A Plexiform Neurofibroma Mimicking a Lipoma: A Rare Case.

Plexiform neurofibromas (PNFs) are rare benign tumors of the peripheral nerve sheath, typically pathognomonic of neurofibromatosis type 1, and rarely isolated. We report the case of a 48-year-old man with a medical history of polycystic renal disease with chronic renal insufficiency with dialysis who presented with a painful, slowly enlarging mass in the lower right leg. The initial clinical examination and ultrasonography misdiagnosed the mass as a lipoma. In the operating room, the lesion was found to develop under the deep fascia with malignant features. Postoperative MRI demonstrated an irregular, lobulated aspect invading multiple muscle compartments and sparing the fibular cortex. Pathology confirmed the diagnosis of PNF, with the classical spindle cells in a myxoid background and positive S-100 staining. The patient declined definitive excision. This case underscores the diagnostic difficulties of adult PNF, especially when it presents as a lipoma mimic. MRI and pathology remain the gold standard for diagnosis.