Malignant involvement of the peripheral nervous system in patients with cancer: Multimodality imaging and pathologic correlation

The clinical and imaging evaluation of peripheral neuropathies in patients with cancer is challenging. It is critically important to differentiate malignant invasion of the peripheral nervous system from nonmalignant causes, such as radiation-induced neuritis, neuropathy associated with chemotherapy, and inflammatory neuropathies. Contrast material–enhanced magnetic resonance (MR) imaging is the initial noninvasive test of choice; however, interpretation can be challenging when the anatomic features are distorted by prior surgery, radiation, or both. Fluorine 18 (¹⁸F)–fluorodeoxyglucose (FDG) positron emission tomography (PET)/computed tomography (CT) is an imaging adjunct to MR imaging that is particularly helpful for evaluating peripheral nerves because the metabolic activity depicted with ¹⁸F-FDG PET/CT helps differentiate malignant from benign disease and assists in making certain management decisions. For example, sites of high ¹⁸F-FDG activity in a peripheral nerve can be targeted to increase the diagnostic yield of a biopsy because malignant involvement of peripheral nerves can be patchy. Of note, ¹⁸F-FDG PET/CT can show clinically unsuspected metastases elsewhere in the body. If cancer is found, ¹⁸F-FDG PET/CT allows excellent assessment of treatment response. ¹⁸F-FDG PET/CT is also useful in evaluating primary nerve sheath tumors in that such tumors with low metabolic activity on FDG PET/CT images are unlikely to be malignant, although the specificity is limited. It is essential to have a good understanding of the imaging characteristics of benign and malignant causes of peripheral neuropathy if ¹⁸F-FDG PET/CT is to be used effectively for accurate diagnosis.