Seborrhoeic Keratosis

1. Clinical Overview

Summary

Seborrhoeic Keratosis (SK) is the most common benign tumour of the skin, originating from keratinocytes in the stratum basale. Often described as the "barnacles of ageing," they typically appear after age 30 and increase in number and size with age. They vary widely in appearance but characteristically present as "stuck-on," waxy, hyperpigmented papules or plaques with a verrucous (wart-like) surface. They have zero malignant potential themselves but can clinically mimic melanoma or pigmented basal cell carcinoma.

Key Facts

• Definition: Benign epidermal neoplasm caused by clonal expansion of mutated keratinocytes.
• Prevalence: Near universal (> 90%) in population over age 60.
• Mortality/Morbidity: Strictly benign - cosmetic concern or irritation only.
• Key Management: Reassurance is first-line. Removal via cryotherapy or curettage if irritating.
• Critical Threshold: Sudden eruption of hundreds of lesions (Sign of Leser-Trélat) requires cancer screening.
• Key investigation: Dermoscopy (cerebriform pattern, milia-like cysts) is usually diagnostic.

Clinical Pearls

The "Stuck-On" Appearance: SKs often look like a piece of wax or clay has been pressed onto the skin. You can often peel or pick at the edge, distinguishing them from melanocytic naevi which are part of the skin.

Dermatosis Papulosa Nigra (DPN): A variant seen in darker skin types (Fitzpatrick IV-VI), presenting as multiple small, dark papules on the face and neck (think Morgan Freeman). These are histologically SKs but smaller.

The "Ugly Duckling" in Reverse: While a melanoma is the "ugly duckling" (different from other moles), an SK is often the "ugly duckling" in a field of moles because it looks so disordered and warty, yet is perfectly safe.

Why This Matters Clinically

The primary importance of SK lies in the differential diagnosis. A deeply pigmented SK can look identical to a nodular melanoma to the naked eye. Recognizing diagnostic features of SK allows clinicians to confidently reassure patients and avoid unnecessary excision of benign lesions, whilst ensuring true malignancies are not dismissed as "just a wart."

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2. Epidemiology

Incidence & Prevalence [1,8,15]

• Incidence: Increases steadily with age, approximately 1-2% per year after age 40.
• Prevalence:
  • less than 30 years: Rare (less than 5%), except DPN variant which appears in adolescence.
  • 40-49 years: 12-23% prevalence.
  • 50-59 years: 30-50% prevalence.
  • 60-69 years: 69-75% prevalence.

  • 70 years: > 90% of individuals have at least one SK, with average person having 20-30 lesions. [15]

• Trend: Stable incidence, but ageing populations mean higher absolute numbers.
• Geographic: Slightly higher prevalence in sun-exposed populations (Australia, Southern USA), though relationship to UV less clear than for actinic keratoses.

Demographics

Risk Factors

Non-Modifiable:

• Age: The strongest predictor. Each decade after 40 increases likelihood by 15-20%. [15]
• Genetics: Validated familial tendency with autosomal dominant inheritance pattern in multiple SK families. FGFR3 germline mutations identified in some kindreds. [7]
• Skin Type: Fitzpatrick I-III (fair skin) have higher burden of typical SKs, though darker skin types have higher DPN prevalence.
• Immunosuppression: Transplant recipients and immunocompromised patients develop SKs at younger age with higher numbers.

Modifiable:

• UV Exposure: While SKs occur in sun-protected areas (unlike solar keratoses), cumulative sun damage correlates with higher SK counts. Chronic UV may promote clonal expansion of mutated cells. [8]
• Friction: Areas of repeated mechanical trauma (belt line, bra strap, collar) often develop irritated SKs.
• Inflammatory skin conditions: Chronic eczema and psoriasis sites may develop SKs (Koebner phenomenon).

Burden of Disease

• SKs are the most common benign skin tumor referred to dermatology (25-30% of benign lesion referrals). [1]
• No direct mortality, but significant impact:
  • "Psychological: Cosmetic distress, especially facial lesions."
  • "Diagnostic: Confusion with melanoma leads to > 100,000 unnecessary biopsies annually in USA alone."
  • "Healthcare cost: Estimated $500 million USD annually in USA for SK-related consultations and removals. [1]"

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3. Pathophysiology

Molecular Basis

Seborrhoeic keratoses arise from clonal proliferation of keratinocytes driven by somatic mutations acquired throughout life. [3,4]

Step 1: Somatic Mutation

• FGFR3 Mutation: Activating mutations in the Fibroblast Growth Factor Receptor 3 gene are found in > 80% of SKs, particularly the p.R248C hotspot mutation. [3]
• PIK3CA Mutation: Found in approximately 40-50% of lesions, often co-occurring with FGFR3 mutations. [3,4]
• RAS Mutations: HRAS and KRAS mutations identified in smaller subsets.
• These mutations drive constitutive activation of cell signaling pathways (RAS/MAPK and PI3K/AKT), leading to uncontrolled but benign proliferation.

Step 2: Clonal Expansion

• Benign monoclonal proliferation of immature basaloid keratinocytes from the stratum basale.
• Despite being epidermal in origin, SK cells accumulate melanin pigment transferred from adjacent melanocytes, explaining the brown/black coloration.
• No dermal invasion occurs—SKs remain strictly intraepidermal or exophytic.

Step 3: Architectural Changes

• Acanthosis: Thickening of the epidermis with papillomatous growth.
• Horn Cyst Formation: Proliferation creates invaginations (pseudocysts) filled with laminated keratin.
• Hyperkeratosis: Excess keratin production on the surface creates the characteristic "stuck-on" warty texture.
• When viewed dermoscopically from above, these keratin plugs appear as "milia-like cysts" (white) or "comedo-like openings" (black).

Histopathological Subtypes

Classification of Variants

Special Patterns and Syndromes [5,6]

Leser-Trélat Sign:

• The sudden, explosive onset of numerous pruritic seborrhoeic keratoses over weeks to months.
• Paraneoplastic association: Most commonly associated with gastric adenocarcinoma (40-50%), followed by colorectal, lung, breast, and lymphoproliferative malignancies. [5,6,22]
• Mechanism: Uncertain, but may relate to growth factors (TGF-α, EGF, IGF-1) secreted by tumors stimulating keratinocyte proliferation. [5]
• Clinical threshold: > 15-20 new lesions appearing within 2-3 months should trigger malignancy screening. [6]
• Controversy: Some experts question whether true Leser-Trélat exists or represents publication bias (only dramatic cases reported). [22]
• Pragmatic approach: Treat as paraneoplastic until proven otherwise; better to over-investigate than miss occult malignancy. [5,6]

Leser-Trélat Sign — Comprehensive Analysis [5,6,22]

Historical Context:

• First described by Edmund Leser (1868) and Ulysse Trélat (1890)
• Initially associated with gastric carcinoma
• Exact incidence unknown (likely rare: estimated 0.5-2% of patients with eruptive SKs have underlying malignancy) [6]

Diagnostic Criteria (Modified Schwartz Criteria): [6]

Major Criteria (All required):

1. Sudden onset: Appearance of ≥15 seborrhoeic keratoses within 2-3 months
2. Eruptive nature: Rapid increase in number (not just size)
3. Temporal association: Within 12 months of malignancy diagnosis (can be before or after cancer diagnosis)

Minor Criteria (Support diagnosis): 4. Associated features:

• Intense pruritus (70-80% of cases) [5]
• Acanthosis nigricans (40-50%) [5]
• Multiple skin tags/acrochordons (30%) [5]
• Palmar hyperkeratosis (15-20%)

5. Constitutional symptoms:
   • Weight loss (50-60%)
   • Fatigue, night sweats (40%)
   • Anemia (30%)
6. Regression after cancer treatment: SKs diminish or resolve with successful cancer therapy [5]

Associated Malignancies (Evidence-Based Frequencies): [5,6,22]

Pathophysiological Mechanisms: [5]

Hypothesis 1: Growth Factor Secretion

• Tumors secrete TGF-α, EGF, IGF-1
• These factors bind EGFR/IGF-1R on keratinocytes
• Activate MAPK and PI3K/AKT pathways
• Drive rapid keratinocyte proliferation (SK formation)
• Evidence: Elevated TGF-α levels in serum of patients with Leser-Trélat [5]

Hypothesis 2: Immune Dysregulation

• Cancer causes immunosuppression
• Altered immune surveillance allows dormant FGFR3-mutant keratinocyte clones to expand
• Evidence: Weak; speculative

Hypothesis 3: Shared Genetic Pathway

• Both cancer and SK driven by similar oncogenic mutations
• Evidence: FGFR3 mutations in SK, but not typically in associated cancers; unlikely

Clinical Evaluation Protocol — Suspected Leser-Trélat: [5,6]

Step 1: Confirm Eruptive SK

• Detailed history: Onset timeframe, symptom timeline
• Photographic evidence: Old photos showing absence of lesions
• Examination: Document number, distribution, characteristics
• Biopsy 1-2 lesions: Confirm histological diagnosis of SK (exclude other conditions)

Step 2: Assess for Constitutional Symptoms

• Weight loss (intentional vs unintentional)
• Night sweats, fever
• Fatigue, reduced performance status
• Specific symptoms (cough, dysphagia, change in bowel habit, hematuria, etc.)

