Seborrheic Keratosis (and Variants)

Elsa F. Velazquez, MD

Key Facts

Macroscopic Features

Sharply delineated plaques with greasy appearance
Verruciform or flat surface
- Surface shows characteristic keratotic plugs
From flesh-colored to brown-black
- Darkly pigmented lesions may be clinically mistaken for melanoma

Microscopic Pathology

Exophytic/endophytic
Ortho-hyperkeratosis typically present
Composed of basaloid and squamoid cells
Horn cysts/pseudocysts
Squamous eddies may be present
Often pigmented
Stromal amyloid deposition may be seen
Variable degree of inflammation
Several variants have been described
- Acanthotic
- Papillomatous (hyperkeratotic)
- Adenoid (reticulated)
- Clonal (Borst-Jadassohn epithelioma)
- Inverted follicular keratosis (irritated SK)
- Lichenoid SK
- Inflammatory
- Desmoplastic
- Adamantinoid
- Overlapping features are common

Top Differential Diagnoses

Verruca vulgaris
Hidroacanthoma simplex
Hypertrophic actinic keratosis
Squamous cell carcinoma in situ

Seborrheic keratosis, acanthotic/endophytic variant. Note the smooth, rounded surface and thickened epithelium. The lesion is predominantly composed of basaloid cells with a prominent horn cyst

Adenoid seborrheic keratosis composed of narrow and hyperpigmented trabeculae

. This variant of SK often evolves from a solar lentigo. Note the “basket weave,” orthokeratotic surface

TERMINOLOGY

Abbreviations

Seborrheic keratosis (SK)

Definitions

Benign epidermal proliferation

ETIOLOGY/PATHOGENESIS

Benign Epidermal Tumor

Monoclonal
Likely neoplasm rather than hyperplasia

CLINICAL ISSUES

Epidemiology

Incidence
- Very common lesions
Age
- Middle-aged and elderly
  - Affect approximately 20% of elderly population

Site

Most common on face, chest, and back
- Unusual locations: Conjunctiva, areola, areas of cleavage
- May be found anywhere except palms and soles

Presentation

Sharply delineated plaques with greasy appearance
Solitary or multiple
Measuring a few mm to a few cm
Verruciform or flat surface
- Surface shows characteristic keratotic plugs
Soft and friable
Round or oval
From flesh-colored to brown-black
- Darkly pigmented lesions may be clinically mistaken for melanoma
Often appearing as stuck on skin surface
Inverted follicular keratosis variant presents as warty papulonodule
Leser-Trélat sign
- Sudden onset of multiple seborrheic keratoses
  - Usually associated with internal malignancies (most commonly gastric adenocarcinoma)

Prognosis

Excellent

MACROSCOPIC FEATURES

General Features

Exophytic tumors
- Verruciform/warty or smooth surface

Size

A few mm to a few cm in diameter

Sections to Be Submitted

As lesions are small, they tend to be almost entirely sampled in most biopsies
Specimens may be bisected or serially sectioned before submission
Curettage specimens are received as multiple fragments

MICROSCOPIC PATHOLOGY

Histologic Features

Sharply defined tumors
Exophytic or endophytic or combination of both
Ortho-hyperkeratosis (“Van Gogh sky”)
- Heavily inflamed, irritated, or infarcted SK may show foci of parakeratosis
Composed of bland-appearing basaloid and squamoid cells (variable amounts of each)
Keratin-filled invaginations and small pseudocysts (pseudohorn cysts) are characteristic
Nests of keratinocytes (squamous eddies) may be seen
- Usually seen in irritated SK
- Apparently related to acrotrichia
Frequently hyperpigmented
- Pigmentation in SK has been linked to increased expression of keratinocyte-derived endothelin 1
- Melanocytes may be increased in number and size
  - Melanoacanthomas are SKs with marked melanocytic proliferation and pigmentation
Tricholemmal differentiation (glycogen-rich) may be focally present
Sebaceous differentiation is uncommon and focal finding
Acantholysis &/or dyskeratosis may be rarely seen
Many different variants of SK have been described
- Acanthotic SK
  - Smooth, rounded and ortho-hyperkeratotic surface
  - Acanthotic epithelium
  - Basaloid cells predominate over squamoid cells
  - Often pigmented
  - Horn cysts tend to be prominent
- Papillomatous (keratotic) SK
  - Ortho-hyperkeratosis
  - Papillomatosis
  - Acanthosis
  - Squamoid cells predominate over basaloid cells
  - Prominent horn cysts
- Adenoid (reticulated) SK
  - Ortho-hyperkeratosis
  - Flat or papillomatous surface
  - Thin proliferating strands emanating from epidermis
  - Basaloid cells predominate over squamoid cells
  - Often pigmented
  - Horn cysts are absent or sparse
- Clonal SK (type of irritated SK)
  - Borst-Jadassohn phenomenon: Oval to round aggregates of intraepidermal keratinocytes
- Inverted follicular keratosis (type of irritated SK)
  - Endophytic growth pattern
  - Prominent squamous eddies
- Lichenoid SK
  - Prominent lymphoid infiltrate
  - Apoptotic cells (Civatte bodies)
  - Some cases probably evolve to lichenoid keratosis (lichen planus-like keratosis)
  - Any of the variants of SK may show lichenoid features
- Inflammatory SK
  - Any variant of SK may be heavily inflamed
  - Rarely, neutrophils may be abundant in inflammatory infiltrate
  - Overlapping features with lichenoid SK
- Desmoplastic SK
  - Irregular cords and strands of squamous cells surrounded by desmoplastic stroma
  - These trapped strands and cords may mimic infiltrative squamous cell carcinoma (SCC)
  - Bulk of lesion is typical SK
  - Analogous to desmoplastic tricholemmoma
- Adamantinoid SK
  - Very unusual
  - Small basaloid keratinocytes with spindled cytoplasm
  - Intercellular mucin
- SK with pseudorosettes
  - Very rare
  - Basaloid cells arranged around central small spaces
Overlapping features of different variants is common finding

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