A solitary dark lesion on the back of an adult is worrisome for melanoma. A scoop-shave biopsy was ordered with the aim of achieving a 1- to 3-mm margin. The biopsy identified the lesion as a benign pigmented seborrheic keratosis (SK).
SKs are a group of common, keratinocyte neoplasms that can occur in large numbers on a patient. They may meet many of the ABCDE criteria (Asymmetry, Border irregularity, Color [varying shades or deep black color], Diameter > 6 mm, or Evolving/changing) used to grossly identify potential melanomas. It is worth noting that not all dark-pigmented lesions arise from melanocytes. In this instance, the dark SK is made of keratinocytes that had accumulated melanin.
Dermoscopy usually helps distinguish SKs from melanocytic neoplasm, which would include nevi and melanoma. Melanocytic lesions (whether benign nevi or malignant melanoma) will display a pigment network, globules, streaks, homogeneous blue or tan color, or characteristic vascular findings. SKs, on the other hand, often demonstrate sharply demarcated borders, milia-like cysts or comedo-like openings, and hairpin vessels.
Both the clinical and dermoscopic photos in this case showed a sharply demarcated border, lack of network, and an absence of any vascular markings. The central scale crust did not exclude a melanocytic lesion and there were peripheral small black dots that could have been asymmetrical globules; however, the biopsy negated those clinical concerns.
Dermoscopy improves diagnostic specificity, but not perfectly. The number of benign lesions biopsied for every malignant lesion confirmed decreases from about 18 without dermoscopy to 8 or fewer for the most experienced dermoscopy practitioners.1 This case highlights one of many instances of a clinically and dermoscopically suspicious lesion that ultimately was benign.
Text courtesy of Jonathan Karnes, MD, medical director, MDFMR Dermatology Services, Augusta, ME. Photos courtesy of Jonathan Karnes, MD (copyright retained).