What does a reactive lymph node in the left axilla indicate and what further testing is required in a patient with unknown age, sex, and medical history?

Reactive Lymph Node in the Left Axilla: Clinical Significance and Required Testing

A reactive lymph node in the left axilla most commonly represents a benign response to infection, inflammation, or vaccination, but requires targeted ultrasound evaluation and tissue diagnosis if suspicious features are present to exclude malignancy, particularly breast cancer or lymphoma. 1, 2

What Does "Reactive" Mean?

A reactive lymph node indicates lymphoid hyperplasia—an immune response to various stimuli rather than malignant infiltration. 3 The most common benign causes include:

• Infections: Upper extremity skin wounds, viral illnesses, or bacterial infections are the most frequent triggers 1, 2
• Recent vaccination: COVID-19 vaccines administered in the left arm can cause hypermetabolic reactive lymphadenopathy that typically resolves within 60 days but may persist up to 7 months 4, 5
• Autoimmune conditions: Systemic inflammatory diseases can produce reactive adenopathy 1, 2
• Silicone adenitis: Ruptured breast implants create characteristic "snowstorm" appearance on ultrasound 1, 2

Critical Initial Assessment

Immediate Clinical Questions to Address:

• Vaccination history: Specifically ask about COVID-19 or other vaccines in the left arm within the past 2-3 months 4, 5
• Breast symptoms or history: Any breast masses, nipple discharge, or personal/family history of breast cancer 6
• Systemic symptoms: Fever, night sweats, weight loss suggesting lymphoma 1
• Skin conditions: Wounds, rashes, or infections on the left arm/hand 6
• Breast implants: History of implants or rupture 1, 2

Required Testing Algorithm

Step 1: Axillary Ultrasound (Primary Modality)

Ultrasound is the first-line imaging test recommended by the American College of Radiology for evaluating axillary lymph nodes. 1, 6 It allows assessment of:

• Node morphology (preserved fatty hilum suggests benign etiology) 4
• Size and cortical thickness 7
• Solid versus cystic nature 1, 6

Step 2: Breast Imaging

Perform diagnostic mammography and/or digital breast tomosynthesis of the ipsilateral (left) breast to exclude occult breast cancer as the primary cause. 1, 6 This is mandatory because:

• Metastatic breast cancer is the most common malignant cause of axillary adenopathy 2
• Occult breast cancer can present with axillary metastases in <1% of cases 2

Step 3: Tissue Diagnosis (When Indicated)

Ultrasound-guided core needle biopsy or fine-needle aspiration is mandatory if suspicious features are present, with specificity of 98-100%. 1, 6

Proceed to biopsy if:

• Loss of fatty hilum 7
• Cortical thickening >3mm 7
• Round rather than oval shape 7
• Absence of clear benign explanation (recent vaccination, known infection) 4, 5
• Persistent enlargement beyond expected timeframe 5

Step 4: Additional Imaging Based on Biopsy Results

If malignancy is confirmed:

• For breast cancer: Breast MRI identifies occult primary lesions in approximately 70% of cases 1
• For lymphoma: CT chest/abdomen/pelvis or PET/CT for staging 1, 6

Critical Pitfalls to Avoid

• Never rely on physical examination alone—both sensitivity and specificity are limited for assessing axillary nodes 1, 6
• Do not delay biopsy of suspicious nodes even if "reactive" features are present, as early diagnosis significantly impacts treatment planning and prognosis 1, 6
• Do not assume benign etiology based on preserved fatty hilum alone if other suspicious features exist 4
• Always inquire about recent vaccination—vaccine-induced lymphadenopathy can mimic metastatic disease on imaging and may persist longer than expected 4, 5
• FDG uptake on PET/CT does not confirm malignancy—multiple benign causes including infection and silicone adenitis demonstrate uptake 1

When Observation is Acceptable

Observation without immediate biopsy is reasonable only if:

• Clear benign explanation exists (recent vaccination in left arm, known infection) 4, 5
• Node maintains normal morphology with preserved fatty hilum 4
• Complete breast imaging is normal 6
• Follow-up ultrasound in 4-6 weeks shows resolution or stability 5

If the node persists beyond 60 days post-vaccination or lacks clear benign etiology, proceed to tissue diagnosis regardless of morphology. 5