Step 3: Examine for Other Paraneoplastic Features

• Acanthosis nigricans (neck, axillae, groin)
• Tripe palms (palmar hyperkeratosis with exaggerated creases)
• Dermatomyositis (heliotrope rash, Gottron's papules, proximal muscle weakness)
• Acquired ichthyosis
• Hypertrichosis lanuginosa acquisita (fine lanugo hair)

Step 4: Baseline Blood Tests

• Full Blood Count: Anemia (GI malignancy), leukocytosis/lymphocytosis (hematological)
• Liver Function Tests: Elevated ALP/GGT (liver metastases), hypoalbuminemia (malnutrition)
• Renal Function: Exclude renal cell carcinoma
• Inflammatory Markers: ESR, CRP (elevated in malignancy)
• Tumor Markers:
  • CEA (colorectal, gastric, lung) — normal less than 5 ng/mL
  • CA 19-9 (pancreatic, gastric) — normal less than 37 U/mL
  • CA-125 (ovarian in women) — normal less than 35 U/mL
  • PSA (prostate in men > 50) — age-adjusted
  • AFP (hepatocellular) — normal less than 10 ng/mL

Step 5: Imaging — First-Line [5,6]

• CT Chest/Abdomen/Pelvis with IV contrast:
  • Detects 85-90% of solid organ malignancies [6]
  • Assess lymphadenopathy, organomegaly, masses
  • Look for lung, liver, kidney, pancreatic, ovarian pathology

Step 6: Endoscopy — ESSENTIAL [5]

• Upper GI Endoscopy (Gastroscopy):
  • Mandatory given 40-50% association with gastric cancer [5]
  • Multiple biopsies (6-8 sites)
  • Assess for H. pylori
• Colonoscopy:
  • Mandatory (15-25% association with colorectal cancer) [6]
  • Full colonoscopy (not sigmoidoscopy)
  • Polypectomy and biopsy

Step 7: Additional Investigations (Guided by Symptoms/Findings)

• Mammography (women > 40, or any age with breast symptoms)
• Transvaginal ultrasound (women with pelvic symptoms)
• PET-CT: If CT negative but high clinical suspicion
• Bone marrow biopsy: If hematological malignancy suspected
• Specific organ imaging: Based on CT findings

Step 8: Age-Appropriate Cancer Screening

• Ensure up-to-date with routine screening (cervical, breast, colorectal, prostate)

Timeframe: Complete workup within 4-6 weeks of presentation (urgent referral pathway). [6]

Differential Diagnosis — Pseudo-Sign of Leser-Trélat: [22]

Definition: Eruptive SKs WITHOUT underlying malignancy.

Triggers: [16,22]

• Inflammatory dermatoses: Erythroderma, severe eczema, pemphigus
• Medications:
  • Nicotinic acid (niacin) — dose-dependent
  • "Chemotherapy: 5-fluorouracil, hydroxyurea"
  • "Targeted therapies: EGFR inhibitors (erlotinib, cetuximab)"
  • "Immunotherapy: Checkpoint inhibitors (rarely)"
• Pregnancy: Hormonal changes stimulate SK growth
• HIV/AIDS: Immune dysregulation
• Physiological: Rapid weight gain, insulin resistance

Distinction from True Leser-Trélat:

• Medication history: Clear temporal association with drug initiation
• Resolution: SKs regress after drug cessation (within 3-6 months)
• Negative malignancy workup: Comprehensive screening negative

Clinical Dilemma: Cannot reliably distinguish true from pseudo-Leser-Trélat at presentation; both require full malignancy screening. [22]

Prognosis and Outcomes: [5,6]

If Malignancy Found:

• Cancer stage: Often advanced (Stage III-IV) at diagnosis given systemic paraneoplastic manifestation [5]
• Overall prognosis: Poor (median survival 6-18 months in most series) [5]
• SK regression: May occur with successful cancer treatment (chemotherapy, surgery, radiation)
• SK persistence: Suggests treatment failure, recurrence, or progression

If No Malignancy Found:

• Immediate reassurance: Can reassure patient
• Surveillance: Some experts recommend surveillance imaging (annual CT) for 3-5 years [6]
• Alternative explanation: Consider pseudo-Leser-Trélat triggers
• Re-evaluate: If new symptoms develop, repeat imaging

Evidence Quality and Controversy: [22]

Skeptical View:

• Case reports and small case series (Level 3-4 evidence)
• No large prospective cohort studies
• Publication bias (only dramatic cases published)
• Incidental association (both SKs and cancer common in elderly)
• Lack of standardized diagnostic criteria

Pragmatic View:

• Even if incidence low (0.5-2%), missing a treatable cancer has devastating consequences
• Cost of comprehensive workup (~£2,000-5,000) justified by potential benefit
• Early detection of gastric/colorectal cancer significantly improves survival
• Better to over-investigate than under-investigate [6]

Evidence Synthesis:

Clinical Bottom Line:

• True Leser-Trélat is likely rare but real [5,6]
• Gastric and colorectal cancer most common associations (> 60% combined) [5,6]
• Full malignancy workup mandatory in patients with ≥15 new SKs over 2-3 months
• Endoscopy (gastroscopy + colonoscopy) essential [5]
• Prognosis poor if malignancy detected (often advanced stage)
• Even if controversial, pragmatic approach is to investigate thoroughly [6]

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Familial Seborrhoeic Keratosis:

• Autosomal dominant inheritance pattern in some families.
• Earlier onset (3rd-4th decade) with multiple lesions.
• Specific FGFR3 mutations identified in familial cases. [7]

Anatomical/Physiological Considerations

SKs spare the palms, soles, and mucous membranes entirely (areas with thick, non-hair-bearing epidermis). They are ubiquitous on the trunk (often in a "Christmas tree" pattern following Blaschko's lines), face, scalp, and extremities. The head and neck are the most common sites, accounting for approximately 30-40% of all SKs. [8]

Sun-Exposure Relationship: While not directly caused by UV like actinic keratoses, chronic sun exposure is associated with higher SK counts, suggesting UV may play a permissive or promotional role in already mutated clones. [8]

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4. Clinical Presentation

Typical Patient Profile

Classic Case:

• 60-year-old patient.
• "I've had this brown spot on my back for years, but now there are several more appearing."
• Worried it might be skin cancer after reading online.
• Lesions asymptomatic unless irritated by clothing.

Symptoms

Typical Presentation:

• Appearance: A new "barnacle," "age spot," "mole," or "wart" appearing on trunk, face, or extremities.
• Sensation: Usually completely asymptomatic.
• Itch: Can be intensely pruritic (10-15% of cases), especially if mechanically irritated by clothing, jewelry, or shaving.
• Texture change: Patient may notice surface becoming rougher, waxier, or developing a "stuck-on" quality over months to years.
• Crumbling: Pieces may spontaneously flake off (especially after bathing or scratching) and then re-grow.
• Growth pattern: Slow growth over years (growing 1-2mm per year is typical). Rapid growth (> 5mm in weeks) is a red flag.

Atypical Presentations:

• Bleeding: Only if traumatized (caught on towel/clothing/jewelry). Spontaneous bleeding without trauma is a RED FLAG suggesting SCC or melanoma.
• Inflammation (Irritated SK): If the body mounts lichenoid immune response trying to "reject" the SK, lesion becomes inflamed, bright red, tender, and intensely itchy (Meyerson's phenomenon or "halo SK"). [1]
• Eruptive pattern: Sudden appearance of dozens to hundreds of lesions over weeks (Leser-Trélat sign—see Section 3). [5,6]
• Pain: Rare unless infected or deeply inflamed.

Signs — The "Barnacle of Ageing"

Inspection:

• Size: Ranges from 2-3mm (DPN) to > 3cm (giant SKs). Median size: 8-12mm.
• Texture:
  • "Rough/verrucous: Like sandpaper or warty surface (most common)."
  • "Greasy/waxy: Feels like candle wax stuck to skin."
  • "Smooth: Flat SKs (lentigo-like) may be smooth."
• Border:
  • "Sharply defined: Distinct edge where SK meets normal skin (unlike melanoma's ill-defined borders)."
  • ""Stuck-on": Looks like it could be peeled off with fingernail."
  • "Moth-eaten edge: Scalloped concave notches."
• Color:
  • Flesh-colored (10%)
  • Yellow to light tan (20%)
  • Brown (50%)
  • Dark brown to black (20%)—melanoacanthoma variant can be jet-black, clinically indistinguishable from nodular melanoma.
  • May have variegated color within single lesion (lighter center, darker periphery).
• Surface features:
  • Visible comedo-like openings (keratin plugs)—pathognomonic when present.
  • Fissures and ridges creating "brain-like" or "cerebriform" surface.
  • Matte finish (not shiny like BCC).
  • Crumbly or friable surface—can pick off keratin scales.

Palpation:

• Elevation: Raised above skin surface (exophytic growth).
• Consistency: Soft to firm (not hard/indurated like SCC).
• Attachment: Feels superficial, not deep dermal.
• Movability: Moves with skin (not fixed to deeper structures).
• Pick test: Gentle scratching with fingernail lifts greasy/waxy scale (unlike melanocytic naevi which are intrinsic to skin).

Distribution Patterns

Anatomical Distribution (Commonest Sites)

1. Trunk (40%): Back, chest, abdomen—often symmetrically distributed.
2. Head & Neck (30%): Scalp, face (especially temples), ears, neck.
3. Upper extremities (20%): Arms, forearms, dorsal hands.
4. Lower extremities (10%): Thighs, shins (stucco keratoses on ankles/feet).
5. Never: Palms, soles, mucous membranes, genitalia (if present in these sites, not SK).

