Seborrheic Keratosis or Melanoma? Telling Them Apart

A seborrheic keratosis (SK) is a benign overgrowth of the top layer of skin cells. The classic appearance is a 'stuck-on' or 'pasted-on' look — as though someone dropped a blob of warm wax or a piece of putty onto the skin that could be peeled off. The surface is often waxy, rough, or warty, and may have a dull, crumbly texture.

Colour ranges from light tan to dark brown or black. Size ranges from a few millimetres to several centimetres. They favour the trunk, face, scalp, and back, become more common from middle age onward, and frequently appear in numbers. They are harmless and require no treatment unless they catch, itch, or bother you cosmetically.

A dark or black, irregular-looking SK can genuinely resemble melanoma at a glance, especially the heavily pigmented ones. Both can be brown-black, both can have an uneven surface, and both can appear on the trunk.

The distinguishing features come from the SK's structure: the stuck-on appearance, the waxy or warty surface, and small surface details dermatologists see with a dermatoscope — tiny keratin-filled cysts (milia-like cysts) and pore-like openings that are characteristic of SKs and absent in melanoma. Still, because the overlap exists, any genuinely uncertain dark lesion should be examined rather than assumed.

Seborrheic keratosis: appears to sit on top of the skin, as if it could be picked off. The edges are often sharply demarcated, and the lesion has a raised, pasted-on quality.

Melanoma: tends to be more integrated into the skin, spreading within it rather than sitting on top. It does not have the waxy, peel-off appearance. The stuck-on quality is one of the most reliable clues that a dark spot is a harmless SK.

Seborrheic keratosis: waxy, rough, warty, or crumbly. Many have a dull, matte surface with visible tiny plugged pores, and they can flake or crumble at the edges.

Melanoma: a smoother or more variably textured pigmented lesion without the characteristic waxy, plugged-pore surface. If a spot has the dry, warty, 'barnacle' texture typical of SKs, that points strongly away from melanoma.

Seborrheic keratosis: usually has a sharply defined, well-demarcated border, even when the overall shape is a bit irregular. The lesion looks like a distinct object placed on the skin.

Melanoma: tends toward an irregular, blurred, or notched border that fades into surrounding skin, and asymmetry where one half does not mirror the other (the A and B in ABCDE). A blurry, asymmetric pigmented spot is more concerning than a sharply edged stuck-on one.

Seborrheic keratosis: grows extremely slowly over years and then stays stable. It may slowly thicken, but it does not rapidly change colour, shape, or size over weeks.

Melanoma: evolves over weeks to months — growing, darkening, or developing new colours and irregular edges (the E in ABCDE). Rapid or recent change in a dark spot is a melanoma signal, whereas a lesion that has looked the same for years is far more likely to be an SK.

One caveat: a sudden eruption of many SKs at once (the Leser-Trélat sign) is rare and occasionally associated with internal illness, so a dramatic crop of new growths is worth mentioning to a doctor.

If you have several seborrheic keratoses, they tend to share a family resemblance. The one to evaluate is the outlier — a single dark lesion that looks different from your other SKs, or that lacks the typical stuck-on, waxy look.

Dermatologists also know that melanoma can occasionally arise next to or within an area of SKs, so a changing, bleeding, or distinctly different lesion among a group of stable warty growths should not be assumed to be 'just another keratosis.'

Most SKs need nothing. See a dermatologist if a dark or waxy spot: is changing in size, shape, or colour over weeks; has an irregular or blurred border with multiple shades; bleeds without being caught or scratched; is the ugly duckling that differs from your other growths; or you are simply unsure whether it is an SK or a mole.

Dermatologists can usually confirm an SK on sight or with a dermatoscope, and uncertain lesions are settled by a quick shave biopsy. Do not try to remove a pigmented growth yourself — both because removal destroys the tissue needed to confirm it is benign, and because what looks like an SK occasionally is not.