Seborrheic Keratosis: Pattern Recognition and Mimickers

01The classic dermoscopic dyad of seborrheic keratosis is milia-like cysts and comedo-like openings. Milia-like cysts appear as round, whitish or yellowish structures corresponding to small intraepidermal keratin-filled cysts. Comedo-like openings (also called pseudofollicular openings or crypts) are sharply circumscribed brown to black round structures representing keratin-filled invaginations of the epidermis. In the Braun series, milia-like cysts were present in 66% and comedo-like openings in 71%, with both criteria heavily skewed toward thicker plaque and papular/nodular SKs.

02Fissures and ridges produce the cerebriform or brain-like pattern that is highly characteristic of acanthotic SK. Fissures are irregular linear keratin-filled depressions; when multiple, they yield a gyriform surface analogous to cerebral cortex on dermoscopy. Fissures were found in 61% of pigmented SKs, with 90% of those occurring in plaque or papular/nodular lesions. The cerebriform pattern is essentially pathognomonic for SK once other criteria align.

03The fingerprint pattern is a delicate light-brown reticulation that resembles a dermatoglyphic fingerprint and is most commonly seen in flat patch SKs and solar lentigines. Fingerprinting was identified in 100% in the patch subtype of SK in the Braun series. It must be distinguished from the true pigment network of melanocytic lesions, which has thin gridlike lines aligned with rete ridges; in SK fingerprinting the network is broader, more delicate, and often abuts a sharp moth-eaten border.

04The moth-eaten border is a sharply demarcated edge with concave indentations resembling fabric eaten by moths. In Braun et al., 46% of SKs displayed a moth-eaten border, and 90% of all SKs were sharply demarcated overall. A moth-eaten edge in a flat brown lesion on photodamaged skin is a strong vote for SK or solar lentigo and weighs against lentigo maligna, which more typically has finely indented but pseudo-network features.

05Hairpin vessels with a whitish halo are the vascular signature of keratinizing tumors. They appear as long capillary loops, often clustered or grape-like, each surrounded by a thin white halo. They were found in 63% of pigmented SKs in the Braun cohort, predominantly in thicker lesions. The whitish halo distinguishes benign hairpin vessels from the irregular hairpin vessels of keratoacanthoma or invasive SCC, which usually lack a clean halo and are more polymorphous.

06Networklike structures are the single most important imitator inside an SK. They were detected in 46% of pigmented SKs but, unlike a true pigment network, the lines are often hyperpigmented, the holes do not correspond to dermal papillae but to keratin-filled fissures or comedo-like openings, and the lines often end abruptly at the periphery. Mistaking networklike structures for a true pigment network is the most common cause of overcalling melanoma in an SK.

07Irritated SK can closely mimic SCC. When an SK becomes inflamed it may show polymorphous vessels, dotted vessels, ulceration, and a reddish background, overlapping with the dermoscopic pattern of invasive SCC. The discriminating features are the persistence of milia-like cysts, comedo-like openings, or sharp demarcation in irritated SK, and the appearance of glomerular vessels and white circles around hair follicles in SCC. When in doubt, biopsy.

08Differentiating clonal SK, pigmented SK, intraepidermal carcinoma (Bowen disease), and Bowenoid papulosis can be challenging. Clonal SK shows nest-like ovoid blue-grey structures within an otherwise typical SK. Pigmented Bowen disease shows clustered glomerular vessels and small brown to grey globules in a linear arrangement. Bowenoid papulosis is genital-site SCC in situ that may show structureless brown areas with dotted vessels. The presence of any of these features in an SK-suspicious lesion warrants biopsy.