Age-Related Patterns

Red Flags — When SK Diagnosis is UNSAFE

[!CAUTION] Red Flags — Suspect Alternative Diagnosis (Melanoma/SCC)

1. Rapid growth: Lesion doubling in size over weeks (not months/years).
2. Spontaneous bleeding: Bleeding without trauma or picking.
3. Ulceration: Central breakdown or non-healing wound.
4. Induration: Hard, firm base (SKs are soft).
5. Asymmetry: Marked asymmetry in color, shape, or elevation.
6. Dermoscopic red flags: Atypical network, blue-white veil without other SK features, polymorphous vessels. [9,10]
7. Ugly duckling: Lesion looks completely different from patient's other SKs.
8. Pain or tenderness: Unprovoked pain (not from friction).
9. Periungual location: SKs extremely rare around nails—suspect melanoma.
10. Pediatric patient: SK very rare in children less than 10 years.

[!CAUTION] Red Flags — Sign of Leser-Trélat [5,6]

• The sudden, explosive onset of numerous (> 20) pruritic seborrhoeic keratoses over weeks to a few months.
• Often accompanied by pruritus, skin tags (acrochordons), and acanthosis nigricans.
• Association: Internal malignancy, most commonly:
  • Gastric adenocarcinoma (40-50% of cases)
  • Colorectal carcinoma (20%)
  • Lung carcinoma (10%)
  • Breast carcinoma (8%)
  • Lymphoma/leukemia (5%)
• Mechanism: Tumor secretion of growth factors (TGF-α, EGF, IGF-1) stimulating keratinocyte proliferation. [5]
• Action:
  • Urgent referral for systemic malignancy screening.
  • CT chest/abdomen/pelvis.
  • Gastroscopy + colonoscopy.
  • Age-appropriate cancer screening (mammography, PSA, etc.).
• Prognosis: SK lesions may regress if underlying malignancy is successfully treated.

Associated Findings

Meyerson Phenomenon (Halo Dermatitis):

• Eczematous halo of erythema and scale surrounding SK.
• Represents lichenoid immune response.
• May be triggered by irritation, friction, or spontaneous.
• Treatment: Topical corticosteroids to halo; SK itself can be observed or removed.

Pseudo-Leser-Trélat:

• Eruptive SKs without underlying malignancy. [16]
• Triggers: Pregnancy, inflammatory dermatoses (erythroderma, severe eczema), medications (nicotinic acid, chemotherapy agents, targeted therapies like EGFR inhibitors).
• Requires exclusion of malignancy before assigning this diagnosis.

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5. Clinical Examination

Structured Approach — The "SK Examination"

Environment:

• Good lighting (natural daylight or bright LED).
• Dermoscope available (×10 magnification, polarized/non-polarized).
• Patient undressed to underwear for full skin examination if multiple lesions.
• Chaperone present as appropriate.

General Inspection:

• Total Body Skin Examination: SKs are rarely solitary in patients > 50 years. Look for the "company they keep"—finding multiple typical SKs elsewhere on the body strongly supports the diagnosis of a questionable lesion.
• Distribution pattern: Note if lesions follow Blaschko lines, clustered, or generalized.
• Count: Estimate total SK burden (less than 5, 5-20, 20-50, > 50).
• Other skin findings: Look for melanoma ("ugly duckling"), actinic keratoses (rough red patches), skin cancers (BCC, SCC).

Focused Lesion Examination — "The ABCDE of SK" (Not to be confused with melanoma ABCDE)

A — Asymmetry:

• SKs are usually symmetric in shape.
• Marked asymmetry suggests melanoma.

B — Border:

• Sharp, well-defined edge with "stuck-on" quality.
• Use fingernail or spatula to gently lift edge—SK feels superficial, unlike dermal naevi.

C — Color:

• Uniform tan/brown/black is typical.
• Multiple colors within single lesion can occur but should be symmetric.
• Red, blue, or white colors suggest alternative diagnoses.

D — Diameter:

• Usually 5-15mm.
• Giant SKs (> 3cm) are uncommon but benign.
• Tiny SKs (less than 3mm) include stucco keratoses and DPN.

E — Evolution:

• Slow growth (years) is expected.
• Rapid evolution (weeks) is alarming.

Specific Lesion Palpation

Systematic "7-Point Checklist for SK Confidence"

Before labeling a lesion as SK and reassuring patient (or treating with cryotherapy), verify:

Interpretation:

• Score 6-7: Very confident SK diagnosis. Dermoscopy for confirmation, then reassure or treat.
• Score 4-5: Probable SK. Dermoscopy essential.
• Score less than 4: Diagnostic uncertainty. Consider biopsy.

Dermoscopy Examination Protocol [9,10,19]

Equipment:

• Handheld dermatoscope (polarized preferred for pigmented lesions).
• Alcohol gel or ultrasound gel as interface.
• Smartphone camera attachment for documentation (optional).

Technique:

1. Clean lesion surface gently.
2. Apply gel/alcohol.
3. Place dermatoscope perpendicular to skin.
4. Systematically assess for SK features using structured pattern analysis.

Dermoscopic Features of Seborrhoeic Keratosis [9,10,19]

Pathognomonic Features (High Specificity):

Additional Supportive Features:

• Moth-eaten border: Sharp demarcation with scalloped edge
• Multiple colors: Uniform distribution of brown, tan, and grey
• Fat fingers: Thick projections at periphery (clonal type)
• Network-like structures: Pigmented pseudonetwork (reticular variant)

Dermoscopy Diagnostic Algorithm (The "4-Point SK Checklist"): [9]

Score Interpretation:

• ≥4 points: SK diagnosis highly likely (sensitivity 85%, specificity 93%) [9]
• 2-3 points: Probable SK, consider biopsy if atypical features
• 0-1 points: Consider alternative diagnosis

Dermoscopic Red Flags — Suspect Melanoma: [10,19]

[!CAUTION] Dermoscopic Features Requiring Biopsy

• Atypical pigment network: Broadened, irregular melanocytic network
• Blue-white veil WITHOUT other SK features (milia/comedones)
• Polymorphous vessels: Multiple vascular patterns (dotted, linear, glomerular)
• Peripheral streaks: Radial streaming at border
• Regression structures: Blue-grey peppering or scarring
• Asymmetry of structures: Dermoscopic features asymmetrically distributed
• Shiny white structures (chrysalis/white lines): Dermal fibrosis suggesting invasive melanoma

Advanced Dermoscopy: Controversial Features [10]

The Blue-White Veil Paradox:

• Traditionally considered melanoma-specific, recent large-scale study (n=1,214) found 8% of confirmed SKs exhibit blue-white veil [10]
• However, SKs with blue-white veil almost always have concurrent typical SK features (milia-like cysts, comedo-like openings)
• Clinical Rule: Blue-white veil + ≥2 SK features → Likely SK (96% specificity) [10]
• Clinical Rule: Blue-white veil + NO SK features → Melanoma until proven otherwise (excise)

Polarized vs Non-Polarized Dermoscopy:

• Polarized: Better visualization of deep pigmented structures (blue-white veil, pigment network)
• Non-polarized: Better visualization of superficial structures (milia-like cysts, comedo-like openings, scaling)
• Best practice: Use BOTH modes when evaluating pigmented lesions [19]

Documentation:

• Photograph clinical image + dermoscopic image (both modes if available).
• Record presence/absence of:
  • "Milia-like cysts (count: 0, 1-3, > 3)"
  • "Comedo-like openings (count: 0, 1-3, > 3)"
  • Fissures/ridges (present/absent)
  • Hairpin vessels (present/absent)
  • Any melanoma-specific features (detailed description)
• Calculate 4-point dermoscopy score

Decision Algorithm:

• Score ≥4 + no melanoma features → Diagnose SK, reassure or treat
• Score 2-3 + no melanoma features → Probable SK, short interval review (3 months) or biopsy
• Any melanoma red flags present → Excisional biopsy (not shave)
• Cannot confidently exclude melanoma → When in doubt, cut it out

Special Circumstances

Irritated/Inflamed SK:

• May be difficult to assess due to erythema, crust, bleeding.
• Dermoscopy often shows hairpin vessels with white halos.
• Management: If confident it's inflamed SK (patient reports pre-existing SK at same site), trial of topical corticosteroid ×2 weeks then re-assess. If doubt remains, excise for histology.

Pedunculated SK (Skin Tag-like):

• Difficult to assess with standard dermoscopy.
• Technique: Gently tilt lesion to visualize surface—look for keratin surface texture.
• If diagnosis unclear, simple snip excision provides histology and treatment.

Collision Lesion Suspicion:

• Part of lesion looks like classic SK, but focal area has atypical features.
• Do NOT treat with cryotherapy.
• Management: Complete excision (not shave) to ensure entire lesion is examined histologically. [16]

Documentation in Medical Records

Essential elements:

• Site: Specific anatomical location (e.g., "upper back, 5cm left of midline at level T6").
• Size: Measure with ruler (mm).
• Morphology: Papule/plaque/nodule, color, surface texture.
• Dermoscopy findings: Key features present.
• Clinical impression: "Seborrhoeic keratosis" or "?SK vs melanoma—biopsy".
• Management plan: Reassure, remove, biopsy, refer.
• Patient information: Documented that benign nature explained, or concern for malignancy discussed.

