What is the likely cause of a bump on the back of the head that has been present for three weeks?

Differential Diagnosis of a Persistent Scalp Bump

A bump on the back of the head persisting for 3 weeks requires clinical evaluation to distinguish between benign causes (most common: sebaceous cyst, lipoma, or pilomatricoma) and less common but serious etiologies including vascular lesions, infection, or neoplasm.

Most Likely Causes

Benign Subcutaneous Lesions (Most Common)

The majority of persistent scalp bumps are benign:

• Sebaceous/epidermoid cysts - Most common scalp masses, typically slow-growing, mobile, and painless 1
• Lipomas - Soft, mobile, subcutaneous masses
• Pilomatricoma - Benign hair follicle tumors, one of the most frequently excised pediatric skin masses, though can occur at any age 2

Key distinguishing features on examination:

• Mobility over underlying structures suggests benign subcutaneous origin
• Firm, non-tender masses are typical of cysts or lipomas
• Fluctuant quality suggests cystic lesion 1

Post-Traumatic Vascular Lesions

If there is any history of trauma (even minor):

• Pseudoaneurysm of superficial temporal artery - Can develop gradually over 3 weeks following head trauma 3
• Presents as progressively enlarging swelling
• Requires ultrasound with color Doppler showing "yin-yang flow pattern" for diagnosis 3
• This is critical to identify as it requires specific treatment (thrombin injection or surgical repair)

Infectious Causes

Consider if associated with:

• Tenderness, warmth, erythema
• Fever or systemic symptoms
• Recent scalp trauma or manipulation
• Carbuncles or abscesses require incision and drainage 4

Neoplastic Lesions (Less Common but Important)

Red flags requiring urgent evaluation:

• Immobile mass fixed to underlying structures 5
• Rapid growth (especially if enlarging from small to large over weeks) 2
• Age >40 years with persistent mass >2 weeks 5
• Associated neurological symptoms
• Overlying skin changes or ulceration

Rare but serious possibilities include:

• Meningioma with extracranial extension - Can erode through skull presenting as scalp mass 6, 7
• Lymphoma - Can present as scalp nodules with lymphadenopathy 8

Recommended Evaluation Approach

Initial Assessment

1. Detailed history:

   • Duration and rate of growth
   • Any preceding trauma (even minor falls)
   • Pain, tenderness, or pulsation
   • Neurological symptoms (headache, vision changes, focal deficits)
   • Constitutional symptoms (fever, weight loss, night sweats)

2. Physical examination specifics:

   • Mobility: Mobile suggests benign subcutaneous; fixed suggests deeper involvement 5
   • Consistency: Soft/fluctuant (cyst), firm (lipoma), hard (bone involvement)
   • Pulsatility: Suggests vascular lesion
   • Tenderness: Suggests infection or rapid expansion
   • Size and borders: Well-circumscribed vs irregular
   • Overlying skin changes
   • Regional lymphadenopathy 5
   • Neurological examination if any concerning features

Imaging Strategy

For typical benign-appearing lesions:

• Ultrasound is first-line - Can characterize cystic vs solid, assess vascularity, and identify pseudoaneurysms 3

If trauma history or pulsatile mass:

• Ultrasound with color Doppler mandatory to exclude pseudoaneurysm 3

If concerning features present (immobility, rapid growth, neurological symptoms, age >40):

• MRI head without and with contrast to evaluate for intracranial extension or underlying pathology 6, 7
• CT may be needed if bone erosion suspected

Management Algorithm

1. If mobile, non-tender, slow-growing: Likely benign - ultrasound for characterization, consider elective excision for diagnosis/cosmesis

2. If any trauma history: Ultrasound with Doppler to exclude pseudoaneurysm before any manipulation

3. If fixed, rapidly growing, or neurological symptoms: MRI brain and urgent referral to neurosurgery

4. If tender with inflammatory signs: Consider infection - may need incision/drainage and cultures 4

5. If age >40 with persistent mass >2 weeks: Maintain high suspicion for malignancy until excluded 5

Critical Pitfalls to Avoid

• Never attempt aspiration or biopsy of a pulsatile mass without imaging - risk of catastrophic hemorrhage from pseudoaneurysm
• Do not dismiss as "just a cyst" in patients >40 without proper evaluation 5
• Do not ignore history of trauma - pseudoaneurysms can present weeks after injury 3
• Fixed masses require imaging before assuming benign etiology 6, 7