What could be the cause of a lump in my armpit?

Armpit Lump: Diagnostic Approach and Causes

For any adult presenting with an armpit lump, the initial evaluation must include age-appropriate imaging—diagnostic mammography plus axillary ultrasound for women ≥30 years, or ultrasound alone for those <30 years—to exclude breast cancer, which is the most common malignant cause of axillary lymphadenopathy. 1

Most Common Causes

Benign Etiologies

• Reactive lymphadenopathy from infections is the most frequent benign cause of axillary lumps 2
• Lipomas and epidermal cysts are commonly encountered soft tissue masses in the axilla 3
• Silicone adenitis from breast implants (ruptured or intact) can produce axillary lymphadenopathy with a characteristic "snowstorm" appearance on ultrasound 2
• Dermatopathic lymphadenopathy from skin conditions may cause axillary swelling 2

Malignant Etiologies

• Metastatic breast cancer is the most common malignancy when cancer is identified in axillary nodes, even when no breast mass is palpable 3, 2
• Occult breast cancer presents with axillary metastases in <1% of breast cancers, with no detectable breast primary on initial examination 2
• Lymphoma (particularly non-Hodgkin's) should be considered, especially with bilateral involvement 2
• Soft tissue sarcomas present as painless enlarging masses, though axillary location is less common 3

Critical Diagnostic Algorithm

Step 1: Age-Stratified Imaging

• Women ≥30 years: Begin with diagnostic mammography (including magnification views to detect microcalcifications) followed immediately by targeted axillary ultrasound at the same visit 1, 4
• Women <30 years: Start with targeted breast and axillary ultrasound to avoid unnecessary radiation in this lower-risk population 1
• Men of any age: Ultrasound is the initial study; mammography is not routinely indicated unless clinical findings are suspicious 3

Step 2: Complete Imaging Before Biopsy

• Always complete all imaging studies before proceeding to tissue diagnosis, as biopsy-related changes will confound subsequent image interpretation 1, 4
• Ultrasound characterizes whether the mass is solid or cystic and evaluates lymph node morphology 1

Step 3: Tissue Diagnosis When Indicated

• Core needle biopsy (not fine needle aspiration) is recommended for any mass that appears suspicious or highly suggestive of malignancy on imaging 3, 4
• For soft tissue masses that are unexplained and increasing in size, consider urgent ultrasound within 2 weeks to assess for soft tissue sarcoma 3
• If lymphoma is suspected, special pathologic evaluation or surgical excision may be required 3

Step 4: Advanced Imaging for Occult Primary

• If malignancy is confirmed in an axillary node but no breast primary is identified on mammography and ultrasound, breast MRI is mandatory 1, 4
• MRI identifies occult breast cancer in approximately 70% of patients with suspicious axillary lymphadenopathy and negative conventional imaging 1, 4

Key Clinical Distinctions

Unilateral vs. Bilateral Presentation

• Unilateral axillary lymphadenopathy more commonly suggests breast cancer or localized infection 1, 2
• Bilateral axillary lymphadenopathy suggests systemic processes including infections, autoimmune diseases, or hematologic malignancies like lymphoma 1, 2

Physical Examination Clues

• Nontender masses are more suspicious for malignancy than tender masses 3
• Normal anatomic structures often mistaken for pathologic masses include submandibular glands, hyoid bone, and carotid bulb 3
• Never rely on physical examination alone, as both sensitivity and specificity are limited 1

Critical Pitfalls to Avoid

• Do not delay biopsy of suspicious nodes, as early diagnosis significantly impacts treatment planning and prognosis 1
• Do not assume benign etiology even when reactive changes are common—bilateral presentation warrants tissue diagnosis to exclude lymphoma 1
• Do not miss the surgical window by dismissing masses in patients with cancer history; subcutaneous metastases from gastric cancer and other malignancies can present in the axilla 5
• For soft tissue masses >5 cm, deep-seated, or increasing in size, consider urgent referral to exclude sarcoma 3

Special Considerations

In Patients with Breast Cancer History

• Any new axillary mass requires immediate evaluation with imaging and likely biopsy 3
• Sentinel lymph node biopsy has replaced axillary lymph node dissection as standard of care for clinically node-negative breast cancer ≤2 cm 1, 4

In Young Women

• While breast cancer is uncommon in young women (80% occurs after age 50), it does occur and has worse outcomes when diagnosed in younger patients 6
• Fibroadenomas and benign breast changes are most common, but malignancy must be excluded with imaging 6, 7