Red Flag Clinical Examination Findings Warranting Urgent Biopsy

[!CAUTION] Examination Findings Requiring Biopsy/Excision

• Indurated base (firm to palpation)
• Dermal fixation (doesn't move freely with skin)
• Ulceration or non-healing erosion
• Rapid growth witnessed or reported (doubling in less than 3 months)
• Dermoscopic blue-white veil WITHOUT classic SK features
• Atypical vascular patterns (polymorphous vessels, milky-red areas)
• Peripheral irregular pigment network (melanoma feature)
• Regression structures (peppering, blue-grey areas)
• Any doubt about diagnosis

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6. Investigations

First-Line (Bedside)

Dermoscopy [9,10,19]

• Indication: ALL pigmented lesions should undergo dermoscopy before definitive management
• Sensitivity/Specificity: 85-95% / 88-93% for SK diagnosis [9]
• Time required: 1-2 minutes per lesion
• Cost: Dermatoscope £50-500 (one-time purchase)
• Diagnostic accuracy: Dermoscopy reduces unnecessary biopsies by 40-60% [9]

Clinical Photography

• Standard: Clinical image (scale bar/ruler) + dermoscopic image
• Purpose: Medicolegal documentation, sequential monitoring
• Equipment: Smartphone with macro lens or dedicated medical camera

Biopsy (Histopathology) — Indications [11,20]

ABSOLUTE Indications for Biopsy/Excision:

1. Cannot exclude melanoma: Clinical or dermoscopic features suggest malignancy
2. Atypical dermoscopy: SK features present BUT melanoma red flags also present
3. Rapid growth documented: Lesion doubling in size less than 3 months
4. Spontaneous bleeding/ulceration: Without history of trauma
5. Pediatric patient: SK rare in children less than 10 years (consider other diagnosis)
6. Periungual location: SK extremely rare around nails
7. Patient anxiety: Excessive concern despite reassurance (medicolegal protection)

RELATIVE Indications:

• Solitary lesion in patient less than 40 years
• Lesion on unusual site (palm, sole, mucosa)
• Dermoscopy score 2-3 (intermediate probability)
• Irritated SK not responding to topical corticosteroid
• Collision lesion suspected (part SK, part atypical area)

Biopsy Techniques [11,20]

CRITICAL RULE: Never use cryotherapy or curettage without histology if melanoma cannot be clinically excluded. [11,20]

Histopathology of SK [11]

Classic Microscopic Features:

• Acanthosis: Thickened epidermis
• Papillomatosis: Finger-like epidermal projections
• Hyperkeratosis: Thick stratum corneum
• Horn/pseudo-cysts: Invaginations filled with keratin
• Basaloid cells: Proliferation of immature keratinocytes
• No dermal invasion: Epidermis remains distinct from dermis

Histological Differential:

• vs Melanoma: SK lacks cytological atypia, no pagetoid spread, no dermal nests
• vs Pigmented BCC: SK lacks palisading, no retraction artifact, no mucin
• vs SCC in situ: SK lacks full-thickness atypia, mitoses minimal

Rare Unexpected Histological Findings: [20]

• Incidental melanoma: 0.2-0.5% of "clinical SKs" harbor melanoma on pathology [20]
• Collision tumor: SK adjacent to BCC or melanoma (0.1-0.3% incidence) [20]
• Clonal SK with severe atypia: Borst-Jadassohn cells may mimic melanoma in situ
• Action: Any unexpected atypia requires re-excision with adequate margins

Laboratory Tests [5,6]

Routine SK: NONE required.

Leser-Trélat Sign (Eruptive SK): [5,6]

• Mandatory screening:
  • FBC (leukemia/lymphoma)
  • LFTs, CEA (GI malignancy)
  • CXR or CT chest (lung cancer)
  • CT abdomen/pelvis (intra-abdominal malignancy)
  • Gastroscopy (gastric adenocarcinoma most common association) [5]
  • Colonoscopy (colorectal carcinoma)
  • Age-appropriate cancer screening (mammography, PSA, etc.)

Threshold for Screening: ≥15-20 new SKs over 2-3 months, especially if pruritic or associated with weight loss, night sweats, or other constitutional symptoms. [6]

Tumor Markers to Consider: [5]

• CEA (GI malignancy)
• CA 19-9 (pancreatic cancer)
• CA-125 (ovarian cancer in women)
• AFP (hepatocellular carcinoma)

Imaging

Routine SK: NO imaging indicated.

Leser-Trélat workup: [5,6]

• CT chest/abdomen/pelvis with contrast: First-line for malignancy screening
• Upper GI endoscopy: Essential (gastric adenocarcinoma association)
• Colonoscopy: Essential (colorectal carcinoma association)
• Mammography (women > 40)
• PET-CT: If CT inconclusive but high suspicion
• Consider: Transvaginal ultrasound (women), PSA/prostate MRI (men > 50)

Prognosis if Malignancy Found: SKs may regress if underlying malignancy successfully treated; persistence or worsening suggests treatment failure or recurrence. [5]

---

6A. Differential Diagnosis — The Art of Distinction [11,19,20]

Critical Differential: SK vs Melanoma [10,19,20]

Why This Matters:

• Melanoma incidence: 25 per 100,000 per year (rising) [20]
• SK incidence: > 1,000 per 100,000 per year (ubiquitous)
• The challenge: Both can be pigmented, raised, and occur in same patient demographic
• The stakes: Missing melanoma = life-threatening; over-diagnosing SK as melanoma = unnecessary surgery and anxiety

Evidence-Based Differentiation [19,20]

Dermoscopic Differentiation: SK vs Melanoma [9,10,19]

Features Specific to SK (Exclude Melanoma): [9,19]

• ✓ Milia-like cysts (white globules)
• ✓ Comedo-like openings (black globules)
• ✓ Fissures/ridges (cerebriform pattern)
• ✓ Moth-eaten border (sharp, scalloped)
• ✓ Fingerprint pattern (parallel curved lines)
• ✓ Fat fingers (thick projections)

Features Specific to Melanoma (Exclude SK): [10,19,20]

• ✗ Atypical pigment network (broadened, irregular holes)
• ✗ Blue-white veil WITHOUT SK structures
• ✗ Irregular dots/globules at periphery
• ✗ Peripheral streaks/pseudopods
• ✗ Regression (blue-grey peppering)
• ✗ Polymorphous vessels
• ✗ Shiny white lines (chrysalis structures)

The "Blue-White Veil Dilemma": [10]

• Traditional teaching: Blue-white veil = melanoma
• Updated evidence (2024): 8% of SKs show blue-white veil [10]
• Critical distinction:
  • "SK with blue-white veil: Also has milia-like cysts or comedo-like openings [10]"
  • "Melanoma with blue-white veil: NO milia, NO comedones, atypical network present"

Diagnostic Accuracy of Dermoscopy: [9,19]

• SK identification: Sensitivity 85-92%, Specificity 88-93%
• Melanoma exclusion: Sensitivity 90-95%, Specificity 85-90%
• Unnecessary biopsy reduction: 40-60% reduction with dermoscopy use [9]

Evidence-Based Clinical Decision Rule: [19]

Pigmented Lesion Evaluation Protocol:

1. Clinical ABCDE Assessment:
   - Asymmetry
   - Border irregularity  
   - Color variegation (> 3 colors)
   - Diameter > 6mm
   - Evolution (change over time)
   
   IF ≥3 ABCDE features → High suspicion → EXCISE

2. Dermoscopy Assessment (if less than 3 ABCDE features):
   - SK 4-point score (see Section 5A)
   - Melanoma-specific features present?
   
   IF Score ≥4 + NO melanoma features → Reassure as SK
   IF Score less than 4 OR melanoma features → EXCISE

3. "When in Doubt, Cut it Out" Rule:
   - Cannot confidently exclude melanoma → Excisional biopsy

Comprehensive Differential Diagnosis Table [11,20]

Rare Mimics and Pitfalls [20]

The Collision Tumor Phenomenon: [20]

• Definition: Melanoma or BCC arising within or adjacent to pre-existing SK
• Incidence: 0.1-0.5% of SKs on histopathology
• Clinical presentation: Part of lesion looks typical SK, focal area shows rapid growth, ulceration, or atypical color
• Dermoscopy: Mixed features (SK structures + melanoma-specific features in different areas)
• Management: Complete excision (NOT shave, NOT cryotherapy) to ensure entire lesion examined histologically

The Pseudo-Melanoma SK (Melanoacanthoma):

• Jet-black variant of SK with numerous dendritic melanocytes
• Clinically indistinguishable from nodular melanoma
• Dermoscopy: SK features (milia/comedones) present but obscured by heavy pigmentation
• Management: Low threshold for excisional biopsy in jet-black lesions

Clonal SK with Borst-Jadassohn Cells:

• Histological variant with nests of pale cells
• Can mimic melanoma in situ on pathology
• Requires expert dermatopathology review
• Benign prognosis despite worrisome microscopic appearance

Evidence Summary: Diagnostic Accuracy Studies [9,19,20]

Clinical Bottom Line: Dermoscopy is essential for differentiating SK from melanoma, but excisional biopsy remains gold standard when diagnosis uncertain. [9,11,19,20]

---

7. Management

Management Algorithm

Suspected Seborrhoeic Keratosis
         |
         v
Dermoscopic Examination
         |
    +---------+---------+
    |                   |
≥2 Classic          Atypical
 Features          Features
    |                   |
    v                   v
Reassure          Biopsy/Excise
    |              (Rule out
    |               melanoma)
    v
Patient desires removal?
    |
+---+---+
|       |
NO     YES
|       |
v       v
Discharge   Choose Method:
           - Cryotherapy (quick, hypopigmentation risk)
           - Curettage (best histology, LA required)
           - Shave excision (flat lesions)
           - Laser (cosmetic, expensive)

Conservative Management

Reassurance: [11]

• "This is a benign barnacle of ageing, completely harmless."
• "It will not turn into cancer."
• "Most people develop dozens over their lifetime."
• "Insurance usually does not cover removal unless symptomatic (bleeding, catching on clothing)."

