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An elderly woman noted growth of a lesion on her chest (Figure 164-1). 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 typical of an SK; the physician was able to convince the patient to not have a biopsy (Figure 164-2). The black comedo-like openings and white milia-like cysts are typical of an SK and can be seen with the naked eye in this case, and even better with a dermatoscope.

FIGURE 164-1

Seborrheic keratosis with associated horn cysts. (Reproduced with permission from Richard P. Usatine, MD.)

FIGURE 164-2

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


  • An 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.


Senile keratoses, age spots.


  • Most common benign tumor in older individuals; frequency increases with age.

  • In a 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 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 has 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 SKs (the sign of Leser-Trélat) have been associated with internal malignancy in case reports (see Figure 164-3),5 although ...

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