Keratoacanthoma is a dome-shaped skin growth that appears rapidly, often reaching one to two centimeters within just a few weeks, then typically shrinks and disappears on its own over the following months. It arises from hair follicle cells and usually shows up on sun-exposed skin, particularly the face, arms, and hands. What makes keratoacanthoma so interesting to dermatologists and so confusing for patients is a long-running debate about whether it is truly benign or actually a form of skin cancer that just happens to behave well most of the time.
What a Keratoacanthoma Looks Like and How It Behaves
A classic keratoacanthoma goes through a predictable life cycle. It starts as a small, firm bump that grows surprisingly fast. Within a few weeks it can reach one to two centimeters across and develop a distinctive volcano-like shape with a central crater filled with a keratin plug, a waxy or horn-like material. After several weeks of sitting at that size, the growth begins to shrink on its own and eventually flattens out, leaving behind a depressed scar. The full cycle from first appearance to complete resolution usually takes about four to six months.1DermNet. Keratoacanthoma
That rapid growth is what alarms most people. A lump that doubles in size over two or three weeks understandably sends patients to the doctor fearing cancer. The speed is actually one of the most useful clinical clues: squamous cell carcinoma, the cancer keratoacanthoma most closely resembles, tends to grow more slowly over months rather than weeks.
Keratoacanthomas most commonly appear on the limbs, especially the hands and arms, with the trunk being the second most common site.2JAMA Dermatology. Keratoacanthoma in Kauai, Hawaii: The First Documented Incidence in a Defined Population The face, particularly the nose, cheeks, and eyelids, is another frequent location. They almost always show up on areas that get regular sun exposure, which is a strong hint about what triggers them in the first place.
The Ongoing Debate: Benign Growth or Skin Cancer?
Ask five dermatopathologists whether keratoacanthoma is a benign tumor or a variant of squamous cell carcinoma and you may get five different answers. This is not a new argument. Some experts view keratoacanthoma as a distinct, self-resolving growth that does not need aggressive treatment, while others classify it as a well-differentiated subtype of squamous cell carcinoma that simply regresses more reliably than other forms.3PubMed Central. Keratoacanthoma versus Squamous-Cell Carcinoma: Histopathological Features and Molecular Markers Gene expression studies using DNA microarrays have tried to settle the question by comparing keratoacanthoma tissue to squamous cell carcinoma tissue and normal skin, but the results have been mixed enough that the debate persists.4Modern Pathology. Gene expression profile of keratoacanthoma
The practical consequence of this debate matters for patients. If your dermatologist considers keratoacanthoma to be a type of squamous cell carcinoma, they will likely recommend removing it. If they see it as benign, they might be comfortable watching it and letting it resolve. In reality, most clinicians today lean toward treating it rather than waiting, partly because the diagnosis is never completely certain until the entire lesion has been examined under a microscope, and partly because even a small risk of an aggressive course is not worth taking when treatment is straightforward.
Who Gets Keratoacanthomas and Why
A large population study in Queensland, Australia, one of the highest-skin-cancer-incidence regions on earth, identified several strong risk factors. Being over 60 roughly tripled the risk compared with being under 50, and men were affected more often than women. Fair-skinned individuals had more than three times the risk compared with people who have olive, dark, or black skin. An inability to tan deeply, a history of prior skin cancer excisions, current smoking, and heavy alcohol use all independently raised the odds as well.5PubMed Central. Assessment of Incidence Rate and Risk Factors for Keratoacanthoma Among Residents of Queensland, Australia
Ultraviolet radiation is the dominant environmental driver, which explains the predilection for sun-exposed skin and the higher rates in fair-skinned populations living at low latitudes. Immunosuppression is another well-established risk factor. People who have received organ transplants and take anti-rejection medications develop keratoacanthomas at higher rates, and those growths can sometimes behave more aggressively than typical cases.
