Neurologic Imaging

Should You Order MRI for Brachial Plexopathy in a Patient with a History of Malignancy?

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Should You Order MRI for Brachial Plexopathy in a Patient with a History of Malignancy?

A 62-year-old man with a history of Stage IIIA non-small cell lung cancer, treated two years ago with chemoradiation, presents to your clinic with three months of progressive right-sided shoulder pain radiating down his arm, accompanied by hand weakness and tingling in his fourth and fifth digits. His oncologic surveillance has been negative, but this new neurologic deficit raises the critical question: is this tumor recurrence invading the brachial plexus, or is it a delayed effect of his life-saving radiation therapy? Deciding on the right initial imaging study is paramount to distinguishing these possibilities and guiding the next steps in his care. This article provides a focused workflow for this exact scenario, explaining why the American College of Radiology (ACR) Appropriateness Criteria rates MRI brachial plexus without IV contrast as Usually Appropriate for the initial evaluation.

Who Fits This Clinical Scenario for Brachial Plexopathy Imaging?

This guidance is specifically for patients presenting with signs and symptoms of brachial plexopathy who have a known history of malignancy or have undergone treatment, particularly radiation therapy, that could affect the plexus.

Inclusion criteria for this workflow:

  • Clinical suspicion of brachial plexopathy (e.g., pain, paresthesias, weakness, or muscle atrophy in a C5-T1 nerve root distribution).
  • A known history of malignancy, especially those prone to local recurrence or metastasis in this region, such as apical lung cancer (Pancoast tumor), breast cancer, or lymphoma.
  • A history of radiation therapy to the neck, supraclavicular region, axilla, or chest wall.

Exclusion criteria (these patients fit a different ACR variant):

  • No History of Malignancy or Radiation: If a patient presents with similar brachial plexus symptoms but has no relevant oncologic history, their workup falls under the nontraumatic, no known malignancy scenario.
  • Acute Trauma: If the symptoms began immediately following a specific traumatic event, such as a motor vehicle collision or a fall, the workup follows the traumatic brachial plexopathy guidelines.
  • Lower Extremity Symptoms: If the patient’s symptoms involve the legs and pelvis, the evaluation should focus on lumbosacral plexopathy, which has its own distinct imaging recommendations.

What Diagnoses Are You Working Up in Brachial Plexopathy After Cancer Treatment?

The primary clinical challenge in this scenario is differentiating between direct neoplastic involvement and the late effects of cancer treatment. The imaging choice is tailored to evaluate for these key diagnostic considerations.

Tumor Recurrence or Metastasis
This is often the most urgent concern. Malignancies like apical lung cancer, breast cancer, and lymphoma can directly invade or compress the brachial plexus. The tumor can encase nerve roots, trunks, divisions, or cords, leading to progressive and often painful neurologic deficits. Imaging must be sensitive enough to detect subtle infiltration along the nerves.

Radiation-Induced Brachial Plexopathy (RIBP)
A delayed, non-neoplastic complication of radiation therapy, RIBP can manifest months to years after treatment. It results from radiation-induced fibrosis and microvascular damage to the nerves and surrounding tissues. Clinically, it can be difficult to distinguish from tumor recurrence, though it is often characterized by more diffuse, non-enhancing thickening on imaging and may present with less severe pain initially compared to neoplastic invasion.

Post-Surgical Changes or Neuroma
Patients who have undergone procedures like axillary lymph node dissection may develop significant scar tissue and fibrosis. This can entrap or tether the plexus, causing symptoms. A neuroma, a disorganized growth of nerve tissue at the site of a prior injury, can also be a source of pain and dysfunction.

Unrelated Cervical Spine Pathology
It is crucial to remember that patients with a history of cancer can still develop common conditions. A large cervical disc herniation or severe foraminal stenosis can cause radiculopathy that mimics the symptoms of a plexopathy. While less likely to be the primary cause in this specific clinical context, it remains an important part of the differential diagnosis.

Why Is MRI of the Brachial Plexus the Recommended Initial Study?

For a patient with a history of malignancy presenting with brachial plexopathy, the ACR designates both MRI brachial plexus without IV contrast and MRI brachial plexus without and with IV contrast as Usually Appropriate. This highlights the central role of Magnetic Resonance Imaging in this diagnostic challenge.

The superior soft-tissue resolution of MRI is essential for visualizing the intricate anatomy of the brachial plexus, from the nerve roots exiting the neural foramina to the terminal branches in the axilla. High-resolution sequences can delineate the individual nerve components and the surrounding fat planes, muscles, and vessels. This capability is critical for identifying the subtle thickening, signal abnormality, or mass effect that characterizes both tumor infiltration and radiation-induced changes.

While a non-contrast MRI is rated as Usually Appropriate and can often identify significant pathology, the addition of gadolinium-based contrast agents frequently provides crucial diagnostic information. Tumor recurrence typically demonstrates avid, often nodular, enhancement. In contrast, radiation-induced fibrosis usually shows more diffuse, less intense enhancement, or no significant enhancement at all. This distinction is fundamental to the workup. Therefore, while a non-contrast study is a valid starting point, many radiology departments will include contrast administration as part of their standard brachial plexus protocol for this indication.

Why are other studies rated lower for this scenario?

  • FDG-PET/CT whole body is rated May be appropriate. While PET/CT is excellent for detecting metabolically active tumor and staging systemic disease, its spatial resolution is lower than MRI. It may not precisely delineate the extent of nerve involvement and can be falsely positive in cases of radiation-induced inflammation. It is often used as a problem-solving tool or for systemic staging rather than as the initial modality for evaluating the plexus itself.
  • CT neck with IV contrast is also rated May be appropriate. CT provides excellent bone detail and can identify large soft-tissue masses, but its ability to resolve the nerves of the plexus is significantly inferior to MRI. It is a reasonable alternative if the patient has a contraindication to MRI, but it is not the preferred first-line study. It also involves ionizing radiation (1-10 mSv).
  • US neck is rated Usually not appropriate. While ultrasound can visualize portions of the plexus, particularly in the supraclavicular region, its field of view is limited, and it cannot assess the entire course of the plexus from the spine to the axilla. It is not comprehensive enough for this clinical question.

