What could cause a tender, mobile 2cm lump in my armpit with no fever or signs of illness?

Evaluation of a Tender Axillary Lump

For a tender, mobile 2cm lump in the armpit present for one week without fever or systemic illness, the most likely diagnosis is reactive lymphadenopathy from a benign cause, but ultrasound evaluation is warranted to characterize the lesion and guide further management.

Initial Clinical Assessment

The clinical presentation suggests a benign process, but several key features must be evaluated:

• Mobility and tenderness favor a benign reactive lymph node over malignancy, as malignant nodes are typically firm, fixed, and painless 1
• Size of 2cm places this in the category requiring imaging evaluation, as nodes >1cm warrant further investigation 1
• Absence of fever or systemic illness makes acute infection less likely but does not exclude reactive lymphadenopathy from a localized process 1

Recommended Imaging Approach

Ultrasound of the axilla is the appropriate initial imaging modality for evaluating a palpable axillary lump 1. The ACR Appropriateness Criteria rates ultrasound as "usually appropriate" for initial imaging of new palpable unilateral axillary lumps 1.

Key ultrasound features to assess include:

• Cortical thickness >0.3cm suggests possible malignancy 1
• Presence or absence of fatty hilum - absence has 90-93% positive predictive value for malignancy 1
• Short-axis diameter and overall morphology 1
• Vascularity pattern on Doppler imaging 2

Differential Diagnosis Considerations

Most Likely: Reactive Lymphadenopathy

• Benign reactive nodes from minor skin infections, folliculitis, or other localized inflammatory processes are the most common cause of tender, mobile axillary lymphadenopathy 1
• The one-week duration and tenderness support this diagnosis 1

Less Likely but Important to Exclude:

Lipoma: Giant axillary lipomas can present as large masses but are typically painless and soft rather than tender 2. MRI would show homogenous fat signal if this were the diagnosis 2.

Malignancy: While less likely given the mobile and tender nature, metastatic disease (including occult breast cancer) can present as axillary lymphadenopathy 3, 4. The absence of a fatty hilum on ultrasound would raise concern 1.

Management Algorithm

1. Obtain ultrasound of the axilla to characterize the lesion 1

2. If ultrasound shows benign features (preserved fatty hilum, cortical thickness <0.3cm, normal vascularity):

   • Observe for 2-4 weeks
   • If persistent or enlarging, proceed to biopsy 1

3. If ultrasound shows suspicious features (absent fatty hilum, cortical thickness >0.3cm, abnormal vascularity):

   • Proceed directly to US-guided core needle biopsy (superior to fine needle aspiration with 88% sensitivity vs 74%) 1
   • Core biopsy provides tissue architecture for definitive diagnosis 1

4. Examine for potential sources:

   • Inspect ipsilateral breast for masses (breast cancer can present with axillary lymphadenopathy) 1
   • Examine ipsilateral arm and hand for skin lesions, cuts, or infections that could cause reactive nodes 1
   • Check for systemic lymphadenopathy in other nodal basins 1

Critical Pitfalls to Avoid

• Do not assume benignity based on tenderness alone - while tenderness favors reactive nodes, malignant nodes can occasionally be tender 1
• Do not delay imaging - a 2cm node warrants ultrasound evaluation regardless of clinical suspicion 1
• Do not perform FNA if core biopsy is feasible - core biopsy has superior sensitivity and provides tissue architecture 1
• Do not forget breast examination - occult breast cancer can present as isolated axillary lymphadenopathy 4
• Inadequate sampling occurs in 5-10% of biopsies - non-diagnostic results do not exclude malignancy and may require repeat sampling 1

Follow-Up Timing

• If observation is chosen after benign ultrasound findings, re-evaluate in 2-4 weeks 1
• Nodes that persist beyond 4-6 weeks or continue to enlarge warrant biopsy even if ultrasound features appear benign 1
• Any node that becomes fixed, painless, or develops suspicious ultrasound features requires immediate tissue diagnosis 1