What are the possible diagnoses and recommended management for a 3 × 5 inch mobile, non‑tender, non‑fluctuant lump in the right lower axillary region without skin changes or lymphadenopathy?

Axillary Mass: Diagnosis and Management

For a 3 × 5 inch mobile, non-tender, non-fluctuant mass in the right lower axillary region without skin changes or lymphadenopathy, ultrasound of the axilla is the most appropriate initial imaging study, followed by diagnostic mammography to evaluate for an occult breast primary. 1

Differential Diagnosis

The differential for this presentation is broad and includes:

Benign etiologies (most common):

• Lipoma (most common benign soft tissue tumor)
• Accessory breast tissue or ectopic breast tissue with fibrocystic changes or fibroadenoma
• Benign reactive lymphadenopathy (though you note no palpable lymphadenopathy)
• Nerve sheath tumors
• Leiomyoma (rare but reported in axilla) 2
• Epidermal/infundibular cysts
• Nodular fibromatosis

Malignant etiologies (less common but critical to exclude):

• Metastatic breast cancer (most common malignant cause if present—26% of axillary lymphadenopathy cases) 3
• Occult breast cancer (can present as isolated axillary mass)
• Lymphoma or leukemia
• Metastatic disease from non-breast primary

The size of this mass (3 × 5 inches, approximately 7.5 × 12.5 cm) is substantial and warrants prompt evaluation. In women without known malignancy presenting with palpable axillary masses, only 7% are malignant 1, but the large size increases concern.

Recommended Diagnostic Approach

Step 1: Axillary Ultrasound (First-Line Imaging)

Axillary ultrasound is the most appropriate initial imaging modality for evaluating a unilateral palpable axillary mass 1. The 2022 ACR Appropriateness Criteria explicitly designate US axilla as "usually appropriate" for this clinical scenario.

Ultrasound will:

• Determine if the mass is solid or cystic
• Characterize nodal architecture (if lymphoid origin)
• Identify features suggesting malignancy vs benignity
• Guide potential biopsy

Key sonographic features to assess:

• For lymph nodes: cortical thickness (>3-4 mm concerning), presence/absence of fatty hilum, shape (round vs oval), echogenicity 1, 4
• For soft tissue masses: echogenicity, vascularity, margins, homogeneity
• Lipomas appear as homogeneous hyperechoic masses and require no further workup 1

Step 2: Diagnostic Mammography ± Digital Breast Tomosynthesis

Mammography should complement axillary ultrasound to evaluate for an occult breast primary, particularly given the size of this mass 1. While mammography cannot adequately visualize the axilla itself, it is essential to exclude breast cancer as the underlying etiology, which is the most common malignant cause of axillary masses 1.

NCCN guidelines specifically recommend diagnostic mammography in patients presenting with axillary lymphadenopathy to provide global breast assessment 1.

Step 3: Image-Guided Biopsy

If ultrasound reveals suspicious features, proceed directly to US-guided core needle biopsy or fine needle aspiration 1, 5. Core needle biopsy is generally preferred over FNA for better tissue architecture assessment.

Biopsy is indicated if:

• Solid mass with indeterminate or suspicious features
• Lymph nodes with cortical thickness ≥4 mm, absent fatty hilum, or diffuse cortical thickening 1, 4
• Any features suggesting malignancy

Biopsy may be deferred if:

• Clear lipoma on ultrasound (homogeneous hyperechoic mass)
• Clearly benign-appearing lymph nodes with preserved fatty hilum and thin cortex (<3 mm)
• In such cases, short-term follow-up ultrasound at 6 months is reasonable 6, 7

Critical Pitfalls to Avoid

1. Do not rely on clinical examination alone. The absence of palpable lymphadenopathy does not exclude nodal disease—this mass may represent a single enlarged node or non-nodal pathology.

2. Do not skip breast imaging. Even with a "normal" breast exam, occult breast cancer can present as isolated axillary disease. Less than 1% of breast cancers present this way, but the consequences of missing it are significant 1.

3. Size matters for malignancy risk. Lymph nodes >33 mm (3.3 cm) have 97% specificity for malignancy when homogeneously dense 3. Your patient's mass is substantially larger, warranting heightened suspicion.

4. Do not order advanced imaging (CT, PET/CT, MRI) as initial tests. These are not appropriate first-line studies for isolated axillary masses without systemic symptoms or known malignancy 1.

Additional Considerations

If initial workup is negative or equivocal:

• MRI breast may be considered if mammography and ultrasound are negative but suspicion remains high for occult breast primary 1
• CT chest may be added if chest wall invasion is suspected or if systemic disease (lymphoma) is in the differential 1

Age and risk factors matter: Malignancy risk increases with age (mean age 55 years for malignant vs 43 years for benign pathology) 5. Incorporate patient's age, breast cancer risk factors, and any systemic symptoms into decision-making.

The mobile, non-tender nature is reassuring but non-specific. Both benign and malignant masses can be mobile. The large size (3 × 5 inches) is the most concerning feature and mandates thorough evaluation rather than observation.