Primary Malignant Peripheral Nerve Sheath Tumor of the Breast in a 19-Year-Old Woman

Primary malignant peripheral nerve sheath tumors (MPNSTs) of the breast are extremely rare neoplasms. Approximately half of the cases are associated with neurofibromatosis type 1 (NF1), whereas the other half occur sporadically. Imaging of breast MPNSTs can be variable and often reveals benign-appearing features, especially early in the disease course. Definitive diagnosis requires biopsy and pathologic analysis with specific immunohistochemistry stains. Treatment standards are not well established due to the rarity of MPNSTs of the breast. We report a 19-year-old woman who presented with a new palpable right breast mass. Ultrasound-guided core biopsy revealed an atypical spindle cell neoplasm with focal/weak S-100 protein expression and loss of histone H3 lysine 27 trimethylation (H3K27me3) expression, consistent with the diagnosis of MPNST. It was subsequently discovered that she had a prior NF1 diagnosis. Because the mass increased in size after external beam radiation, total mastectomy was performed. This case contributes to the limited literature on breast MPNSTs and highlights the importance of including MPNST in the differential diagnosis of an atypical breast mass in a young woman, particularly in the setting of NF1.