Keratoacanthoma
Summary
Keratoacanthoma (KA) is a rapidly growing, dome-shaped skin tumour with a characteristic central keratin plug, giving it a "volcano-like" appearance. It arises from hair follicles on sun-exposed skin. The classic natural history is rapid growth over weeks, a stable phase, and spontaneous resolution over months, leaving a pitted scar. However, KA is histologically and clinically indistinguishable from well-differentiated squamous cell carcinoma (SCC), and many pathologists consider KA a variant of SCC. For this reason, most KAs are excised rather than observed.
Key Facts
- Appearance: Dome-shaped nodule with central keratin plug ("volcano")
- Growth Pattern: Rapid growth (2-8 weeks) → Stable → Spontaneous involution (months)
- Location: Sun-exposed areas (face, hands, forearms)
- Key Issue: Clinically and histologically hard to distinguish from SCC
- Treatment: Excision is standard (treats as SCC to be safe)
- Natural History: If left, resolves leaving pitted scar
Clinical Pearls
"Volcano Sign": The classic KA has a dome shape with a central keratin-filled crater - like a volcano.
"Rapid Growth = KA or SCC": If a nodule appears and grows rapidly over weeks, think KA or aggressive SCC. Biopsy or excise.
"Treat as SCC": Because KA and SCC can be impossible to distinguish, most are excised completely to avoid missing an SCC.
"Multiple KAs = Syndrome": Multiple keratoacanthomas raise suspicion for Muir-Torre syndrome (sebaceous tumours + visceral malignancy).
Incidence
- Common (similar incidence to SCC in some series)
- More common in elderly
Demographics
- Peak age: 60-70 years
- M > F (2:1)
- More common in fair-skinned individuals
Risk Factors
| Factor | Notes |
|---|---|
| Sun exposure | Major risk factor |
| Fair skin | Fitzpatrick I-II |
| Immunosuppression | Organ transplant, HIV |
| Smoking | Associated |
| Tar exposure | Occupational |
| HPV | Possible role |
Syndromes
- Muir-Torre syndrome: Multiple KAs + sebaceous tumours + visceral malignancy (Lynch syndrome)
- Ferguson-Smith syndrome: Multiple self-healing KAs (autosomal dominant)
Origin
- Arises from hair follicle (follicular keratinisation)
- Considered by many to be a variant of well-differentiated SCC
- Debate continues about whether truly benign or low-grade malignant
Natural History
- Proliferative phase: Rapid growth (2-8 weeks)
- Mature phase: Stable (weeks)
- Involution phase: Spontaneous resolution over months
- Scar: Leaves depressed, pitted scar
Why the Controversy?
| Pro-Benign | Pro-SCC Variant |
|---|---|
| Spontaneous resolution | Histologically identical to SCC |
| Rarely metastasises | Rare cases metastasise |
| Follicular origin | Shares molecular features |
Appearance
| Feature | Description |
|---|---|
| Shape | Dome-shaped nodule |
| Central crater | Filled with keratin plug ("volcano") |
| Size | Usually 1-2 cm; can be larger |
| Surface | Smooth, often with crust |
| Colour | Skin-coloured or pink |
Distribution
History
Differential Diagnosis
| Condition | Distinguishing Features |
|---|---|
| SCC | Cannot reliably distinguish clinically |
| BCC (nodular) | Pearly, telangiectasia, no central keratin |
| Molluscum contagiosum | Multiple, umbilicated, viral |
| Nodular melanoma | Dark pigmentation, bleeds |
Inspection
- Well-defined dome with central keratin-filled crater
- Surrounding skin usually normal
- No ulceration (unless traumatised)
Palpation
- Firm
- Non-tender (usually)
- Mobile over deeper tissues
Dermoscopy
- White/yellow central keratin
- Blood vessels at periphery
- "White circles" (follicular pattern)
Clinical Diagnosis
- Often suspected clinically based on classic appearance
- BUT cannot rule out SCC without histology
Biopsy
| Type | Notes |
|---|---|
| Excision biopsy | Preferred - removes lesion entirely |
| Incisional/punch | May miss SCC; sampling error |
Histology
- Crateriform architecture
- Well-differentiated squamous epithelium
- Glassy eosinophilic cytoplasm
- Sharp demarcation at base
- Often indistinguishable from SCC
Standard Approach
┌──────────────────────────────────────────────────────────┐
│ KERATOACANTHOMA MANAGEMENT │
├──────────────────────────────────────────────────────────┤
│ │
│ STANDARD (MOST CASES): │
│ • Complete excision with margins │
│ • Send for histology │
│ • Treats as SCC to avoid missed diagnosis │
│ │
│ ALTERNATIVES (Selected cases): │
│ • Curettage and cautery (if diagnosis certain) │
│ • Intralesional methotrexate or 5-FU │
│ • Radiotherapy (if surgery not possible) │
│ │
│ OBSERVATION: │
│ • Only if patient declines surgery AND diagnosis very │
│ confident │
│ • Will leave pitted scar after spontaneous resolution │
│ │
│ MULTIPLE KAs: │
│ • Consider acitretin (systemic retinoid) │
│ • Investigate for Muir-Torre syndrome │
│ │
└──────────────────────────────────────────────────────────┘
Surgical Margins
- Standard: 4mm clinical margins
- Mohs micrographic surgery for high-risk sites (face)
Of KA
- Cosmetic (scar even if self-resolves)
- Misdiagnosis of SCC (if observed)
- Rare metastasis (debated)
Of Treatment
- Scarring
- Recurrence (rare if completely excised)
Natural History
- If untreated: Resolves in 4-6 months leaving pitted scar
- Low metastatic potential
With Treatment
- Excellent prognosis after complete excision
- Recurrence rare
Key Guidelines
- BAD: Patient Information Leaflet on KA
- NICE: Skin Cancer Guidelines (excise as SCC)
Key Evidence
KA vs SCC
- Studies show histological overlap
- Many pathologists prefer diagnosis "SCC, keratoacanthoma-type"
What is a Keratoacanthoma?
A keratoacanthoma is a fast-growing skin lump that looks like a small volcano with a crusty centre. It appears over a few weeks, usually on sun-exposed areas like the face or hands.
Is it Dangerous?
Keratoacanthomas are usually harmless and can disappear on their own over several months. However, they look very similar to a type of skin cancer called squamous cell carcinoma (SCC), so doctors usually remove them to be safe.
How is it Treated?
Most keratoacanthomas are surgically removed. This allows the doctor to check under the microscope that it isn't skin cancer and prevents a scar from forming if it were left to heal on its own.
Primary Guidelines
- British Association of Dermatologists. Patient Information: Keratoacanthoma.
Key Studies
- Savage JA, Maize JC. Keratoacanthoma clinical behaviour: a systematic review. Am J Dermatopathol. 2014;36(5):422-429. PMID: 24556899