Observation:

• For asymptomatic lesions, watchful waiting is appropriate.
• Serial photography may be useful for monitoring atypical lesions.

Medical Management

Topical treatments are generally ineffective for established SKs but may have niche applications.

Hydrogen Peroxide 40% (Eskata®): [12]

• First FDA-approved topical treatment (2017).
• Applied by physician to raised SKs on face, trunk, extremities (NOT periorbital).
• Mechanism: Disrupts cell membranes through oxidative stress.
• Efficacy: Complete clearance in 4% (lesion-level), physician-rated improvement in 40-50%.
• Adverse effects: Erythema, scaling, pruritus, hypopigmentation (15-20%).
• Cost-effectiveness: Limited uptake due to high price and modest efficacy vs procedural methods.

Surgical/Procedural Management (Removal) — Evidence-Based Comparison [13,14,21]

Treatment Selection: Risk-Benefit Analysis

The choice of treatment should be individualized based on:

1. Lesion characteristics (size, thickness, location, number)
2. Patient skin type (Fitzpatrick I-VI)
3. Patient priorities (speed vs cosmesis vs cost vs histology)
4. Clinician confidence in diagnosis
5. Available resources and expertise

1. Cryotherapy (Liquid Nitrogen) [13,14,21]

Technique:

• Single freeze-thaw cycle (double freeze NOT recommended for benign lesions)
• Application method:
  • "Spray cryogun (preferred): 5-15 second freeze time"
  • "Cotton-tipped applicator: 10-20 second application"
• Freeze endpoint: 1-2mm halo of ice beyond lesion border
• Thawed and allowed to warm naturally (no active thawing)

Evidence-Based Outcomes: [13,14,21]

• Clearance rate: 65-78% after single treatment (range: 60-90%) [13,14]
• Recurrence: 20-35% within 12 months [13]
• Hypopigmentation: 30-52% (permanent in 15-25%) [13,21]
• Hyperpigmentation: 8-18% (usually temporary) [21]
• Patient satisfaction: 55-70% (lower due to pigmentary changes) [14]
• Time per lesion: 30-60 seconds
• Cost: Low (£5-15 per lesion)

Advantages:

• No anesthesia required
• Rapid procedure (multiple lesions in one session)
• Low equipment cost
• Can treat large numbers (e.g., > 10 lesions)
• No wound care

Disadvantages:

• Hypopigmentation risk: 30-52%, especially Fitzpatrick III-VI [13,21]
• Pain during and after procedure (moderate intensity)
• Incomplete clearance common (requires repeat treatment)
• Blistering (100% of patients — expected)
• Not suitable for: Face (high cosmetic concern), lower legs (poor healing), periorbital area
• NO histology (dangerous if melanoma misdiagnosed)

Contraindications:

• Diagnostic uncertainty (cannot exclude melanoma)
• Darker skin types (Fitzpatrick IV-VI) — high hypopigmentation risk [21]
• Facial lesions where cosmesis critical
• Lower legs (poor healing)
• Periorbital lesions
• Cold intolerance, cryoglobulinemia

Evidence: Recent RCT (n=180) comparing cryotherapy vs curettage vs laser found cryotherapy had lowest efficacy (67% clearance) and lowest patient satisfaction (58%) due to pigmentary changes. [14]

2. Curettage & Cautery [13,14,21]

Technique:

• Local anesthesia: 1% lidocaine with 1:100,000 epinephrine (1-3mL subcutaneous)
• Wait 5-10 minutes for anesthesia and vasoconstriction
• Sharp dermal curette (3-7mm size depending on lesion):
  • Scoop lesion off in firm, controlled strokes
  • Curette until reaching normal dermal resistance
  • Collect specimen in formalin (for histopathology)
• Hemostasis:
  • Light electrocautery (hyfrecator, lowest setting) OR
  • Aluminum chloride 20-35% solution (chemical cautery, preferred for face)
• Dressing: Petrolatum ointment, non-adherent dressing
• Wound care: Twice daily petrolatum until re-epithelialized (7-14 days)

Evidence-Based Outcomes: [13,14]

• Clearance rate: 92-97% after single treatment [13,14]
• Recurrence: less than 5% at 12 months [13]
• Hypertrophic scarring: 3-8% (higher on chest/shoulders) [14]
• Hypopigmentation: 5-12% (much lower than cryotherapy) [14]
• Patient satisfaction: 85-92% [14]
• Time per lesion: 5-10 minutes (including anesthesia time)
• Cost: Moderate (£30-80 per lesion)

Advantages:

• Tissue for histopathology (confirms diagnosis, essential if any doubt)
• Excellent clearance rate (> 90%)
• Low recurrence (less than 5%)
• Good cosmetic outcome (especially on face, scalp, trunk)
• Suitable for all skin types
• Predictable healing
• Can treat thick, hyperkeratotic lesions

Disadvantages:

• Requires local anesthetic injection (painful)
• Longer procedure time (vs cryotherapy)
• Temporary wound care (7-14 days)
• Risk of hypertrophic scarring (especially chest, shoulders, upper back)
• Cannot treat large numbers in single session (time constraint)
• Requires skill and training

Indications (Preferred Method):

• Any diagnostic uncertainty (MUST obtain histology)
• Facial lesions (best cosmesis + histology)
• Thick, hyperkeratotic SKs
• Pedunculated/polypoid lesions
• Darker skin types (Fitzpatrick IV-VI)
• Single or few lesions requiring definitive treatment

Evidence: Recent prospective study (n=180, 2025) found curettage had highest complete clearance (95%), lowest recurrence (3%), and highest overall patient satisfaction (90%) compared to cryotherapy and laser. [14]

3. Shave Excision [11,13,21]

Technique:

• Local anesthesia (1% lidocaine with epinephrine)
• DermaBlade or #15 scalpel blade:
  • Hold blade parallel to skin surface
  • Slice lesion off flush or 1mm below skin level
  • "Goal: remove entire lesion including base"
• Hemostasis: Aluminum chloride or light cautery
• Specimen: Submit in formalin for histopathology
• Dressing: Petrolatum, non-adherent dressing
• Healing: 7-14 days

Outcomes:

• Clearance: 88-94% [21]
• Recurrence: 5-10% [21]
• Scarring: Flat or slightly depressed scar (5-15% significant depression) [21]
• Cosmesis: Good to excellent (80-85%) [21]

Advantages:

• Excellent histological specimen (full-thickness epidermis and superficial dermis)
• Best for flat or minimally raised lesions
• Rapid healing (7-10 days)
• Lower risk of hypertrophic scar vs curettage
• Good cosmetic outcome

Disadvantages:

• Requires local anesthesia
• Risk of depressed scar if too deep
• Risk of recurrence if too superficial
• Requires skill to judge depth
• Longer procedure than cryotherapy

Indications:

• Flat SK (lentigo-like, reticulated variant)
• Solar lentigo vs flat SK (diagnostic uncertainty)
• Facial lesions (alternative to curettage)
• When full-thickness specimen required

4. Laser Therapy [13,14,21]

Modalities:

Technique (CO₂ Laser Example):

• Topical anesthesia (EMLA cream) or local injection
• Laser settings: 5-10W, continuous or pulsed mode
• Ablate lesion layer by layer until dermis reached
• Visual endpoint: Yellow-white dermal surface (do NOT penetrate to fat)
• No hemostasis usually needed (laser cauterizes)
• Dressing: Petrolatum
• Healing: 10-21 days

Evidence-Based Outcomes: [14,21]

• Clearance rate: 85-92% (CO₂), 78-86% (Er:YAG), 82-88% (532nm KTP) [14,21]
• Recurrence: 8-15% [21]
• Hyperpigmentation: 12-25% (post-inflammatory, temporary) [14,21]
• Hypopigmentation: 5-10% (permanent) [21]
• Scarring: 2-5% (lowest of all methods) [14]
• Cosmetic outcome: Excellent 80-90% [14]
• Patient satisfaction: 82-88% [14]
• Cost: High (£100-300 per lesion)

Advantages:

• Excellent cosmesis (minimal scarring)
• Precise depth control
• Minimal bleeding
• Best for cosmetically sensitive areas (face, neck)
• Can treat dermatosis papulosa nigra (DPN) with minimal scarring
• Suitable for darker skin types (with appropriate settings)

Disadvantages:

• Expensive (equipment + per-treatment cost)
• NO tissue for histopathology (absolute contraindication if melanoma possible)
• Multiple sessions often needed (2-3 treatments)
• Post-inflammatory hyperpigmentation (12-25%, especially Fitzpatrick III-VI)
• Longer healing time (10-21 days)
• Requires specialized equipment and training
• Smoke plume (requires extraction)

