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Skin - Nonmelanocytic tumors

Benign nonmelanotic epidermal tumors / tumor-like lesions

Seborrheic keratosis

Reviewer: Christopher Hale, M.D. (see Reviewers page)
Revised: 25 June 2012, last major update June 2012
Copyright: (c) 2001-2012, PathologyOutlines.com, Inc.


● Common
● Usually age 40+ years
● Benign, although may coexist with malignancy
● Usually affects trunk, head and neck, extremities
● Only hair bearing skin
● Not HPV related, although HPV present in morphologically similar cases of epidermodysplasia verruciformis and bowenoid changes
● FGFR-3 mutations in subset of cases (Mod Pathol 2007;20:895), PIK3CA also

Dermatosis papulosa nigra:
● In blacks, younger age, multiple

Stucco keratosis:
● Caucasians, men >> women, extensor surfaces of forearms/lower legs

Leser-Trelat sign:
● Sudden appearance or increase in number and size of seborrheic keratoses, associated with internal malignancy
● 65 year old man with sign of Leser-Trélat (N Engl J Med 2007;356:2184)

Acanthotic – most common, rounded verrucous surface; thick layer of basal cells mixed with horn cysts (contain keratin) and pseudohorn cysts (downgrowth of keratin into tumor mass); no prominent granular layer; some cells contain melanin due to transfer from neighboring melanocytes
Irritated – pronounced squamous metaplasia with abundant eosinophilic cytoplasm and whorled squamous eddies; often atypia and mitotic figures; resembles carcinoma
Inverted follicular keratosis – irritated seborrheic keratosis that grows downward and involves hair follicles
● Also hyperkeratotic, adenoid, acantholytic and desmoplastic patterns


● Superficial curettage, freezing

Gross description

● Exophytic, sharply demarcated, pigmented lesions that protrude above surface of skin, appear to be stuck to skin
● Single or multiple, soft, tan-black, “greasy” surface

Micro description

● Basal keratinocyte proliferations, no atypia, “string sign” (smooth demarcation of lesion from dermis)

Micro images

Acanthotic seborrheic keratosis

Positive stains

● Low molecular weight keratin

Negative stains

● High molecular weight keratin (usually), HPV

Differential diagnosis

● Squamous cell carcinoma (particularly desmoplastic pattern)

Additional references

Proc Natl Acad Sci USA 2007;104:13450

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

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