Verrucous Keratosis vs. Seborrheic Keratosis: A Comparison

Verrucous keratosis and seborrheic keratosis are warty-looking skin growths that can be difficult to tell apart on sight, yet they carry different clinical implications. Seborrheic keratosis is one of the most common benign skin tumors in adults and almost never turns cancerous, while verrucous keratosis refers to a hyperkeratotic, wart-like papule whose biology is considerably more ambiguous and, in certain clinical settings, may represent an early step toward squamous cell carcinoma. The distinction matters because it changes whether a dermatologist monitors, treats, or biopsies a lesion.

What Each Term Actually Means

Seborrheic keratosis (SK) is a benign epidermal growth made up of keratinocytes, the cells that form the skin’s outer layer. SKs are extremely common, appearing as waxy, stuck-on-looking bumps that range in color from tan to dark brown or nearly black. They show up on the face, trunk, and extremities, and most adults over 50 have at least a few. Under a microscope, the classic hallmarks are a proliferation of basaloid cells, the presence of pseudo-horn cysts (small round keratin-filled pockets), and an absence of the cellular atypia that would suggest cancer.¹ Despite the name, they have nothing to do with sebum or oily skin.

Verrucous keratosis (VK), by contrast, is a less well-defined clinical entity. The term describes hyperkeratotic papules with verruca-like (wart-like) surface features that can look nearly identical to an SK or a common wart. The most clinically significant form of VK arises in patients taking BRAF inhibitor drugs for melanoma, where these lesions harbor upstream mutations in RAS and PIK3CA and appear to be an early clinical presentation of cutaneous squamous cell carcinoma (cuSCC).² Because the term is used somewhat loosely in dermatology, some providers apply it to any warty keratotic growth that doesn’t fit neatly into the SK or verruca vulgaris category, which creates confusion.

How They Look on the Skin

Both growths share a raised, rough surface that laypeople and even clinicians can confuse at a glance. SKs tend to have a well-defined border, a “stuck on” quality as though they could be peeled off, and a surface that may show small pits or fissures giving a brain-like or cerebriform pattern under magnification. They are often multiple, scattered across the body, and can occur in a wide range of skin tones. A study comparing flat wart-like SKs to actual flat warts found that the SK variants showed a significantly more scattered distribution across the skin, while true flat warts clustered in groups.³

Verrucous keratoses look more like spiky or filiform warts. They tend to be flesh-colored to pink, with a rougher, more irregular surface than a classic SK. In the BRAF inhibitor setting, they often appear as crops of new keratotic papules relatively quickly after starting therapy, which itself is a clue. Without that clinical context, though, VK and SK can be virtually indistinguishable on visual inspection alone, which is exactly why biopsy and dermoscopy play such important roles.

What Drives Each Growth

The genetic underpinnings of seborrheic keratosis are surprisingly well characterized for a supposedly mundane skin bump. Somatic activating mutations in FGFR3, a growth-factor receptor gene, are the most frequent finding, detected in roughly half of all SKs in several studies.⁴ These mutations are somatic, meaning they arise in individual skin cells rather than being inherited. They have been found even in flat, early-stage SKs, suggesting they help initiate the growth rather than appearing later as a byproduct.⁵ Additional mutations in PIK3CA, the TERT promoter, and DPH3 promoter genes have also been identified, sometimes co-occurring in the same lesion.⁶

A separate line of research has explored whether human papillomavirus (HPV) contributes. HPV DNA has been detected in some SKs, including co-infection with HPV types 6 and 18, leading some investigators to suggest the virus plays a role in pathogenesis.⁷ A case report also identified HPV type 20 in an SK lesion, though HPV-20 is considered low risk and generally causes tumors only in people with compromised immune systems.⁸ The HPV connection remains debated; most dermatologists regard SKs primarily as age- and sun-related growths rather than viral lesions.

