Pelvic Malignant Peripheral Nerve Sheath Tumor Revealing Previously Undiagnosed Familial Neurofibromatosis Type 1 in a Nine-Year-Old Girl: A Case Report.

Malignant peripheral nerve sheath tumors (MPNSTs) are malignant neoplasms of peripheral nerve sheath differentiation and may occur in association with neurofibromatosis type 1 (NF1). We report a case of a nine-year-old girl who presented with a rapidly enlarging left pelvic mass evolving over two months, accompanied by weight loss, pain, and functional limitation of the left lower limb with ipsilateral edema. Abdominopelvic imaging revealed a large heterogeneous mass centered on the left iliopsoas with pelvic and proximal thigh extension (108×100×151 mm), and thoracic staging identified bilateral pulmonary nodules suspicious for metastases. Careful dermatologic examination demonstrated numerous café au lait macules and axillary/inguinal freckling, strongly suggestive of NF1; targeted examination of the mother revealed similar pigmentary findings, consistent with previously unrecognized familial NF1. Biopsy showed a high-grade malignant spindle cell neoplasm with extensive necrosis and high mitotic activity. Immunohistochemistry was negative for desmin, myogenin, cytokeratin, and CD99, with focal SOX10 nuclear positivity, supporting the diagnosis of MPNST in the appropriate clinical and radiologic context. The patient was started on doxorubicin and ifosfamide chemotherapy. This case emphasizes that systematic cutaneous assessment in children with deep, rapidly growing soft tissue masses can be pivotal for identifying NF1, refining the differential diagnosis toward MPNST, and accelerating appropriate staging and multidisciplinary management.