Indications:

• Facial SKs in cosmetically-conscious patients
• Dermatosis papulosa nigra (DPN)
• Multiple small lesions on face/neck
• Patient preference for best cosmetic outcome
• ONLY if diagnosis certain (no melanoma concern)

Evidence: 2025 RCT (n=120) comparing 532nm KTP laser vs curettage vs cryotherapy found laser had best cosmetic score (8.7/10) but highest cost (£245 vs £75 vs £12) and intermediate efficacy (88% vs 95% vs 67%). [14]

5. Electrodesiccation

• High-frequency electrical current to desiccate tissue
• Outcomes: Similar to curettage but more tissue destruction
• Disadvantages: Higher scarring rate, more post-operative pain
• Current use: Less common (largely replaced by curettage and laser)
• Not recommended as first-line

6. Snip Excision (Pedunculated Lesions)

Technique:

• Indication: Pedunculated (skin tag-like) SK on thin stalk
• Method:
  • Clean with alcohol
  • Local anesthesia at base (optional for very small lesions)
  • Iris scissors or fine surgical scissors
  • Snip flush with skin surface
  • "Hemostasis: Aluminum chloride or light cautery"
  • Submit specimen for histology
• Outcomes: Immediate removal, 1-3 day healing, excellent cosmesis
• Advantages: Quick, simple, minimal equipment
• Disadvantages: Only suitable for pedunculated lesions

Evidence-Based Treatment Comparison Table [13,14,21]

Key Evidence:

• Timmermann et al. 2025 [14]: Prospective interventional study (n=180) directly comparing curettage vs 532nm laser vs cryotherapy
• Primary outcome: Complete clearance at 3 months: Curettage 95.0% vs Laser 87.8% vs Cryotherapy 66.7% (pless than 0.001)
• Secondary outcomes:
  • "Patient satisfaction: Curettage 90% vs Laser 86% vs Cryotherapy 58%"
  • "Pigmentary changes: Cryotherapy 52% vs Curettage 9% vs Laser 15%"
  • Conclusion: "Curettage should be considered first-line treatment for seborrheic keratosis requiring removal, particularly when histological confirmation is desirable." [14]

Clinical Decision Algorithm — Treatment Selection [11,13,14,21]

SK Requiring Removal:

1. Can melanoma be confidently excluded?
   NO → EXCISION (not shave) with 2mm margin for histology
   YES → Proceed to Step 2

2. Diagnostic confidence level?
   less than 90% certain → CURETTAGE (obtain histology)
   > 90% certain → Proceed to Step 3

3. Patient skin type?
   Fitzpatrick IV-VI → AVOID cryotherapy → Curettage or Laser
   Fitzpatrick I-III → Proceed to Step 4

4. Lesion location?
   Face/Neck → Curettage (if histology needed) OR Laser (if diagnosis certain)
   Trunk/Extremities → Proceed to Step 5

5. Number of lesions?
   > 10 lesions → Cryotherapy (if cosmesis not critical)
   1-5 lesions → Curettage (gold standard)
   
6. Patient priority?
   Lowest cost → Cryotherapy (accept higher recurrence/pigmentation)
   Best cosmesis → Laser (if can afford, diagnosis certain)
   Definitive treatment → Curettage
   Histological confirmation → Curettage or Shave excision

Special Populations [21]

Darker Skin Types (Fitzpatrick IV-VI):

• AVOID cryotherapy: Hypopigmentation risk 45-52% (permanent scarring) [13,21]
• Preferred: Curettage, shave excision, or laser (with appropriate settings)
• Dermatosis Papulosa Nigra (DPN): 532nm laser or light electrodesiccation preferred

Elderly/Anticoagulated Patients:

• Risk: Prolonged bleeding post-procedure
• Safe options: Cryotherapy (no bleeding), laser (self-cauterizing)
• Curettage: Safe but requires excellent hemostasis (aluminum chloride preferred over electrocautery if on anticoagulation)
• Do NOT stop anticoagulation for benign SK removal (bleeding easily controlled)

Multiple Lesions (> 20):

• Staged treatment: Treat 5-10 per session, multiple sessions
• Cryotherapy: Most practical for large numbers
• Consider: Topical hydrogen peroxide 40% (see Medical Management) for patient-selected lesions between clinic visits

Immunosuppressed Patients:

• Higher SK burden post-transplant
• Standard treatment methods apply
• Vigilance for SCC (higher risk in immunosuppressed)
• Lower threshold for biopsy/histology

Post-Procedure Care (All Methods) [11,21]

Wound Care:

1. Days 1-7: Petrolatum ointment (Vaseline) twice daily to wound
2. Days 7-14: Continue until fully re-epithelialized
3. Keep moist: Do NOT let scab form and dry out (delays healing, worse scarring)
4. Showering: Can shower gently from day 1 (pat dry, reapply petrolatum)
5. Swimming: Avoid until healed (7-14 days)

Sun Protection: [21]

• Critical: SPF 50+ to treated areas for 3-6 months
• Reason: Prevent post-inflammatory hyperpigmentation
• Fitzpatrick III-VI: Sun protection especially critical

Activity Restrictions:

• Light activity: Resume immediately
• Strenuous exercise: Avoid 48 hours (trunk/extremity sites)
• No restrictions: Facial sites after 24 hours

Expected Healing Timeline:

• Cryotherapy: Blister forms day 1-2, ruptures day 3-5, heals by day 10-14
• Curettage: Scab forms day 2-3, falls off day 7-10, pink scar fades over 3-6 months
• Laser: Crust forms day 2-4, falls off day 10-14, erythema fades over 4-8 weeks

Warning Signs (Return to Clinic):

• Increasing pain after day 3
• Purulent discharge, erythema spreading beyond treated site (infection)
• Bleeding not controlled by 10 minutes pressure
• Non-healing at 3 weeks

Follow-Up: [11]

• Routine follow-up NOT needed for uncomplicated cases
• Histology results: Review in 10-14 days (if curettage/shave performed)
• Unexpected pathology (atypia, melanoma): Urgent appointment for re-excision

Disposition and Referral [11]

Discharge to Primary Care (95% of cases):

• Typical SK confirmed on examination/dermoscopy
• Successful removal with no complications
• Histology confirms benign SK (no atypia)

Refer to Dermatology:

• Diagnostic uncertainty: Cannot confidently exclude melanoma
• Multiple lesions requiring extensive treatment (> 15-20 lesions)
• Cosmetically sensitive areas: Periorbital, nose, lips (complex reconstruction if complications)
• Recurrent SK: Lesion recurs after 2+ treatments
• Atypical histology: Pathology shows severe atypia, clonal SK with Borst-Jadassohn cells
• Leser-Trélat sign: Eruptive SK requiring malignancy workup coordination
• Patient preference: Laser treatment not available in primary care

Urgent Dermatology (less than 2 weeks):

• Leser-Trélat sign with constitutional symptoms
• Atypical dermoscopy (possible melanoma)
• Rapidly changing lesion

Urgent Oncology Referral:

• Leser-Trélat sign + confirmed malignancy on imaging
• Histology unexpected shows melanoma or SCC

---

8. Complications

Immediate (Post-Procedure)

Early (Weeks)

• Recurrence: Incomplete removal (common with Cryo) leads to re-growth.
• Pyogenic Granuloma: Rare vascular overgrowth at site of curettage.

Late (Months)

• Post-Inflammatory Hypopigmentation: White mark where lesion was. Very common after Cryo. Patients must be warned ("Would you rather a brown wart or a white flat spot?").
• Scarring: Hypertrophic scarring (especially on chest/shoulders).

---

9. Prognosis & Outcomes

Natural History

• SKs persist indefinitely. They do not spontaneously resolve.
• They tend to grow slowly thicker and darker over years.
• New lesions appear continuously.

Outcomes with Treatment

Prognostic Factors

Not applicable for benign lesions.

---

10. Evidence & Guidelines

Key Guidelines

1. British Association of Dermatologists (BAD) — Patient Information on Seborrhoeic Keratoses (2023). Emphasizes reassurance and non-intervention as first-line. Recommends biopsy for any atypical features.

2. American Academy of Dermatology (AAD) — Clinical Practice Guidelines for Benign Lesions (2022). Supports dermoscopy as standard-of-care for SK diagnosis to reduce unnecessary biopsies.

3. German Society of Dermatology (DDG) — Seborrheic Keratosis Guideline [11]. Comprehensive German-language guideline covering diagnosis, differential, and treatment modalities with evidence grading.

4. DermNet NZ — Seborrhoeic Keratoses (2024 update). Gold standard online resource for clinical and dermoscopic features.

Differential Diagnosis — Critical Distinctions

The "Collision Tumour" Phenomenon: [16]

• Rarely, a melanoma or BCC can arise within or adjacent to a pre-existing SK.
• Histopathological studies report this in less than 1% of SKs, but it represents a diagnostic pitfall.
• Clinical implication: This is why dermoscopic examination showing atypical features (even in an otherwise classic SK) warrants biopsy/excision. Blind destruction with cryotherapy of an atypical pigmented lesion is dangerous.

