What imaging modalities can be used to evaluate a lump in the axilla (under the armpit)?

Imaging for an Axillary Lump

Axillary ultrasound (US) is the initial imaging modality of choice for evaluating a lump under the armpit, regardless of whether it is unilateral or bilateral. 1

Primary Recommendation

The 2022 ACR Appropriateness Criteria definitively establishes that US of the axilla is "usually appropriate" as the first-line imaging test for both unilateral and bilateral palpable axillary lumps in women. 1 This recommendation is based on multiple studies demonstrating US's ability to characterize axillary findings, differentiate solid from cystic masses, assess lymph node morphology, and guide biopsy when needed. 1

Why Ultrasound First?

• Direct visualization: US can characterize the mass as solid, cystic, or nodal, and identify accessory breast tissue, lipomas, or lymphadenopathy 1
• Biopsy capability: If suspicious features are identified, US-guided core biopsy can be performed immediately for definitive diagnosis 1
• Low malignancy rate: In women without breast cancer history, only 7% of abnormal axillary nodes biopsied prove malignant, making invasive workup unnecessary in most cases 1
• High diagnostic performance: Recent meta-analyses show US maintains good sensitivity (79%) and specificity (86%) for detecting axillary pathology 2

Complementary Imaging Based on Clinical Context

For Women (Breast Cancer Concern)

Add diagnostic mammography and/or digital breast tomosynthesis (DBT) as complementary studies—not alternatives—when:

• There is personal history of breast cancer
• Clinical suspicion exists for axillary tail breast carcinoma
• The axillary mass might represent metastatic adenopathy from occult breast primary 1

These studies evaluate the entire breast for underlying lesions and microcalcifications that might be associated with the axillary finding. However, mammography or DBT alone should never be used as the sole initial test for an axillary lump, as they provide insufficient visualization of the axillary region. 1

Advanced Imaging: When to Escalate

MRI of the breast is reserved for specific scenarios after initial US evaluation:

• When US reveals suspicious adenopathy but mammography shows no breast primary (MRI detects occult breast cancer in over two-thirds of such cases) 1
• To define disease extent and characterize chest wall or vascular involvement 1
• Not routinely performed as initial assessment 1

CT chest should be considered when:

• Chest wall invasion is suspected
• Bony involvement needs assessment
• Systemic disease (lymphoma) is in the differential diagnosis 1

What NOT to Order Initially

FDG-PET/CT is not beneficial as initial imaging for an axillary mass of unknown etiology because:

• Low yield for detecting occult primary malignancy without first confirming malignant etiology
• Less than 1% of breast cancers present as isolated axillary adenopathy 1
• If PET/CT incidentally detects FDG-avid nodes, follow up with axillary US for characterization and biopsy guidance 1

Clinical Algorithm

1. Start with axillary ultrasound for all palpable axillary lumps
2. If female and concern for breast-related pathology exists: Add diagnostic mammography/DBT to evaluate the breast parenchyma
3. If US shows suspicious features: Proceed directly to US-guided core biopsy (superior to fine-needle aspiration with 88% vs 74% sensitivity) 1
4. If malignancy confirmed and breast primary suspected but not found: Consider breast MRI
5. If chest wall involvement or systemic disease suspected: Add CT chest

Important Caveats

• Bilateral axillary lumps have broader differential including reactive lymphadenopathy from infection, inflammatory processes, lymphoma, or leukemia—but US remains the initial test 1
• Advanced techniques like elastography, contrast-enhanced US, and photoacoustic imaging are investigational and not recommended for routine clinical use 1
• In obese patients, US diagnostic performance remains preserved, though one study suggests caution when interpreting negative US findings in very high BMI patients 3
• The wide differential for axillary masses includes benign entities (accessory breast tissue, lipomas, nerve sheath tumors, infection) and malignant causes (metastatic breast cancer, lymphoma, other primaries) 1