What is the best imaging modality for the axilla (armpit)?

Best Imaging Modality for the Axilla

Ultrasound (US) is the most appropriate first-line imaging modality for evaluation of the axilla, providing high-quality visualization of axillary structures with the added benefit of enabling image-guided biopsy when necessary. 1

Advantages of Axillary Ultrasound

• High Accessibility: Widely available, fast, and cost-effective
• No Radiation: Safe for all patients including pregnant women
• Real-time Imaging: Allows dynamic assessment of structures
• Biopsy Capability: Enables immediate tissue sampling of suspicious nodes
• Good Diagnostic Performance: Sensitivity ranges from 52% to 90% and specificity from 98% to 100% when combined with biopsy 1

Ultrasound Characteristics of Axillary Nodes

When evaluating axillary nodes with ultrasound, assess for:

1. Cortical morphology: Thickening or eccentric contour suggests metastatic involvement 2
2. Fatty hilum: Loss of the fatty hilum is suspicious for malignancy 2
3. Size: Disparity in size compared to other nodes may indicate pathology
4. Cortical thickness: Greater thickness (>7mm) correlates with higher likelihood of positive cytology 2

Supplementary Imaging Modalities

While ultrasound is the primary modality, other imaging techniques may be appropriate in specific clinical scenarios:

MRI of the Breast

• May be appropriate when evaluating patients with breast cancer >2cm with clinical node-negative disease who have completed neoadjuvant chemotherapy 1
• Better visualizes level III and interpectoral nodes 1
• Not recommended as initial imaging for axillary evaluation alone

FDG-PET/CT

• Not recommended for routine axillary evaluation 1
• May be useful in patients with suspected metastatic disease or locally recurrent breast cancer 1
• High specificity (90-100%) but variable sensitivity (48-87%) for detecting lymph node metastases 1

Mammography/Digital Breast Tomosynthesis (DBT)

• Not primarily used for axillary evaluation as they incompletely visualize the axilla 1
• May complement axillary US by evaluating the breast for underlying lesions 1

Biopsy Considerations

When suspicious nodes are identified on ultrasound:

• US-guided core needle biopsy is superior to fine-needle aspiration (FNA) with sensitivity of 88% vs 74% 1
• US-guided FNA remains useful for patients who cannot discontinue anticoagulants 1

Advanced Techniques

Ultrasound elastography shows promise as an adjunct to conventional ultrasound:

• Improves sensitivity (90% vs 76%) and specificity (86% vs 78%) compared to conventional ultrasound alone 3, 4
• Particularly useful for detecting abnormal nodes in suspected breast cancer 3
• Not yet standard of care but emerging as a valuable supplementary technique

Clinical Algorithm for Axillary Imaging

1. Initial presentation: Begin with axillary ultrasound
2. Suspicious findings: Proceed to US-guided core biopsy or FNA
3. Breast cancer staging: Consider supplementary MRI if needed for complete nodal evaluation
4. Post-treatment evaluation: US remains first choice for monitoring response to therapy

Common Pitfalls to Avoid

1. Relying solely on node size: Morphology and cortical features are more reliable indicators of malignancy than size alone 2
2. Inadequate sampling during biopsy: Ensure at least 3 sentinel nodes are sampled when evaluating post-neoadjuvant chemotherapy patients 1
3. Overlooking deep nodes: Standard US may miss level III nodes that are better visualized with MRI 1
4. False negatives: Negative ultrasound findings do not exclude axillary lymph node metastases 5

Ultrasound remains the cornerstone of axillary imaging due to its excellent visualization capabilities, ability to guide interventions, and favorable risk profile compared to other modalities.