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