Physical trauma to the skin is an underappreciated trigger. Keratoacanthomas have been reported at sites of surgical scars, burns, and other injuries.6Journal of the American Academy of Dermatology. Keratoacanthoma developing in sites of previous trauma: A report of two cases and review of the literature Laser skin resurfacing, particularly fractional ablative laser procedures, has also been linked to eruptive keratoacanthomas, with most cases appearing within the first month after treatment. People who already had a history of precancerous or cancerous skin lesions were at the highest risk of this complication.7PubMed. Spontaneous resolution of eruptive papules following ablative laser resurfacing-Case report and review of laser-associated eruptive keratoacanthomas The takeaway is that disrupted skin, whether from injury or a cosmetic procedure, seems to create a favorable environment for these growths in people who are already predisposed.
How the Immune System Drives Regression
The fact that keratoacanthomas shrink without treatment is remarkable, and the mechanism behind this self-destruction appears to be immunological. Researchers comparing the immune environments of keratoacanthomas and squamous cell carcinomas found that keratoacanthomas attracted significantly more T cells into the tumor tissue, particularly a type called CD4-positive T cells. These immune cells were also more active in keratoacanthomas, showing higher levels of a signaling molecule associated with immune activation. Tumor cells within keratoacanthomas expressed higher levels of adhesion molecules that help immune cells stick to and attack cancer cells.8British Journal of Dermatology. Evidence that regression in Keratoacanthoma is immunologically mediated: a comparison with squamous cell carcinoma
In other words, keratoacanthoma cells seem to keep their immune visibility in a way that squamous cell carcinoma cells do not. The tumor essentially paints a target on itself. This immune-mediated regression explains why immunosuppressed patients, such as transplant recipients, sometimes see keratoacanthomas that do not regress on schedule or that behave more aggressively. Without a fully functional immune system, the built-in off switch does not work as well.
Genetic Instability and Molecular Findings
Despite their reputation as benign, keratoacanthomas show a surprising degree of genetic scrambling. A study using comparative genomic hybridization found chromosomal abnormalities in roughly a third of cases examined. The same frequency of genetic instability appeared regardless of whether the patient was immunosuppressed, suggesting a shared underlying pathway of tumor development. These chromosomal changes were found in both early and late stages of growth, indicating that genetic instability plays a role in both the initial expansion and the eventual involution of the tumor.9PubMed. Keratoacanthomas frequently show chromosomal aberrations as assessed by comparative genomic hybridization
Mutations in the p53 gene, one of the most commonly altered genes in skin cancer, have also been identified in keratoacanthomas, though at a lower rate than in squamous cell carcinoma. In one study, p53 gene mutations were found in a small fraction of keratoacanthomas evaluated.10JAMA Dermatology. p53 Oncoprotein Expression and Gene Mutations in Some Keratoacanthomas This finding blurs the clean line between “benign” and “malignant” even further. Keratoacanthomas carry some of the same molecular scars as skin cancer but still manage to regress. The biology here is genuinely unusual.
Telling It Apart From Squamous Cell Carcinoma
The clinical overlap between keratoacanthoma and squamous cell carcinoma is the central problem in managing these growths. Under the microscope, keratoacanthomas can look very similar to well-differentiated squamous cell carcinomas, and various staining techniques have been proposed to help pathologists tell them apart, though mixed expression patterns limit the reliability of any single test.11PubMed Central. Keratoacanthoma versus Squamous-Cell Carcinoma: Histopathological Features and Molecular Markers
More recent work has identified a promising combination of markers. A study examining staining patterns for three proteins found that keratoacanthomas showed a characteristic mosaic pattern for one marker (p16) and a graded pattern for another (p53) in the vast majority of cases, while squamous cell carcinomas showed highly abnormal, often absent, staining for one or both of those proteins. Additionally, abnormal distribution of a cell proliferation marker called Ki-67 was uncommon in keratoacanthomas but present in the majority of squamous cell carcinomas.12Pathology. Aberrant p16, p53 and Ki-67 immunohistochemistry staining patterns can distinguish solitary keratoacanthoma from cutaneous squamous cell carcinoma If validated in larger studies, this combination approach could significantly improve diagnostic accuracy. For now, when there is any doubt, the standard practice is to treat the lesion as if it were squamous cell carcinoma.
