Methotrexate-associated Peripheral Neuropathy on 18F-FDG PET/CT and MRI Masquerading as Neurolymphomatosis.

A 53-year-old woman with primary central nervous system lymphoma with vitreal involvement presented with upper extremity weakness, pain, and contractures following methotrexate infusion. Electromyography showed mononeuropathies of the upper extremities, suspicious for neurolymphomatosis versus vasculitis. An 18F-FDG PET CT indicated neurolymphomatosis, while an MRI of the bilateral forearms demonstrated intramuscular fluid collections, suspicious for abscesses or infarcts. Given the diagnostic dilemma, a right ulnar nerve biopsy was performed and showed findings most consistent with toxic/drug-related axonal neuropathy. Methotrexate was discontinued, and the patient was switched to TEDDI-R therapy, achieving complete pain relief.