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An elderly woman noted a growth of a lesion on her chest (Figure 158-1). She was afraid that it might be melanoma. Her family physician recognized the typical features of a seborrheic keratosis (SK) (stuck-on with visible horn cysts) and attempted to reassure her. Dermoscopy was performed and the features were so typical of an SK; the physician was able to convince the patient to not have a biopsy (Figure 158-2). The black comedonal-like openings and white milia-like cysts are typical of an SK and can be seen with the naked eye and magnified with a dermatoscope.

Figure 158-1

Seborrheic keratosis with associated horn cysts. (Courtesy of Richard P. Usatine, MD.)

Figure 158-2

Dermoscopy of the seborrheic keratosis in the previous figure showing comedo-like openings (black-like blackheads) and milia-like cysts (white-like milia). (Courtesy of Richard P. Usatine, MD.)

  • A SK is a benign skin tumor and a form of localized hyperpigmentation as a result of epidermal alteration; it develops from the proliferation of epidermal cells, although the cause is unknown.

  • Most common benign tumor in older individuals; frequency increases with age.
  • In an older study of individuals older than age 64 years in North Carolina, 88% had at least one SK. Ten or more SKs were found in 61% of the black men and women, 38% of the white women, and 54% of the white men in the study.1
  • In an Australian study preformed in 2 general practices, 23.5% (40 out of 170) of individuals between ages 15 and 30 years had at least 1 SK; prevalence and size increased with age.2
  • Approximately half of cases of multiple SKs occur within families, with an autosomal dominant mode of inheritance.3

  • In pigmented SKs, the proliferating keratinocytes secrete melanocyte-stimulating cytokines triggering activation of neighboring melanocytes.3
  • A high frequency of mutations have been found in certain types of SKs in the gene encoding the tyrosine kinase receptor fibroblast growth factor receptor 3 (FGFR3).3 One study found that FGFR3 and transcription factor forkhead box N1 (FOXN1) were highly expressed in SKs but close to undetectable in squamous cell skin cancer.4 This may represent a positive regulatory loop between FGFR3 and FOXN1 that underlies a benign versus malignant skin tumor phenotype.
  • Reticulated SKs, usually found on sun-exposed skin, may develop from solar lentigines.3
  • Multiple eruptive seborrheic keratoses (the sign of Leser-Trélat) can be associated with internal malignancy (most often adenocarcinoma of the GI tract) (Figure 158-3),5 although this association has been questioned.6
  • An eruption of seborrheic keratoses may develop after an inflammatory dermatosis such as severe sunburn or eczema.3 There is also a report of exacerbation of a SK by topical fluorouracil.7...

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