In dermatology, SK stands for seborrheic keratosis, the most common benign skin tumor. These growths affect over 80 million Americans and become increasingly common after age 30. They are noncancerous, made up of immature skin cells, and while they can look alarming, they are almost always harmless.
What Seborrheic Keratoses Look Like
The hallmark of an SK is its “stuck-on” appearance. These growths look like someone pressed a waxy, slightly raised patch onto the skin’s surface. They’re roundish or oval, and even flat ones feel distinct when you run a finger over them. The surface is often dull and scaly, covered in keratin, the same fibrous protein that makes up fingernails and animal horns. That keratin layer gives them their characteristic texture.
Color varies widely. Most are brown, but they can also be black, tan, yellow, gray, or occasionally pink or white. Some look scabby or wart-like. Others have tiny bubble-like cysts visible within the growth. A single person might have just one or develop hundreds over time, and the growths can range from a few millimeters to over a centimeter across.
They show up almost anywhere on the body, most commonly on the face, scalp, trunk, chest (especially under the breasts), arms, and legs. They don’t appear on the palms, soles, or inside the mouth.
Who Gets Them and Why
SKs become far more common with age. In Australian studies, about 24% of people aged 15 to 30 already had at least one. In the UK, 17% of women under 40 had one or more. After age 30, prevalence climbs steadily, with studies showing SKs present in roughly 40% of people in their 40s, 50s, and 70s.
The underlying cause involves specific genetic mutations in the skin cells themselves. These are somatic mutations, meaning they happen in individual cells over your lifetime rather than being inherited from your parents. Unlike cancer-related mutations, SKs remain genetically stable and don’t involve the loss of tumor-suppressing genes. That’s the key biological reason they stay benign. Sun exposure, friction, and simple aging all appear to play a role in triggering their development, but the exact mechanism isn’t fully understood.
How Doctors Tell SKs Apart From Skin Cancer
The concern most people have is whether a growth might be melanoma rather than a harmless SK. This is a legitimate worry because some melanomas can mimic the appearance of seborrheic keratoses. Dermatologists use a handheld magnifying tool called a dermoscope to examine growths closely. Under magnification, benign SKs typically show distinctive features: tiny white cyst-like structures and dark plug-like openings on the surface, along with patterns described as “fat fingers” (thick, ridge-like structures).
Melanomas, even those disguised as SKs, tend to reveal warning signs under dermoscopy. Irregular pigmentation appears in about 87% of melanoma cases that look like SKs, along with abnormal pigment networks, unusual blood vessel patterns, and blue-white veils. Dermatologists use standardized scoring checklists to flag these features. When any melanoma-specific sign is present, the growth is typically removed and sent for biopsy. If a growth changes rapidly in color, shape, or size, or starts bleeding without being bumped, having it evaluated is worthwhile.
The Leser-Trélat Sign
One rare but important scenario involves the sudden eruption of many seborrheic keratoses over a short period, typically within a year. This is called the Leser-Trélat sign, and it can be a marker of an internal cancer. The most commonly associated cancers are gastrointestinal (especially stomach and colon), breast, and lymphomas, though it has been linked to cancers in the pancreas, kidney, lung, prostate, and other organs. This is very different from the gradual, one-at-a-time appearance of ordinary SKs, which develop slowly over years. The abrupt onset is the distinguishing factor, and it prompts doctors to investigate for an underlying malignancy.
Removal Options
Because SKs are benign, removal is optional. Most people seek it for cosmetic reasons or because a growth is irritated by clothing or jewelry. There are three standard in-office procedures:
- Cryotherapy (freezing): Liquid nitrogen is applied to the growth. This works well for thinner, flatter SKs but is less effective on raised, thicker ones. On darker skin tones, it carries a risk of permanent pigment loss in the treated area.
- Curettage (scraping): After numbing the skin, a doctor uses a blade to scrape the growth off the surface. This is sometimes combined with cryotherapy for flatter lesions.
- Electrocautery (burning): An electric current destroys the growth after numbing. This can be used alone or paired with scraping, and it’s particularly useful for thicker growths.
All three methods can potentially cause temporary or permanent changes in skin color at the treatment site, as well as minor scarring.
Topical Treatment
A 40% hydrogen peroxide solution (brand name Eskata) is the first FDA-approved topical treatment for raised SKs. It’s applied in a doctor’s office using a disposable pen-like applicator. The solution is rubbed onto each growth four times per visit, with about a minute between applications. Up to 10 small growths can be treated in a single session.
In two large clinical trials involving 937 patients, about 51% of treated SKs cleared or nearly cleared after one to two applications, compared to 7% with a placebo. Results were best on the face, where 65% of growths resolved, compared to 46% on the trunk and 38% on the arms and legs. If growths remain after three weeks, a second treatment session may be needed. In published case reports, full resolution has been documented within about three months of the initial treatment.
Living With Seborrheic Keratoses
SKs don’t become cancerous, and they don’t need treatment unless they bother you. They can itch, catch on clothing, or simply be cosmetically unwelcome. New ones will likely continue to appear as you age, and removed growths won’t grow back in the same spot, though new ones can develop nearby. If a growth looks different from your others, changes quickly, or you’re simply unsure what it is, a dermatologist can evaluate it in a routine visit.