Extensive Plexiform Neurofibroma of the Right Thigh and Pelvis in a Child with Genetically Confirmed Neurofibromatosis Type 1: A Case Report

A bstract Neurofibromatosis type 1 (NF1) is an autosomal dominant neurocutaneous disorder; plexiform neurofibromas are pathognomonic, and extensive disease may mimic malignancy. An 11-year-old girl had a congenital, gradually progressive, painful hyperpigmented swelling over the right thigh and gluteal region. She had multiple café-au-lait macules, axillary freckling, a large “bag-of-worms” plexiform neurofibroma, clitoromegaly, and Lisch nodules. Contrast-enhanced magnetic resonance imaging showed diffuse infiltrative T2/short tau inversion recovery hyperintense, minimally enhancing soft-tissue thickening involving the right thigh, gluteal region, perineum, and extraperitoneal pelvis, with an elongated lesion posterior to the urinary bladder and clitoral involvement. Whole-body 18 F-fluorodeoxyglucose positron emission tomography–computed tomography demonstrated only low-grade uptake without a hypermetabolic focus. Clinical exome sequencing identified a heterozygous truncating NF1 variant. This rare pelvic and external genital plexiform neurofibroma in a child with NF1 highlights the value of multimodality imaging and genetic confirmation for diagnosis, counseling, and surveillance. Early recognition helps avoid unnecessary biopsy and supports multidisciplinary follow-up planning.