Skin nonmelanocytic tumor
Benign (nonmelanotic) epidermal tumors or tumor-like lesions
Seborrheic keratosis

Topic Completed: 1 June 2016

Minor changes: 19 February 2021

Copyright: 2001-2021,, Inc.

PubMed search: Seborrheic keratosis[title]

Hillary Rose Elwood, M.D.
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Cite this page: Elwood H. Seborrheic keratosis. website. Accessed April 14th, 2021.
Definition / general
  • Common, benign lesion developing in middle aged and elderly, all races and genders
Clinical features
  • Single or multiple, sharply demarcated pigmented lesion that protrudes above surface of skin
  • Appears to be stuck to skin
  • Soft, tan-black, "greasy" surface
  • Few millimeters to several centimeters in size
  • Can occur anywhere except palms and soles
  • Commonly on trunk

Leser-Trélat sign:
  • Sudden appearance or increase in number and size of seborrheic keratoses, associated with internal malignancy
  • A paraneoplastic phenomenon typically associated with GI malignancy
  • Case report: 65 year old man with sign of Leser-Trélat (N Engl J Med 2007;356:2184)
Clinical images

Images hosted on other servers:

Various images

Leser-Trélat sign

  • Superficial curettage, freezing, excision
Microscopic (histologic) description
  • Acanthotic proliferations of small cuboidal keratinocytes without cytologic atypia
  • Usually has "horn pseudocysts" (round intralesional cysts of loose keratin)
  • String sign: lesion extends to a uniform depth and one could draw a horizontal line parallel to the epidermal surface underlying the lesion

  • Histologic patterns: there are numerous histologic patterns or variants of seborrheic keratosis, which have no clinical significance
    • Acanthotic: most common; rounded verrucous surface, thick layer of basal cells mixed with pseudohorn cysts
    • Clonal: intraepidermal proliferation of keratinocytes, can have Borst-Jadassohn effect (intraepidermal nests of basaloid to epithelioid cells), horn pseudocysts usually absent
    • Flat / macular: barely protrude above the adjacent epidermis
    • Hyperkeratotic: prominent hyperorthokeratosis with verrucous papillomatosis; can form a cutaneous horn
    • Irritated: pronounced squamous metaplasia with abundant eosinophilic cytoplasm and whorled squamous eddies; often mitotic figures and parakeratosis
    • Reticulate / adenoid: thin anastomosing strands of basaloid cells arising from the epidermis
  • May have mixture of patterns, or other patterns
  • Malignant change rare but has occasionally been documented - more common is a collision tumor between a neoplasms and seborrheic keratosis
Microscopic (histologic) images

Contributed by Hillary R. Elwood, M.D.



Irritated with squamous eddies


Contributed by Yuri Tachibana, M.D.

Acanthotic proliferations

Pseudohorn cyst

Contributed by Sara Shalin, M.D., Ph.D.

Seborrheic keratosis

Contributed by Jijgee Munkhdelger, M.D., Ph.D. and Andrey Bychkov, M.D., Ph.D.

Low power

Pseudohorn cysts

Inflamed pseudohorn cyst

Differential diagnosis
  • Compound or intradermal melanocytic nevus: can have seborrheic keratosis-like change overlying it, but also has nests of melanocytes throughout dermis
  • Condyloma acuminatum: can resemble seborrheic keratosis on genital skin; clinical findings and HPV studies may be helpful
  • Epidermal nevus: can appear identical; consider this diagnosis in lesion resembling a seborrheic keratosis but in a young patient
  • Hidroacanthoma simplex: see Eccrine poroma; can resemble seborrheic keratosis at first glance; look for rudimentary duct formation or intracytoplasmic lumina, nests of smaller basaloid cuboidal cells
  • Squamous cell carcinoma: full thickness squamous atypia, atypical mitoses
  • Verruca vulgaris: has hypergranulosis, tiers of parakeratosis, dilated papillary blood vessels, intracorneal hemorrhage; some benign lesions have intermediate overlapping features of verruca vulgaris and seborrheic keratosis, and may be termed "verrucous or verrucoid keratosis"
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