What’s Next After MRI of the Brachial Plexus? Downstream Workflow

The results of the initial MRI will guide the subsequent clinical pathway. The goal is to confirm a diagnosis and direct the patient toward appropriate management, whether that involves oncology, radiation oncology, neurosurgery, or supportive care.

If the MRI is positive for tumor recurrence:
A finding of a distinct, enhancing mass encasing or invading the plexus is highly suggestive of malignancy. The next step is typically a biopsy to obtain a tissue diagnosis, which is essential before initiating further cancer treatment like re-irradiation, systemic therapy, or surgical debulking. The MRI findings are critical for planning a safe biopsy approach, often performed under CT or ultrasound guidance.

If the MRI suggests radiation-induced plexopathy:
Findings of diffuse, non-enhancing thickening and fibrosis of the plexus, in the absence of a discrete mass, point toward RIBP. This is often a diagnosis of exclusion. Management is primarily supportive and may include physical therapy, pain management, and symptomatic treatment. In these cases, close clinical and imaging follow-up is necessary to ensure a small, underlying tumor was not missed.

If the MRI is indeterminate or negative:
When MRI findings are equivocal, further evaluation may be necessary. An FDG-PET/CT scan can be a valuable problem-solving tool to assess for metabolic activity that would favor tumor over fibrosis. If both MRI and PET/CT are unrevealing but clinical suspicion remains high, electrodiagnostic studies (EMG/NCS) can help confirm the presence and severity of a plexopathy and localize the lesion. If symptoms persist despite a negative workup, referral to a neurologist or neurosurgeon specializing in peripheral nerve disorders is warranted.

Pitfalls to Avoid (and When to Get Help)

Navigating this diagnostic pathway requires careful attention to detail to avoid common errors.

  • Inadequate MRI Protocol: Ordering a “routine” neck or shoulder MRI is insufficient. The order must specifically request a dedicated “brachial plexus MRI” to ensure the correct high-resolution sequences and field of view are used.
  • Misinterpreting Post-Treatment Changes: Differentiating radiation fibrosis from tumor can be challenging even for experienced radiologists. The radiologist must have access to the full clinical history, including the original tumor type, treatment dates, and radiation fields.
  • Over-reliance on a Single Modality: In ambiguous cases, do not hesitate to proceed to a complementary study. An indeterminate MRI may be clarified by the metabolic information from a PET/CT.
  • Delaying Biopsy for a Clear Lesion: If imaging reveals a clear, accessible lesion suspicious for recurrence, delaying tissue diagnosis can postpone necessary oncologic treatment.

If the clinical picture and imaging findings are discordant, or if the diagnosis remains unclear after initial studies, escalate by consulting with a multidisciplinary tumor board or referring the patient to a tertiary care center with expertise in peripheral nerve oncology.

Related ACR Topics and Tools

For a comprehensive overview of imaging for all types of plexopathy and to explore adjacent clinical scenarios, please consult our parent topic guide. Additional tools are available to help you select the right test and understand the technical details.

Frequently Asked Questions

Should I always order the MRI with and without contrast for this scenario?

The ACR rates both ‘MRI brachial plexus without IV contrast’ and ‘MRI brachial plexus without and with IV contrast’ as ‘Usually Appropriate.’ While a non-contrast study can be sufficient, adding contrast is often critical for differentiating enhancing tumor recurrence from non-enhancing radiation fibrosis. Most specialized centers will perform the study with contrast unless there is a specific contraindication, such as severe renal impairment or a known gadolinium allergy.

What if my patient has a pacemaker or other contraindication to MRI?

If a patient cannot undergo an MRI, ‘CT neck with IV contrast’ is rated as ‘May be appropriate’ and is the best alternative. While its soft-tissue resolution is lower than MRI, it can still identify large masses or significant bony erosion. In complex cases, an FDG-PET/CT scan may also provide valuable information, though it should be correlated with the CT findings for anatomical localization.

How soon after radiation therapy can radiation-induced brachial plexopathy (RIBP) occur?

RIBP is a delayed effect of radiation. It typically manifests anywhere from 6 months to over 20 years after treatment completion, with a median onset of several years post-therapy. Symptoms appearing within the first few months of completing radiation are much more suspicious for tumor recurrence.

The MRI was negative, but my clinical suspicion for plexopathy is very high. What’s next?

If a high-quality, dedicated brachial plexus MRI is negative but the patient has clear clinical signs of plexopathy, the next step is often electrodiagnostic testing (EMG/NCS). These tests can confirm the presence of a plexopathy, assess its severity, and help localize the site of injury, even when structural changes are not visible on imaging. A negative MRI in the face of a positive EMG may suggest a diagnosis like RIBP, where functional impairment precedes gross anatomical changes.

Can an MRI reliably distinguish tumor recurrence from radiation fibrosis?

In many cases, yes. Key MRI features favoring tumor include a focal mass, T2 signal hyperintensity, and avid contrast enhancement. Features favoring radiation fibrosis include more diffuse tissue thickening, T2 signal hypointensity (due to fibrosis), and minimal to no contrast enhancement. However, there can be significant overlap, especially in cases with radiation-induced inflammation. This is why a multidisciplinary approach combining imaging, clinical history, and sometimes biopsy is essential.

Reviewed by Pouyan Golshani, MD, Interventional Radiologist — May 26, 2026