Verrucous keratosis, at least in the BRAF inhibitor context, runs through a different pathway. These lesions harbor RAS mutations and PIK3CA mutations rather than the FGFR3 mutations typical of SK.² BRAF inhibitors block one arm of a growth-signaling cascade, but in doing so they can paradoxically activate the same cascade through alternative routes, pushing pre-existing RAS-mutant skin cells to proliferate. The shared mutation profile between VK and cuSCC in these patients is what makes the distinction from an ordinary SK so consequential.

The Role of Sunlight in Seborrheic Keratosis

Cumulative sun exposure is a well-supported risk factor for SK. A study of Korean men found that SKs were far more concentrated on sun-exposed areas like the face and the backs of the hands, and that people with a lifetime history of more than six hours of daily sun exposure had roughly a twofold higher risk of developing them compared to those with less than three hours a day.⁹ Australian data reinforced this, showing higher SK density on exposed body sites when adjusted for surface area.¹⁰ A retrospective review additionally found SKs are more common in males and favor the back and face.¹¹

Verrucous keratosis doesn’t have the same kind of sun-exposure data behind it, largely because the lesion is defined more by its clinical context (especially BRAF inhibitor use) than by environmental triggers. When VK appears in melanoma patients on targeted therapy, it can crop up on both sun-exposed and non-exposed skin, reflecting a drug-driven mechanism rather than a UV-driven one.

Microscopic Features That Separate Them

Histopathology is where the two growths diverge most clearly. A classic SK shows a proliferation of basaloid keratinocytes, acanthosis (thickening of the epidermis), and horn cysts or pseudohorn cysts scattered within the thickened skin. There is no mitosis, no nuclear atypia, and no koilocytosis. The growth pattern is predominantly exophytic, meaning it bulges outward above the level of the surrounding skin rather than invading downward.¹²

Verrucous keratosis, on the other hand, typically shows hyperkeratosis and papillomatosis but may lack the horn cysts that serve as the SK’s microscopic fingerprint. In BRAF-inhibitor-associated VK, pathologists look for features that shade toward dysplasia or early squamous cell change. The histologic findings in VK were described as “benign” in isolation, but the underlying RAS and PIK3CA mutations shared with cuSCC suggest they sit on a continuum rather than in a separate biological category.² In tricky cases where histology alone isn’t enough to distinguish warty growths, immunohistochemical stains for markers like Ki-67 and p21 can help clinch the diagnosis.¹³

Why the Distinction Matters for Malignant Potential

Standard seborrheic keratosis is considered benign, and decades of dermatologic practice back that up. But the picture becomes more complicated with certain subtypes. Clonal-pattern seborrheic keratosis (CPSK), a variant in which the cells grow in nested clusters, carries a meaningful recurrence and upgrade risk. A study of 244 CPSKs found that about 7% recurred after partial removal, compared with under 2% of ordinary SKs. Among the recurrent CPSKs, most came back as squamous cell carcinoma in situ or invasive SCC rather than as another benign growth. CPSKs with cellular atypia were especially prone to this, with an upgrade rate to at least SCC in situ of 78%.¹⁴ That data point is a reminder that not all SKs are equally harmless, even if the vast majority are.

Verrucous keratosis in the BRAF inhibitor setting has a more overt link to malignancy. The shared RAS and PIK3CA mutations between VK and cuSCC suggest that VK in these patients is not a parallel but benign track — it’s the same track, just caught earlier.² This is why oncologists and dermatologists managing melanoma patients on BRAF inhibitors perform regular skin checks and biopsy suspicious keratotic lesions rather than assuming they are garden-variety SKs.

There is also the phenomenon of eruptive seborrheic keratoses, sometimes called the sign of Leser-Trélat, where dozens of SKs appear rapidly and may signal an internal malignancy. The association has been debated for decades. A systematic review found that while a co-occurring malignancy was reported in most published cases, the case-control studies that attempted to test the link had sample sizes too small to confirm it.¹⁵ The honest read of the literature is that the sign exists as a clinical observation, but its reliability as a paraneoplastic marker remains unproven.

