Skin nonmelanocytic tumor

Benign (nonmelanotic) epidermal tumors or tumor-like lesions

Seborrheic keratosis



Minor changes: 4 October 2021

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PubMed search: Seborrheic keratosis [TI] pathology "last 5 years" [DP]

Caroline I.M. Underwood, M.D.
Elizabeth Boswell, M.D.
Page views in 2020: 54,437
Page views in 2021 to date: 58,238
Cite this page: Underwood CIM, Boswell E. Seborrheic keratosis. PathologyOutlines.com website. https://www.pathologyoutlines.com/topic/skintumornonmelanocyticSK.html. Accessed October 25th, 2021.
Definition / general
  • Common, benign keratinocyte proliferation of middle aged and elderly
Essential features
  • Benign
  • Clinical: waxy, brown slow growing papule
  • Histologic:
    • Proliferation of basaloid keratinocytes without atypia
    • Acanthosis and hyperkeratosis most often with horn pseudocysts
Terminology
  • Senile wart
  • Seborrheic wart
ICD coding
  • ICD-10: L82 - seborrheic keratosis
Epidemiology
Sites
Pathophysiology
  • Immature benign keratinocyte proliferation
  • Multiple somatic mutations (Oncotarget 2017;8:36639)
    • Most common: FGFR3, PIK3CA and HRAS
  • Amyloid precursor protein (APP) (Acta Derm Venereol 2018;98:594)
    • Expression increases in sun exposed areas and with age
    • May contribute to seborrheic keratosis formation
Etiology
  • Aging
  • Chronic UV light exposure
Clinical features
  • Single or multiple
  • Papules or plaques with a stuck on appearance (well demarcated edges)
  • Brown-black or gray in color
  • Waxy or greasy with cerebriform surface
  • Millimeters, up to a centimeter
    • Larger have been reported (rare)
  • Slow growing
  • Leser-Trélat sign: (Cleve Clin J Med 2017;84:918)
    • Sudden appearance of multiple seborrheic keratoses, rapid increase in size, pruritic
    • Paraneoplastic phenomenon typically associated with gastrointestinal adenocarcinoma
Diagnosis
  • Clinical:
    • Dermoscope: small keratin filled cysts, fissures, ridges, small vessels with perivascular halo (F1000Res 2019;8:1520)
  • Biopsy often performed on irritated lesions or those with rapid growth (Ann Dermatol 2016;28:152)
  • Histologic findings confirm diagnosis
Prognostic factors
  • Benign
  • Leser-Trélat sign may indicate underlying malignancy
Case reports
Treatment
Clinical images

Images hosted on other servers:
Seborrheic keratosis papules

Papules

Various images

Sharply circumscribed

Leser-Trélat sign

Leser-Trélat sign

Microscopic (histologic) description
  • General (shared) features:
    • Intraepidermal, well demarcated edges with a flat base
      • String sign: can draw a horizontal line along the base of the lesion
    • Basaloid keratinocyte proliferation without dysplasia
    • Hyperkeratotic with horn pseudocyst formation (intralesional cysts of loose keratin)
    • Multiple variants (no clinical or prognostic significance)
      • Often overlapping features
  • Acanthotic type:
  • Keratotic (papillomatous) type (Indian J Sex Transm Dis AIDS 2017;38:176):
    • Marked hyperorthokeratosis and papillomatosis
    • Can form a cutaneous horn
  • Reticulated (adenoid) type:
    • Thin, anastomosing strands of basaloid cells
    • Small horn cysts
    • May have increased pigment
  • Clonal type (Dermatol Pract Concept 2015;5:5, Pan Afr Med J 2019;34:54):
    • Pale basaloid keratinocytes in nests (Borst-Jadassohn phenomenon)
    • Horn pseudocysts may be absent
  • Irritated type (Arch Plast Surg 2017;44:570, Dermatol Online J 2019;25:13030):
    • Squamous metaplasia and whorled squamous eddies
    • Reactive squamous atypia
    • Scattered keratinocyte apoptosis and dyskeratosis
    • Spongiosis
    • Scale crust and parakeratosis in the stratum corneum
    • Lichenoid infiltrate in the superficial dermis (variable)
  • Pigmented:
    • Increased melanin pigmentation in the keratinocytes
    • Increased melanophages, mostly in the basal layer
  • Macular:
    • Minimal / mild acanthosis
    • Absent horn pseudocysts
    • Usually basal pigmentation increased
Microscopic (histologic) images

