Temporal evolution of tumefactive neurolymphomatosis with serial MRI and positron emission tomography: illustrative case.

backgroundNeurolymphomatosis (NL), characterized by lymphoma infiltration of peripheral nerves, can be diagnostically challenging because of nonspecific clinical presentations and imaging findings. Tumefactive NL refers to circumferential subparaneurial tumor deposition beneath the paraneurium, outside the epineurium, producing an "encased" nerve appearance on MRI and at surgery. OBSERVATIONS: A 76‑year‑old man with treated large B‑cell lymphoma developed a painful left lower trunk brachial plexopathy with numbness of the ring and little fingers and progressive weakness in finger flexors and intrinsic hand muscles. Serial brachial plexus MRI and 18F-fluorodeoxyglucose positron emission tomography over 4 months demonstrated evolution from a thin, circumferential subparaneurial rim of enhancement with mild nerve enlargement to a homogeneously enhancing, tumefactive lesion of the C8 nerve root and lower trunk. Targeted fascicular biopsy of a nonfunctioning C8 fascicle confirmed recurrent B‑cell lymphoma. LESSONS: This case illustrates temporal evolution along the tumefactive NL spectrum, from early, subtle subparaneurial involvement to a frankly tumefactive lesion. Recognizing this early MRI pattern, particularly in patients with a history of lymphoma and focal plexopathy, can guide targeted biopsy and facilitate earlier diagnosis and treatment. The clinicoradiological findings suggest that subtle peripheral/subparaneurial enhancement may precede overt tumefactive enlargement. https://thejns.org/doi/10.3171/CASE26139.