Treatment Options
Even though keratoacanthomas can resolve on their own, most dermatologists prefer active treatment. The reasons are straightforward: waiting means months of uncertainty, the scar from untreated regression is often cosmetically worse than a surgical scar, and there is always the lingering question of whether the lesion might actually be squamous cell carcinoma.
Surgical Removal
Standard surgical excision is the most common approach and has an excellent track record. In a series of 84 keratoacanthomas treated by surgical excision, there were no recurrences.13Dermatologic Surgery. Management of Keratoacanthoma: 157 Tumors Treated With Surgery or Intralesional Methotrexate Mohs micrographic surgery, a technique where tissue is removed in thin layers and examined under a microscope during the procedure, is sometimes used for keratoacanthomas in cosmetically sensitive areas. In a study of 43 keratoacanthomas treated with Mohs surgery, only one recurred, for an overall recurrence rate of about two percent.14PubMed. Keratoacanthomas treated with Mohs’ micrographic surgery (chemosurgery). A review of forty-three cases Surgical removal also has the advantage of providing tissue for pathological examination, which helps confirm the diagnosis.
Injection Therapy
When surgery is impractical or the patient prefers a less invasive approach, injecting medication directly into the tumor is an alternative. The two most studied drugs for this are methotrexate and 5-fluorouracil, both of which are chemotherapy agents that interfere with rapidly dividing cells. A head-to-head comparison found complete clearance in about 70 percent of keratoacanthomas treated with methotrexate injections and 80 percent treated with 5-fluorouracil injections, with 5-fluorouracil requiring fewer treatment sessions.15PubMed Central. Intralesional methotrexate versus 5-flurouracil in the treatment of keratoacanthoma Other case series have reported complete resolution with methotrexate alone after several injection sessions.16PubMed Central. Intralesional Methotrexate for the Treatment of Keratoacanthoma: The Neapolitan Experience Intralesional methotrexate has also been used successfully for large keratoacanthomas in difficult locations, such as the nose, where surgery might be disfiguring.17PubMed. Intralesional methotrexate as non-surgical therapy for giant keratoacanthoma on the nose
Injection therapy is particularly useful for elderly patients or those on blood thinners who may have higher risks from surgery. The main downside is that it requires multiple visits and does not provide the complete tissue sample that surgical removal does, which means the diagnosis cannot be confirmed with the same certainty.
Drug-Induced Keratoacanthomas
One of the more fascinating developments in dermatology over the past decade has been the recognition that certain targeted cancer therapies can trigger keratoacanthomas. BRAF inhibitors, drugs designed to treat melanoma by blocking a specific cancer-driving protein, can paradoxically cause keratoacanthomas and squamous cell carcinomas to pop up on the skin. The mechanism involves a signaling pathway that gets suppressed in melanoma cells but inadvertently activated in normal skin cells that carry pre-existing mutations in a related gene called RAS.18PubMed Central. RAS mutations in cutaneous squamous-cell carcinomas in patients treated with BRAF inhibitors
The paradox is striking: a drug designed to fight one type of skin cancer ends up fueling another. The accelerated growth happens because when the drug blocks one member of a signaling partnership, the partner protein can become hyperactive in cells that have the right pre-existing mutation. This was identified as a RAS-dependent process where the drug, while suppressing one part of the pathway, triggers overactivation through an alternative route.19JAMA Dermatology. Cutaneous Toxic Effects Associated With Vemurafenib and Inhibition of the BRAF Pathway Patients on BRAF inhibitors are now monitored regularly with skin checks, and the addition of a second drug called a MEK inhibitor has been shown to dramatically reduce this side effect.