Contributed by Sara Shalin, M.D., Ph.D. and Caroline I.M. Underwood, M.D.
Acanthotic (regular) Acanthotic (regular)

Acanthotic (regular)

Hyperkeratosis

Hyperkeratosis

Papillomatosis without atypia

Papillomatosis without atypia

Reticulated pattern

Reticulated pattern


Adenoid with increased pigment

Adenoid with increased pigment

Clonal nests

Clonal nests

Pale keratinocytes

Pale keratinocytes

Clonal nests and horn pseudocyts

Clonal nests and horn pseudocyts


Irritated Irritated

Irritated

Increased pigmentation

Increased pigmentation

Minimal acanthosis

Minimal acanthosis

Virtual slides

Images hosted on other servers:
Acanthotic type

Acanthotic type

Clonal type

Clonal type

Irritated type

Irritated type

Macular

Macular

Videos

Seborrheic keratosis overview

Seborrheic keratosis and variants

Sample pathology report
  • Left chest, shave of skin:
    • Seborrheic keratosis
Differential diagnosis
  • Melanocytic nevus:
    • Nests of melanocytes in dermis or at dermal epidermal junction
    • Also present in younger patients
  • Solar lentigo:
    • Similar to reticulated seborrheic keratosis
    • Lentigo has pigmented, nonanastomotic, bulbous rete and no horn cysts
  • Malignant melanoma:
    • Malignant melanocytes with invasion into the dermis
  • Pigmented basal cell carcinoma:
    • Clefting, peripheral palisading, mucin, apoptosis and mitosis
  • Condyloma acuminatum:
    • Koilocytes
    • Dysplastic changes
    • Clinical findings (genital areas) and HPV positivity helpful in distinguishing
  • Verruca vulgaris:
    • Hypergranulosis, tiers of parakeratosis, dilated papillary blood vessels, intracorneal hemorrhage
    • Clinical features (present on hands and feet) and HPV positivity helpful in distinguishing
  • Actinic keratosis:
    • Atypia of basal keratinocytes
  • Squamous cell carcinoma:
    • Full thickness squamous atypia
    • Increased, often atypical mitoses
Board review style question #1

Which feature helps distinguish the above lesion from squamous cell carcinoma?

  1. Acanathosis and papillomatosis
  2. Atypical mitotic figures
  3. Lack of atypia
  4. Molecular testing for FGFR3
Board review style answer #1
C. Lack of atypia. Seborrheic keratosis is a benign keratinocyte proliferation that lacks atypia and dysplasia. Acanthosis and papillomatosis (A) are features of seborrheic keratosis but can also be seen in squamous cell carcinoma. Atypical mitotic figures (B) are a feature of squamous cell carcinoma. Many seborrheic keratoses have mutations in FGFR3 (D) but it is not diagnostic or ubiquitous.

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Reference: Seborrheic keratosis
Board review style question #2
Sudden eruption of seborrheic keratosis on the trunk should prompt which clinical response?

  1. Evaluation for underlying colonic carcinoma
  2. Full body skin exam for atypical nevi
  3. Immediate treatment with cryotherapy
  4. No further workup is necessary
Board review style answer #2
A. Evaluation for underlying colonic carcinoma. Sudden eruption of multiple seborrheic keratoses, the Leser-Trélat sign, is a paraneoplastic phenomenon. It is associated with underlying malignancy, most often colonic adenocarcinoma and should prompt a clinical workup to evaluate (D). The Leser-Trélat sign is not associated with increased risk for atypical nevi or melanoma (B). While seborrheic keratoses can be treated with cryotherapy (C), they are benign and treatment is not necessary.

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Reference: Seborrheic keratosis
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