18F-FDG PET/CT Helps Differentiate Peripheral Nerve Myeloid Sarcoma From a Presumed Benign Nerve Sheath Tumor

A 21-year-old man with NF1 (neurofibromatosis type 1) mutation and in remission from acute myeloid leukemia presented with a painless mass in the left upper limb. MRI showed a soft-tissue mass involving the ulnar nerve presumed to be a nerve sheath tumor. F-FDG PET/CT was performed demonstrating high FDG avidity in the mass, prompting a biopsy. Histopathology and immunohistochemistry of the biopsy sample demonstrated myeloid sarcoma of the ulnar nerve. This case highlights the role of F-FDG PET/CT in raising the suspicion of malignancy in otherwise presumably benign lesions of the nerve.