What is the differential diagnosis for a 20-year-old female presenting with a large lump under her armpit?

Differential Diagnosis for Axillary Lump in a 20-Year-Old Female

In a 20-year-old woman with a large axillary lump, the most likely diagnoses are benign reactive lymphadenopathy, fibroadenoma of accessory breast tissue, or lipoma, with malignancy being uncommon but requiring exclusion through ultrasound evaluation. 1, 2

Most Common Benign Etiologies

Reactive Lymphadenopathy

• Reactive lymph node enlargement is the most common cause of axillary masses in young women, typically from infection, inflammatory processes, or autoimmune conditions 1, 2
• The risk of malignancy in women with no personal history of breast cancer is low (only 7% in one study of abnormal axillary nodes) and decreases further with younger age 1
• Look for recent infections, skin lesions in the drainage area, or systemic symptoms like fever 2

Fibroadenoma in Accessory Breast Tissue

• Accessory breast tissue occurs in the axilla and can develop benign lesions including fibroadenomas 1
• Fibroadenomas are the most common solid breast masses in women under 30 years, appearing as oval or round masses with well-defined margins on ultrasound 1
• The likelihood of malignancy in probably benign masses is particularly low in young women (only 0.3% in patients under 25 years) 1

Lipoma

• Giant lipomas can occur in the axilla, though they are uncommon in this location 3
• These present as soft, mobile masses that may cause local compression symptoms if large 3
• Ultrasound readily identifies lipomas as homogeneous fatty masses 2

Less Common but Important Diagnoses

Hidradenitis Suppurativa or Abscess

• Infection or inflammatory conditions can present as painful axillary masses 1
• Look for skin changes, warmth, erythema, or drainage 2

Nerve Sheath Tumors

• Schwannomas or neurofibromas can present as axillary masses 1
• These are typically firm and may cause neurologic symptoms 1

Malignant Etiologies (Less Likely but Must Exclude)

Lymphoma

• Lymphoma should be considered in any persistent lymphadenopathy lasting more than 2-3 weeks 4, 5
• Look for B symptoms (fever, night sweats, weight loss), multiple nodal sites, or systemic symptoms 2, 4

Metastatic Breast Cancer

• Less than 1% of breast cancers initially present as isolated axillary adenopathy 1
• Assess for breast masses, nipple discharge, skin changes, or family history of breast/ovarian cancer 2
• In women under 30 with suspicious findings, ultrasound-guided core biopsy is superior to fine needle aspiration for sensitivity, specificity, and histological grading 1

Leukemia

• Can present with generalized or localized lymphadenopathy 2
• Look for systemic symptoms, easy bruising, or other nodal sites 4

Critical Red Flags Requiring Urgent Evaluation

• Duration greater than 2-3 weeks warrants investigation 5
• Progressive enlargement over time 2, 5
• Hard, fixed, or matted consistency 2
• Associated breast changes, nipple discharge, or skin alterations 2
• Constitutional symptoms (fever, weight loss, night sweats) 2, 4
• Personal or family history of malignancy 2

Initial Diagnostic Approach

Start with ultrasound of the axilla as the primary imaging modality to differentiate solid from cystic masses and assess lymph node architecture 1, 2

• Ultrasound determines if the mass is a lymph node, solid lesion, cyst, or lipoma 2
• Assess cortical thickness and preservation of fatty hilum in lymph nodes 2
• If ultrasound shows clearly benign features (simple cyst, normal lymph node, lipoma), return to clinical follow-up only with no further imaging needed 1, 2

Proceed to image-guided core needle biopsy if ultrasound findings are suspicious or uncertain, as this provides superior diagnostic accuracy compared to fine needle aspiration 1, 2

• Indications for biopsy include abnormal cortical thickening, loss of fatty hilum, or solid masses that cannot be definitively characterized 2
• Core biopsy allows assessment of hormone receptors and HER2 status if malignancy is found 2

Mammography is not routinely indicated in women under 30 years unless there is high clinical suspicion for breast malignancy or personal history of breast cancer 1