Abdominal Wall Schwannoma: A Rare Case of Target Sign on Ultrasonography

Schwannoma is a benign tumor derived from Schwann cells of peripheral nerves. Schwannomas arising within muscle or fat exhibit characteristic magnetic resonance imaging (MRI) findings called the target sign, split fat sign, or fascicular sign. We present a case in which ultrasound findings revealed a well-defined encapsulated mass showing the "target sign," which is commonly associated with MRI diagnosis of schwannomas. A 71-year-old man was referred to our hospital after a clearly demarcated abdominal wall mass was detected on plain computed tomography (CT) during a medical checkup at another hospital. The patient's chief complaint was mild discomfort and slight pain. He had a history of appendicitis surgery and mild insomnia, and no related family history was noted. There were no principal features that would raise suspicion of von Recklinghausen's disease (NF1) or neurofibromatosis type 2 (NF2). Ultrasonography revealed a well-defined, capsular mass with a diameter of just under 3 cm within the rectus abdominis. Therefore, we performed MRI, which showed the same target sign observed on ultrasonography. Our imaging findings strongly supported the diagnosis of schwannoma. Tumor resection was planned; however, due to the ongoing coronavirus outbreak, the patient strongly preferred to postpone treatment until the outbreak had subsided. He was followed up in the outpatient clinic, which was performed approximately one year later, and no change was observed during follow up. Histopathological examination revealed schwannoma. The patient was discharged 2 days after surgery, and no evidence of recurrence was found on imaging one year later.