Metastatic brachial plexopathy presenting as radiation-induced plexopathy in metastatic lung cancer: a case report and literature review.

BACKGROUND: Metastatic brachial plexopathy (MBP) is a rare but debilitating complication of advanced malignancies, most commonly associated with lung and breast carcinomas. Distinguishing MBP from radiation-induced brachial plexopathy (RIBP) represents a significant clinical challenge due to their overlapping clinical presentations and similar appearances on structural imaging. While magnetic resonance imaging (MRI) is considered the gold standard for evaluating the brachial plexus (BP), it often lacks the sensitivity required to definitively differentiate between malignant infiltration and post-radiation fibrosis.

CASE DESCRIPTION: A 75-year-old male with a history of cT4N3M1c squamous cell lung carcinoma was treated with chemo-immunotherapy followed by consolidating radiotherapy (46.75 Gy in 17 fractions). After 2 years of maintenance pembrolizumab and achieving a complete response, the patient presented with progressive right arm paralysis and neuropathic pain. Initial electromyography and MRI were inconclusive, with the latter suggesting RIBP as a potential etiology. However, a subsequent [18F]fluorodeoxyglucose-positron emission tomography/computed tomography (FDG-PET/CT) revealed intense hypermetabolic uptake along the C4-C5 nerve roots extending into the right BP, confirming a diagnosis of MBP. The patient subsequently underwent palliative radiotherapy (30 Gy in 10 fractions) targeted at the [18F]FDG-avid lesion. This intervention resulted in substantial pain relief, although the motor deficits remained persistent.

CONCLUSIONS: This case highlights an atypical presentation of MBP that closely mimicked RIBP through its slowly progressive nature. It underscores the pivotal role of [18F]FDG-PET/CT in establishing the correct diagnosis and guiding palliative management.