. The tumor was composed of ganglion cell-like short spindle cells with lymphocytic infiltration in the collagenous background. Tumor cells with large bizarre nuclei were occasionally observed, and multinucleated giant cells were scattered at the periphery. Hemangiopericytoma-like patterns and adipose tissue elements were not evident, and mitotic figures were rarely observed (<1 per 10 high-power fields). Immunohistochemically, the tumor cells were positive for S-100 and CD34 and focally positive for epithelial membrane antigen and AE1/AE3. RNA sequencing and subsequent reverse-transcription polymerase chain reaction revealed alternative splicing forms of LMNA-NTRK1 fusion (Ex2-Ex10 and Ex2-Ex15).