Evidence Synthesis

Recent Advances (2022-2025)

Dermoscopy-AI Algorithms:

• Convolutional neural networks (CNNs) trained on > 50,000 dermoscopic images now achieve 92-96% accuracy in distinguishing SK from melanoma.
• Smartphone-based apps incorporating AI may assist primary care triage. [17]

Targeted Therapies (Experimental):

• FGFR3 inhibitors (e.g., erdafitinib) theoretically could treat SKs, but no trials exist given benign nature. May be relevant for severe familial cases. [18]

Blue-White Veil in SK:

• Recent large study (n=1,214 lesions) found 8% of SKs exhibit blue-white veil on dermoscopy, historically considered melanoma-specific. However, presence of other SK features (milia-like cysts) still distinguished SK from melanoma with 96% accuracy. [10]

Controversies & Knowledge Gaps

Is Leser-Trélat Real?

• Controversial. Some authors argue it's publication bias (only dramatic cases reported).
• Counter-argument: Multiple case series show temporal association with malignancy diagnosis.
• Practical approach: Treat sudden eruptive SKs as paraneoplastic until proven otherwise. [5,6]

UV Role:

• SKs occur in sun-protected areas (buttocks, breasts), unlike AKs.
• Yet, epidemiological data show UV correlation.
• Hypothesis: UV may not initiate but may promote clonal expansion of pre-existing FGFR3-mutant cells. [8]

Why are SKs benign despite oncogenic mutations?

• FGFR3 and PIK3CA mutations are oncogenic in bladder cancer and other malignancies.
• In skin, these mutations drive proliferation but remain constrained to epidermis without invasion.
• Proposed mechanisms: Keratinocyte differentiation programs, lack of stromal remodeling, immune surveillance. [4]

Quality of Life Impact

While medically insignificant, SKs have measurable impact on QoL:

• Dermatology Life Quality Index (DLQI) scores elevated in patients with multiple facial SKs (mean 8.2 vs 2.1 controls).
• Cosmetic concern drives 60-70% of removal requests.
• Misdiagnosis as melanoma causes significant anxiety (30% of patients report persistent fear despite reassurance).

---

11. Clinical Scenarios & Exam Preparation

OSCE Station: "The Worried Patient with a Brown Spot"

Candidate Instructions: You are an FY2 doctor in GP. A 62-year-old patient presents worried about a "changing mole" on their back. Take a focused history, examine the lesion, and provide appropriate management advice. (8 minutes)

Model Approach:

History (2 minutes):

• "When did you first notice this?"
• "Has it changed in size, color, or symptoms?"
• "Any bleeding, itching, or pain?"
• "Any similar spots elsewhere?"
• "Personal or family history of skin cancer?"
• "Occupation/sun exposure?"

Examination (3 minutes):

• Expose area with chaperone.
• Good lighting.
• Inspect: Size, color, border, surface texture, symmetry.
• Palpate: Consistency, elevation, pick test.
• Dermoscopy if available: Look for milia-like cysts, comedo-like openings.
• Check for other lesions (total body exam if time permits).

Findings (Example):

• 10mm brown plaque on upper back.
• Well-defined border, stuck-on appearance.
• Verrucous surface with visible keratin plugs.
• Soft to touch, no induration.
• Multiple similar lesions elsewhere.
• Dermoscopy: Milia-like cysts ++, comedo-like openings +, fissures +.

Diagnosis & Explanation (2 minutes):

• "This is a seborrhoeic keratosis, commonly called a 'barnacle of ageing.'"
• "It's completely benign and very common as we get older."
• "It won't turn into cancer."
• "It's caused by a harmless overgrowth of skin cells."

Management (1 minute):

• Reassure.
• Options: Leave alone (most common) or remove if symptomatic/cosmetic concern.
• Removal methods: Freezing, scraping, cutting.
• Expected to get more over time—this is normal.
• Return if any lesion bleeds spontaneously, grows rapidly, or looks different.

OSCE Mark Scheme:

• History taking (2 marks)
• Examination technique (2 marks)
• Correct diagnosis (1 mark)
• Patient-centered explanation (2 marks)
• Appropriate management plan (2 marks)
• Professional manner (1 mark) Total: 10 marks

---

SBA Question 1: Dermoscopy

A 58-year-old woman presents with a 12mm brown plaque on her shoulder. Dermoscopy shows multiple bright white round structures and dark brown irregular openings. What is the most likely diagnosis?

A. Melanoma in situ
B. Pigmented basal cell carcinoma
C. Seborrhoeic keratosis
D. Solar lentigo
E. Dermatofibroma

Answer: C. Seborrhoeic keratosis

Explanation: The dermoscopic features described are milia-like cysts (bright white round structures) and comedo-like openings (dark brown irregular openings), which are pathognomonic for seborrhoeic keratosis. [9,10] Melanoma would show atypical network and blue-white veil; pigmented BCC shows arborizing vessels and blue-grey ovoid nests; solar lentigo shows uniform pigment without keratin structures; dermatofibroma shows peripheral pigment network with central white scar-like area.

---

SBA Question 2: Leser-Trélat Sign

A 67-year-old man presents with sudden onset of > 30 pruritic brown papules across his trunk over 6 weeks. He has lost 8kg in 3 months. What is the most appropriate investigation?

A. Full skin biopsy of all lesions
B. CT chest, abdomen, pelvis + gastroscopy
C. Reassure and arrange follow-up in 6 months
D. Treat all lesions with cryotherapy
E. Topical corticosteroids for pruritus

Answer: B. CT chest, abdomen, pelvis + gastroscopy

Explanation: This presentation is highly suggestive of the Sign of Leser-Trélat (sudden eruptive seborrhoeic keratoses associated with internal malignancy, most commonly gastric adenocarcinoma). [5,6] The weight loss further raises concern. Urgent malignancy screening is mandated. Treating the SKs or providing symptomatic relief without investigating the underlying cause would be dangerous. Full skin biopsy of all lesions is unnecessary and impractical.

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SAQ Question: Management of Seborrhoeic Keratosis

A 55-year-old woman has a 10mm brown warty plaque on her face that she finds cosmetically distressing. Dermoscopy confirms seborrhoeic keratosis.

a) List THREE treatment options and give ONE advantage and ONE disadvantage of each. (6 marks)

Model Answer:

1. Cryotherapy (liquid nitrogen)

   • Advantage: Quick, no anaesthetic needed, low cost
   • Disadvantage: Risk of hypopigmentation (white scar), especially on face

2. Curettage with cautery

   • Advantage: Provides tissue for histology; excellent clearance rate (> 95%)
   • Disadvantage: Requires local anaesthetic injection; 7-14 day healing time

3. Laser ablation (CO₂ or Er:YAG)

   • Advantage: Best cosmetic outcome; precise depth control
   • Disadvantage: Expensive; no tissue for histology

Award 2 marks per treatment option (0.5 for naming, 0.5 for advantage, 0.5 for disadvantage, 0.5 for relevant detail)

b) What specific advice should be given regarding cryotherapy on the face? (2 marks)

Model Answer:

• High risk of hypopigmentation (permanent white mark) on facial skin [13]
• Alternative methods (curettage or laser) preferred for facial lesions where cosmesis is important
• Patient must be counseled about cosmetic outcomes before proceeding

Award 1 mark for hypopigmentation risk, 1 mark for counseling importance

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Viva Question: Differential Diagnosis

Examiner: "A 70-year-old presents with a 15mm black lesion on the back. What are your differential diagnoses?"

Model Answer: "My key differentials for a black lesion in an elderly patient include:

Benign:

1. Seborrhoeic keratosis (melanoacanthoma variant)—most likely given age; would expect stuck-on appearance, keratin plugs
2. Melanocytic naevus—less common de novo at this age
3. Solar lentigo—usually flat, uniform brown (not black)

Malignant: 4. Melanoma (nodular type)—must always exclude; would look for asymmetry, ulceration, rapid growth 5. Pigmented basal cell carcinoma—rolled border, telangiectasia

Approach:

• Full history: Duration, change, symptoms, risk factors
• Examination: Palpation (SK is soft, not indurated), dermoscopy
• Dermoscopy: Look for SK features (milia-like cysts, comedo-like openings) vs melanoma features (atypical network, blue-white veil)
• If any doubt: Biopsy or excise [9,10]
• Never treat with blind cryotherapy if diagnosis uncertain"

Examiner: "Dermoscopy shows milia-like cysts and comedo-like openings. What is your management?"

Model Answer: "These are pathognomonic features of seborrhoeic keratosis. [9] My management would be:

1. Reassure the patient this is benign and won't become cancerous
2. Explain natural history—likely to persist and may slowly enlarge
3. Discuss options:
   • Observation (most appropriate if asymptomatic)
   • Removal if symptomatic (bleeding, catching on clothing) or cosmetic concern
4. Removal methods: Cryotherapy, curettage, or shave excision
5. Counsel regarding new lesions appearing over time (expected)
6. Safety-net: Return if any lesion changes rapidly or bleeds spontaneously"

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Clinical Pearls — "The 10 Commandments of SK Diagnosis"

1. "Stuck-on" is Stuck in Your Brain: If it looks like wax stuck to the skin, it's probably SK.

2. Milia + Comedones = SK: The dermoscopic duo of milia-like cysts and comedo-like openings is > 90% specific. [9]

3. Never Cryo What You Can't Diagnose: Blind cryotherapy of black lesions without dermoscopy is medico-legally dangerous.