More recently, immune checkpoint inhibitors, a different class of cancer therapy that works by unleashing the immune system against tumors, have also been associated with eruptive keratoacanthomas. A review identified 19 reported patients who developed eruptive keratoacanthomas after starting checkpoint inhibitor therapy, with a mean age of about 79 years.20PubMed. Eruptive keratoacanthoma secondary to immune checkpoint inhibitors: a narrative review This is somewhat ironic given that immune activation is thought to be the mechanism behind natural keratoacanthoma regression. The relationship between immune stimulation and keratoacanthoma development is clearly more complex than a simple “more immunity equals fewer tumors” equation.
Hereditary Syndromes With Multiple Keratoacanthomas
Most keratoacanthomas are isolated events. When someone develops multiple keratoacanthomas, especially at a young age, it raises the possibility of an underlying hereditary condition. Several syndromes are associated with multiple keratoacanthoma-like lesions, including Ferguson-Smith syndrome (multiple self-healing squamous tumors), Grzybowski syndrome (eruptive keratoacanthomas that can number in the hundreds or thousands), the multiple familial keratoacanthoma of Witten and Zak, and Muir-Torre syndrome.21PubMed Central. Multiple Keratoacanthoma-like Syndromes: Case Report and Literature Review
Muir-Torre syndrome deserves special attention because it is associated with internal cancers, particularly of the colon and genitourinary tract. It results from inherited defects in DNA mismatch repair genes, the same genetic pathway behind Lynch syndrome. Keratoacanthomas occur in up to about 20 percent of Muir-Torre syndrome patients.22JAAD Case Reports. Genetic heterogeneity in a patient with Muir-Torre syndrome The skin lesions sometimes appear before the internal cancers do, which means a dermatologist who recognizes the pattern can potentially trigger life-saving screening. When Muir-Torre syndrome is suspected, immunohistochemical staining of the skin tumor for mismatch repair proteins can help confirm the diagnosis, and molecular testing has identified concordance between the protein losses seen in skin lesions and those found in patients’ internal tumors.23PubMed. Identification of Muir-Torre syndrome among patients with sebaceous tumors and keratoacanthomas: role of clinical features, microsatellite instability, and immunohistochemistry
When Watchful Waiting Might Be Reasonable
Given that most keratoacanthomas resolve without treatment, there are specific situations where observation rather than intervention can be a legitimate choice. A patient whose biopsy clearly shows a classic keratoacanthoma pattern with no features worrisome for squamous cell carcinoma, who understands the timeline of regression, and who has the lesion in a location where a slightly larger scar from natural involution is acceptable might reasonably choose to wait. Elderly patients for whom surgery carries more risk, or those whose keratoacanthoma is in a location where excision would require significant tissue removal, sometimes do better with monitoring or injection therapy.
The critical caveats: watchful waiting only works when the clinician is confident in the diagnosis, the patient can reliably follow up, and the lesion does actually begin to shrink within the expected timeframe. A keratoacanthoma that is still growing after eight or more weeks, or one that does not begin to flatten by the three to four month mark, needs to be reconsidered and likely removed. Any keratoacanthoma in an immunosuppressed patient warrants a lower threshold for treatment, given the higher chance of atypical behavior in that population.
Giant and Subungual Variants
Most keratoacanthomas are under two centimeters, but a small percentage grow far beyond that threshold. Giant keratoacanthomas, loosely defined as those exceeding three centimeters, present unique management challenges because of the tissue destruction they can cause before they resolve, if they resolve at all. These oversized variants are more common on the nose and dorsum of the hands. They are also more likely to be found on closer inspection to be squamous cell carcinoma rather than true keratoacanthoma, so giant variants warrant a higher index of suspicion and more aggressive treatment.
Subungual keratoacanthomas, those that arise under the fingernail or toenail, are another unusual variant. They typically cause pain and can erode into the underlying bone of the fingertip. They are easily mistaken for other conditions including fungal infection or melanoma. Because of the confined space under the nail, subungual keratoacanthomas do not form the classic crater-shaped appearance and can be difficult to diagnose without biopsy. Surgical removal, sometimes including amputation of the fingertip in severe cases, is usually needed because spontaneous regression in this location is unreliable.