Radiation therapy to the neck and axilla may result in brachial plexopathy either due to radiation damage of the nerve or due to nerve compression by surrounding fibrous connective tissue. The median interval between irradiation and occurrence of brachial neuropathy has been reported to be one to four years, although neuropathy has been seen even 21 years after radiation therapy. Two phases of neuropathy following irradiation have been described where early changes include electrophysiological and histochemical damage in the nerves themselves and the later phases include fibrosis of the soft tissue surrounding the nerves. Clinical symptoms and electromyography cannot always differentiate a recurrence from brachial plexus injury, although painless upper trunk lesions with lymphedema suggest radiation injury, and painful lower trunk lesions with Horner syndrome are usually indicative of tumor infiltration.MRI plays an important role in determining the diagnosis. Imaging features suggestive of radiation-induced plexopathy include diffuse, uniform, symmetric swelling, and T2 hyperintensity of the plexus within the radiation field. Mild contrast enhancement may also be present, sometimes making differentiation from tumor difficult. Nonuniform, asymmetric, and focal enlargement, and the presence of a mass with postcontrast enhancement, usually indicate tumor recurrence. Radiation fibrosis often shows low SI on both T1W and T2W images. T2 fat-suppressed and STIR sequences help to differentiate radiation fibrosis from tumor.
A 60-year-old female presenting with right upper limb weakness 18 months after radiation therapy for breast cancer. Coronal fat-suppressed T2W image (A) shows radiation plexopathy appearing as an abnormal thickening, and increased signal in the right brachial plexus (arrow). Sagittal T1W image (B) shows radiation-induced fat marrow replacement in the cervical vertebrae (arrows)
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