Mimickers — When Seb K Is Not Seb K

This is the most clinically important section of this module. Seborrhoeic keratosis is so common and so recognisable that there is a real risk of anchoring — seeing the pattern you expect rather than the pattern that is actually there.

Two malignant lesions in particular can mimic seborrhoeic keratosis:

Melanoma mimicking seborrhoeic keratosis

Melanoma can occasionally develop features that resemble seborrhoeic keratosis, particularly in older patients on sun-damaged skin. This is one of the most dangerous mimics in dermoscopy.

How the mimicry works:

• Some melanomas develop pseudo-cysts or pseudo-openings — structures that superficially resemble the cysts and openings of seborrhoeic keratosis
• Heavily pigmented melanomas on the trunk can appear dark and well-defined, mimicking a darkly pigmented seb K
• On the face, lentigo maligna can mimic flat seborrhoeic keratosis

Red flags that suggest melanoma rather than seb K:

• Asymmetric distribution of structures — classic seb K features tend to be evenly scattered; melanoma pseudo-structures are often clustered or irregular
• Multiple colours beyond the expected tan-to-brown range — particularly blue, grey, or red
• Atypical network visible in parts of the lesion
• Areas that lack any structure (structureless zones of varying colour)
• The lesion has areas that look like seb K alongside areas that look different — a mixed pattern

Pigmented basal cell carcinoma mimicking seborrhoeic keratosis

Pigmented BCC can appear as a dark, well-defined lesion that at first glance resembles a seborrhoeic keratosis — particularly in its darker, more pigmented variants.

How the mimicry works:

• Pigmented BCC can be well-circumscribed and darkly pigmented
• The blue-grey ovoid nests of BCC can superficially resemble the dark plugs of comedo-like openings
• On quick examination, the overall impression may suggest a darkly pigmented seb K

Red flags that suggest pigmented BCC rather than seb K:

• Arborising vessels — these bright red, branching vessels are the signature of BCC and are never seen in seborrhoeic keratosis
• Blue-grey ovoid nests — well-circumscribed blue-grey structures, unlike the dark brown/black of true comedo-like openings
• Leaf-like areas or spoke-wheel structures
• Shiny white structures — visible under polarised dermoscopy
• Absence of true milia-like cysts — BCC does not produce the bright keratin pearls of seb K

The key distinction: True seborrhoeic keratosis features (milia-like cysts, comedo-like openings, cerebriform surface) are epidermal structures — they sit on the surface. BCC features (arborising vessels, ovoid nests, shiny white structures) are dermal — they originate from deeper in the skin. If you see dermal features in a lesion you thought was a seb K, reconsider.

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