Neurolymphomatosis Involving the Brachial Plexus: Correlative MRI and FDG PET-CT Findings

Neurolymphomatosis is a rare manifestation of lymphoma characterized by infiltration of peripheral nerves, nerve roots, or plexuses, often presenting with painful neuropathy and diagnostic difficulty. We report a case of an 18-year-old patient presenting with progressive pain and weakness of the right upper limb over 10 days. Magnetic resonance imaging (MRI) revealed an ill-defined infiltrative soft tissue lesion in the right posterior cervical region extending into the thoracic outlet and axilla, with extensive perineural involvement of the brachial plexus and encasement of adjacent vascular structures. The lesion was hypointense on T1-weighted images, hyperintense on short tau inversion recovery and T2-weighted images, demonstrated restricted diffusion, and showed heterogeneous postcontrast enhancement. Fluorodeoxyglucose (FDG) positron emission tomography (PET)-computed tomography (CT) demonstrated intense linear FDG uptake along the involved cervical nerve roots and brachial plexus. No additional systemic nodal or extranodal disease was identified. A neoplastic etiology, likely a lymphomatous involvement, was considered and subsequently confirmed via ultrasound-guided core biopsy as diffuse large B-cell lymphoma. Follow-up PET-CT after systemic chemotherapy demonstrated interval reduction in lesion size and metabolic activity. This case highlights the complementary role of MRI and FDG PET-CT in early diagnosis, biopsy guidance, accurate disease mapping, and treatment response assessment of this rare entity.