Differential Diagnosis for Palpable Lump on Bilateral Posterior Scalp of a 6-Week-Old Baby

Single Most Likely Diagnosis

• Branchial Cleft Cysts: These are congenital anomalies that can present as palpable lumps along the side of the neck or scalp, close to the ear. They are relatively common and can become apparent in early infancy.

Other Likely Diagnoses

• Sebaceous Cysts: These are benign cysts that can occur anywhere on the skin, including the scalp. They might become more noticeable in early infancy due to the natural fluctuations in hormone levels after birth.
• Dermoid Cysts: Similar to sebaceous cysts, dermoid cysts are benign growths that can appear on the scalp. They are present at birth but may not be noticeable until later.
• Lymphadenopathy: Enlarged lymph nodes can present as lumps and could be due to infection or other inflammatory processes. Given the location close to the ear, it's plausible in the context of an ear infection or other local infections.

Do Not Miss Diagnoses

• Neuroblastoma: Although rare, neuroblastoma can present with masses in the neck or scalp. Early detection is crucial for treatment outcomes, making it a "do not miss" diagnosis.
• Rhabdomyosarcoma: A type of soft tissue cancer that can occur in infants, presenting as a mass. It's critical to consider this in the differential diagnosis due to its potential for aggressive behavior.
• Langerhans Cell Histiocytosis (LCH): LCH can present with skin lesions, bone lesions, or masses, including in the scalp. It's a condition that requires early diagnosis and treatment.

Rare Diagnoses

• Epidermoid Cysts: Similar to sebaceous cysts but filled with keratin, these are less common and can appear on the scalp.
• Teratomas: These are rare tumors that can contain several different types of tissues and can occur in the scalp, presenting as a lump.
• Hemangiomas: While more commonly associated with the skin, deep hemangiomas can present as palpable masses and could potentially occur in the scalp.

Given the presentation, referral to a Pediatric Surgeon or a Pediatric Dermatologist would be appropriate for further evaluation and management. If there's suspicion of a malignant process or neuroblastoma, urgent referral to Pediatric Oncology might be necessary.