We report a case of a 47-year-old man with neurofibromatosis type 1 presenting with a growing and painful lesion within the right thigh, suggesting a malignant peripheral nerve sheath tumor. MRI showed a T2-weighted hyperintense lesion with surrounding edema and contrast enhancement. (18)F-FDG PET/CT demonstrated inhomogeneously increased tracer uptake within the right thigh. Histopathologic evaluation revealed a foreign body with purulent fibroinflammatory reaction. (18)F-FDG PET/CT is a highly sensitive tool for detection of malignant transformation in neurofibromatosis type 1, but false-positive findings may be observed in benign lesions, for example, inflammatory processes.