What are the etiologies of a breast lump in a 5‑month‑old female infant?

Causes of Breast Lumps in a 5-Month-Old Female Infant

In a 5-month-old female infant, a breast lump is almost always benign and most commonly represents normal physiologic breast tissue development (thelarche variant) or a benign cyst; malignancy is extraordinarily rare in this age group. 1, 2

Most Common Etiologies

Physiologic Breast Development

• Premature thelarche is the most common cause of bilateral breast enlargement in prepubertal girls, representing a benign, transient, and incomplete form of precocious puberty that requires no intervention 2
• Neonatal breast tissue can persist or transiently enlarge in infants due to residual maternal hormones, typically resolving spontaneously 1
• This physiologic enlargement may present as a palpable "lump" but represents normal glandular tissue rather than a true mass 2

Benign Cystic Lesions

• Simple cysts can occur even in infancy, though they are more common after puberty 1
• These appear as well-circumscribed fluid collections and require no treatment if confirmed benign 3

Infection or Inflammation

• Neonatal mastitis can present as a tender, indurated breast mass with or without erythema 1
• Breast abscesses may develop, particularly if there is skin breakdown or infection 4

Rare Benign Tumors

• Fibroadenomas, while the most common breast mass in adolescents (accounting for 95% of lesions in older children), are exceedingly rare in infants 5, 2
• Hemangiomas or other soft tissue tumors of the chest wall may be mistaken for breast masses 1

Malignancy Risk

• Primary breast carcinoma is extraordinarily rare in the pediatric age group, and essentially nonexistent in infants 6, 1
• When malignancy does occur in children, it is most commonly metastatic disease from other primary sites (leukemia, lymphoma, rhabdomyosarcoma) rather than primary breast cancer 1
• The risk of malignancy in a 5-month-old is so low that aggressive diagnostic interventions must be carefully weighed against the risk of damaging the developing breast bud 6, 1

Diagnostic Approach

Initial Clinical Assessment

• Determine whether the finding is unilateral or bilateral, as bilateral enlargement strongly suggests physiologic thelarche 2
• Assess for signs of infection: erythema, warmth, tenderness, or systemic symptoms 4
• Examine for skin changes, nipple discharge, or lymphadenopathy that would raise concern 1

Imaging Strategy

• Ultrasound is the only appropriate initial imaging modality in this age group if imaging is deemed necessary after clinical examination 3, 6
• Mammography has no role in infants and should never be performed 3
• Most cases of physiologic breast tissue in infants require no imaging at all—clinical observation and reassurance are sufficient 2

When to Image

• Image only if there is a discrete, unilateral mass that is not clearly physiologic breast tissue 6
• Image if there are concerning features such as rapid growth, skin changes, or signs of infection 1
• Do not image bilateral, symmetric breast enlargement in an otherwise well infant—this is premature thelarche and requires only reassurance 2

Management Principles

• Conservative observation is the cornerstone of management for breast findings in infants, given the overwhelming likelihood of benign etiology and the risk of damaging the developing breast 6, 1
• Biopsy should be avoided unless there are highly suspicious features, as interventional procedures can cause permanent breast deformity in this age group 6, 1
• If infection is suspected clinically, treat with antibiotics and consider drainage only if an abscess is confirmed on ultrasound 4

Critical Pitfalls to Avoid

• Do not perform biopsy or surgical excision of physiologic breast tissue—this will permanently damage the breast bud and cause future breast asymmetry or absence 6, 1
• Do not order mammography, MRI, or other advanced imaging in infants—ultrasound is the only appropriate modality if imaging is needed 3, 6
• Do not assume all breast enlargement requires imaging—bilateral symmetric enlargement in an infant is almost always benign thelarche 2
• Do not delay evaluation if there are red flags such as rapid unilateral growth, skin changes, or systemic symptoms, as these may indicate infection or (very rarely) malignancy 6, 1