4. The Pick Test: If you can lift greasy scale with gentle scratching, it's SK (not a melanocytic naevus).

5. Leser-Trélat is Real Until Proven Otherwise: Eruptive SKs deserve malignancy work-up. [5,6]

6. Age Matters: SK in a 25-year-old is rare—think twice. SK in a 65-year-old is expected—think once.

7. Location, Location, Location: Palms, soles, mucosa? NOT SK. Trunk, scalp, face? Classic SK territory.

8. The Ugly Duckling Paradox: An SK often looks "ugly" and disordered, yet is benign. Trust dermoscopy, not your aesthetic sense.

9. Hypopigmentation > SK: Before cryotherapy, ask: "Would you prefer a brown warty spot or a white flat scar?" (Many choose the SK!)

10. When in Doubt, Cut it Out: If you cannot confidently exclude melanoma, excise for histology. Reassurance without certainty is medicolegally perilous.

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11. Patient/Layperson Explanation

What is a Seborrhoeic Keratosis?

It is a harmless skin growth that is incredibly common as we get older. We often call them "senile warts" or "wisdom spots." It is basically a build-up of skin cells that pile up on the surface like a barnacle on a ship. They are NOT caused by the sun, and they are NOT skin cancer.

Why does it matter?

They don't matter for your health, but they can be annoying. They can catch on bra straps or belts, become itchy, or bleed if scratched. The main reason to see a doctor is just to double-check it isn't a bad mole, especially if it is dark black.

How is it treated?

Most of the time, we leave them alone. If one is bothering you or looks unsightly:

1. Freezing (Cold Spray): We spray it with liquid nitrogen. It creates a scab and falls off in a week or two.
2. Scraping: We numb the skin and gently scrape it off. This usually heals very well.

What to expect

• If we remove it, you might be left with a white mark or a small scar.
• You will likely get more of these spots as you get older. This is normal.
• They do not turn into cancer.

When to seek help

• If a spot bleeds without being scratched.
• If it changes shape or colour very quickly.
• If you suddenly get dozens of them appearing all at once over a few weeks.

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12. References

Primary Literature

1. Wollina U. Seborrheic Keratoses - The Most Common Benign Skin Tumor of Humans. Clinical features and diagnosis. Open Access Maced J Med Sci. 2018;6(11):2270-2275. doi:10.3889/oamjms.2018.527. PMID: 30559863

2. Hafner C, Hartmann A, Vogt T. FGFR3 mutations in benign skin tumors. Cell Cycle. 2006;5(23):2723-2728. doi:10.4161/cc.5.23.3497. PMID: 17172864

3. Gorai S, Ahmad S, Raza SSM, et al. Update of pathophysiology and treatment options of seborrheic keratosis. Dermatol Ther. 2022;35(12):e15934. doi:10.1111/dth.15934. PMID: 36226729

4. Hafner C, Groesser L. Mosaic RASopathies. Cell Cycle. 2013;12(1):43-50. doi:10.4161/cc.23108. PMID: 23255107

5. Geber A, Hadžavdić A, Ljubojević Hadžavdić S. The Leser-Trélat Sign in a Patient with Gastric Adenocarcinoma. Acta Dermatovenerol Croat. 2023;31(1):51-52. PMID: 37843094

6. Bernett CN, Davidson CL, Schmieder GJ. Leser-Trélat Sign. StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2025 Jan. PMID: 29261959

7. Agustí Martínez J, Bella-Navarro R, García-García AB, et al. Familial seborrhoeic keratosis associated with multiple 'pure reticulated acanthomas' and infundibulocystic basal cell carcinomas. Br J Dermatol. 2017;177(6):1654-1663. doi:10.1111/bjd.15736. PMID: 28627087

8. Huang J, Zhang L, Shi L, et al. An epidemiological study on skin tumors of the elderly in a community in Shanghai, China. Sci Rep. 2023;13(1):4441. doi:10.1038/s41598-023-29012-1. PMID: 36932111

9. Minagawa A. Dermoscopy-pathology relationship in seborrheic keratosis. J Dermatol. 2017;44(5):518-524. doi:10.1111/1346-8138.13657. PMID: 28447350

10. Dana IN, Kurtansky NR, Pastore LM, et al. Differentiating seborrheic keratosis from melanoma among lesions exhibiting blue-white veil: A retrospective study. J Am Acad Dermatol. 2025;92(3):480-486. doi:10.1016/j.jaad.2024.10.070. PMID: 39528164

11. Barthelmann S, Butsch F, Lang BM, et al. Seborrheic keratosis. J Dtsch Dermatol Ges. 2023;21(3):265-277. doi:10.1111/ddg.14984. PMID: 36892019

12. Funkhouser CH, Coerdt KM, Haidari W, Cardis MA. Hydrogen Peroxide 40% for the Treatment of Seborrheic Keratoses. Ann Pharmacother. 2021;55(2):216-221. doi:10.1177/1060028020941793. PMID: 32646224

13. Timmermann V, Haase O, Krengel S. Comparison of cryotherapy, curettage, and laser therapy for seborrheic keratoses: efficacy and patient satisfaction. Dermatol Surg. 2024;50(9):816-822. doi:10.1097/DSS.0000000000004198. PMID: 38743570

14. Timmermann V, Krengel S, Mrowka P, Haase O. Practical Approaches for Seborrheic Keratosis Treatment: Curettage Versus 532-nm Lithium Borate Laser Versus Cryotherapy: A Prospective Interventional Study. Lasers Surg Med. 2025;57(7):581-589. doi:10.1002/lsm.70042. PMID: 40619638

15. Yeatman JM, Kilkenny M, Marks R. The prevalence of seborrhoeic keratoses in an Australian population: does exposure to sunlight play a part in their frequency? Br J Dermatol. 1997;137(3):411-414. doi:10.1111/j.1365-2133.1997.tb03748.x. PMID: 9349338

16. Suening BS, Neidenbach PJ. The Pseudo-Sign of Leser-Trélat: A Rare Presentation. Cureus. 2023;15(2):e35155. doi:10.7759/cureus.35155. PMID: 36819974

17. Esteva A, Kuprel B, Novoa RA, et al. Dermatologist-level classification of skin cancer with deep neural networks. Nature. 2017;542(7639):115-118. doi:10.1038/nature21056. PMID: 28117445

18. Grünewald S, Wojas-Pelc A, Kreutzer K, et al. Update on hereditary disorders with hyperpigmented macules: genodermatoses with excess skin pigmentation. J Dtsch Dermatol Ges. 2020;18(4):273-290. doi:10.1111/ddg.14072. PMID: 32301272

19. Tognetti L, Cinotti E, Farnetani F, et al. Development and Implementation of a Web-Based International Registry Dedicated to Atypical Pigmented Skin Lesions of the Face: Teledermatologic Investigation on Epidemiology and Risk Factors. Telemed J E Health. 2023;29(9):1356-1365. doi:10.1089/tmj.2022.0456. PMID: 36752711

20. Jaimes N, Halpern AC, Marghoob AA. Collision tumors: melanoma and seborrheic keratosis. Dermatol Surg. 2013;39(10):1516-1524. doi:10.1111/dsu.12292. PMID: 23915332

21. Werner RN, Sammain A, Erdmann R, et al. Evidence- and consensus-based (S3) Guidelines for the Treatment of Actinic Keratosis - International League of Dermatological Societies in cooperation with the European Dermatology Forum - Short version. J Eur Acad Dermatol Venereol. 2015;29(11):2069-2079. doi:10.1111/jdv.13180. PMID: 26370093

22. Khemakhem R, Kallel N, Jarraya R, Yangui I, Kammoun S. Leser-Trélat syndrome secondary to non-small-cell lung carcinoma. Clin Case Rep. 2022;10(8):e6069. doi:10.1002/ccr3.6069. PMID: 35937026

Additional Key References

23. Conic RZ, Cabrera CI, Khorana AA, Gastman BR. Determination of the impact of melanoma surgical timing on survival using the National Cancer Database. J Am Acad Dermatol. 2018;78(1):40-46.e7. doi:10.1016/j.jaad.2017.08.039. PMID: 28941590

24. Roh MR, Eliades P, Gupta S, Tsao H. Genetics of melanocytic nevi. Pigment Cell Melanoma Res. 2015;28(6):661-672. doi:10.1111/pcmr.12412. PMID: 26300491

Guidelines & Reviews

• British Association of Dermatologists. Patient Information Leaflet: Seborrhoeic Keratoses. 2023. Available at: www.bad.org.uk
• DermNet NZ. Seborrhoeic keratoses. Updated 2024. Available at: https://dermnetnz.org/topics/seborrhoeic-keratoses
• Primary Care Dermatology Society (PCDS). Seborrhoeic Keratosis Clinical Guidance. Available at: www.pcds.org.uk
• American Academy of Dermatology (AAD). Seborrheic Keratosis: Diagnosis and Treatment. www.aad.org
• International Dermoscopy Society. Atlas of Dermoscopy Patterns in Seborrhoeic Keratosis. www.dermoscopy-ids.org

Further Reading

• Braun RP, Rabinovitz HS, Oliviero M, et al. Dermoscopy of pigmented skin lesions. J Am Acad Dermatol. 2005;52(1):109-121.
• Argenziano G, Soyer HP, Chimenti S, et al. Dermoscopy of pigmented skin lesions: results of a consensus meeting via the Internet. J Am Acad Dermatol. 2003;48(5):679-693.

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Medical Disclaimer: MedVellum content is for educational purposes and clinical reference. Clinical decisions should account for individual patient circumstances. Always consult appropriate specialists.