Brachial plexopathy is a form of peripheral neuropathy. Cardinal symptoms include pain, paraesthesia, and weakness across C5 to T1 nerve root distribution. The two major causes of brachial plexopathy in breast cancer patients include tumour recurrence along the path of the brachial plexus and radiation damage to the plexus. Differentiation between the two pathologies is important to guide treatment, but is difficult to make clinically. Magnetic resonance imaging with or without fluorine-18 fluorodeoxyglucose positron emission tomographycomputed tomography is a standard approach for differentiation. We report the use of this approach to diagnose metastasis of breast cancer at the brachial plexus.