Diagnostic Tools Beyond the Naked Eye

When a warty lesion sits in diagnostic limbo between SK and VK (or a true viral wart), several technologies can help. Dermoscopy, which uses a handheld magnifier with polarized light, reveals characteristic patterns in SKs: comedo-like openings, milia-like cysts, and gyriform (brain-like) structures that are easy to recognize once you know what to look for. A study comparing several verrucous skin diseases under dermoscopy confirmed that SK shows gyriform structures along with its signature comedo-like openings and milia-like cysts, while other verrucous conditions display distinct vascular patterns like dotted vessels and frogspawn-like structures.¹⁶ Those same researchers found that under dermoscopy, SK featured brain-like dermoscopic patterns combined with pseudocysts, while flat warts showed frogspawn-like structures and dotted vessels instead.

Reflectance confocal microscopy (RCM) takes this a step further. RCM is a non-invasive imaging tool that generates near-cellular-resolution images of living skin, and studies have shown it can reliably distinguish SK from flat warts and other verrucous mimics in vivo.¹⁷ Another evaluation concluded that RCM’s ability to pick up pseudocysts and gyriform ridges specific to SK, versus the evenly distributed ring-like structures of flat warts, could make it a useful tool for avoiding unnecessary biopsies.¹⁸ RCM is not yet widely available outside academic centers, but it is increasingly recognized as helpful in ambiguous cases.

Treatment of Seborrheic Keratosis

Because SKs are benign, treatment is elective and driven by cosmetic preference, irritation from clothing or jewelry, or diagnostic uncertainty. The standard first-line approaches are all minor office procedures: cryotherapy (liquid nitrogen), shave excision, and laser removal.¹⁹ A randomized trial comparing plasma exeresis to cryotherapy found that at six weeks, plasma exeresis cleared a significantly higher proportion of lesions, though both methods reduced the growths over time. The most common side effects were redness after plasma and lighter skin patches after cryotherapy.²⁰

For patients who want to avoid procedures, topical options have been gaining attention. A high-concentration hydrogen peroxide solution (40%, marketed under a brand name in the U.S.) is the most studied topical agent, and a smaller study of 30% hydrogen peroxide found that about half of treated lesions cleared completely after a single in-office application, with mild and transient side effects like burning and redness.²¹ A systematic review of topical treatments confirmed that hydrogen peroxide had the strongest evidence base among topicals, though no head-to-head trials have yet compared it directly to cryotherapy or shave excision.²² Other topicals showing some promise in that review included tazarotene cream, potassium dobesilate cream, and trichloroacetic acid, but none has yet crossed the threshold of reliably matching procedural removal.

Verrucous keratosis, particularly in the BRAF-inhibitor context, demands a different approach. Because VK may be a precursor to squamous cell carcinoma, excision with histologic examination is the standard. The goal is both to remove the growth and to confirm its biology on the pathologist’s slide. In melanoma patients on targeted therapy, multiple VK-like lesions may develop, and close dermatologic surveillance is part of routine care.

Immunosuppression and Overlapping Presentations

Both SK and verrucous growths can behave differently in immunosuppressed people. Organ transplant recipients, HIV-positive individuals, and patients on immunosuppressive medications tend to develop more viral warts and keratotic lesions, making the clinical landscape even harder to parse. SK itself has been described as an acquired lesion that can appear in the setting of immunosuppression, pregnancy, or benign tumors.²³ And because HPV-driven warts are more numerous and more persistent in immunocompromised patients, the overlap between true warts, wart-like SKs, and verrucous keratoses becomes especially pronounced. Biopsy thresholds tend to be lower in this population for exactly that reason — the consequences of missing a dysplastic or malignant lesion are higher when the immune system cannot help keep abnormal cells in check.

The clinical bottom line for anyone reading a pathology report that mentions “verrucous keratosis” is that the term sits in a grayer zone than “seborrheic keratosis.” An SK diagnosis is reassuring; a VK diagnosis warrants a conversation with your dermatologist about whether follow-up, additional sampling, or excision is appropriate, especially if you are on any form of immunosuppressive or targeted cancer therapy.