Retromammary Schwannoma Mimicking a Breast Tumor in a Patient with Neurofibromatosis Type 1: A Case Report Demonstrating Surgical Avoidance Using Contrast-Enhanced Ultrasound-Informed Biopsy.

INTRODUCTION: Retromammary schwannomas are rare peripheral nerve sheath tumors that may clinically and radiologically mimic breast lesions, occasionally leading to unnecessary surgical intervention. CASE PRESENTATION: We report a young Japanese woman with previously undiagnosed neurofibromatosis type 1 who presented with breast pain. US revealed a mobile, well-circumscribed hypoechoic mass located in the retromammary space. Contrast-enhanced US (CEUS) using a second-generation contrast agent demonstrated a characteristic dynamic target sign, and MRI showed concordant findings. Vacuum-assisted biopsy performed with reference to the CEUS target-like heterogeneous enhancement pattern confirmed a benign schwannoma. CONCLUSIONS: Dynamic US assessment combined with CEUS-informed biopsy target selection may improve diagnostic confidence for retromammary lesions and help avoid unnecessary surgical resection.