Skin - Nonmelanocytic tumors
Benign nonmelanotic epidermal tumors / tumor-like lesions
Seborrheic keratosis

Authors: Hillary R. Elwood, M.D. (see Authors page)

Revised: 29 August 2016, last major update June 2016

Copyright: (c) 2001-2016,, Inc.

PubMed search: Seborrheic keratosis[title]
Cite this page: Seborrheic keratosis. website. Accessed March 23rd, 2018.
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

Images hosted on PathOut server:

Courtesy of Hillary R. Elwood, M.D.



Irritated with
squamous eddies


Courtesy of Sara Shalin, M.D., Ph.D.

Images hosted on other servers:

Missing Image


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"