Infantile NTRK‐rearranged spindle cell neoplasm on the scalp

Summary: A 7-month-old boy presented for a suspected hemangioma that was first noted shortly after birth and was enlarging. Previous ultrasound showed a non-vascular small superficial anechoic area. Exam revealed a 1.0 x 1.0 cm firm dark-pink blanchable round non-vascular plaque on the left frontal scalp. A punch biopsy was obtained. Pathology revealed a dermal spindle cell proliferation with positive pan-TRK and CD34 on immunohistochemical evaluation. Next generation sequencing (NGS) revealed an LMNA::NTRK1 [del(1;1)(q22;q23.1)] fusion alteration. A diagnosis of NTRK-rearranged spindle cell neoplasm was favored. Mohs surgical excision was planned. The patient underwent resection in the operating room under general anesthesia with intraoperative tissue read by a Mohs micrographic surgeon and a dermatopathologist. The tumor was excised down to pericranium and cleared after one stage. Surveillance imaging with PET MRI and CT chest was obtained 3 months post-operatively. Imaging did not show significant hypermetabolic activity nor evidence of metastatic disease. The patient remains healthy without complication or recurrence. This case highlights an emerging group of rare spindle cell tumors. Molecular characterization with NGS may be helpful in spindle cell tumors that cannot be clearly diagnosed by morphology and immunohistochemistry. Gene fusions involving NTRK support a diagnosis of NTRK‐rearranged spindle cell neoplasm.