Keratoacanthoma and squamous cell carcinoma can look remarkably similar under a microscope and to the naked eye, yet they typically behave in dramatically different ways. A keratoacanthoma (KA) is a rapidly growing skin tumor that, in most cases, will shrink and disappear on its own within a few months, while a squamous cell carcinoma (SCC) is a genuine skin cancer that can invade surrounding tissue, spread to lymph nodes, and sometimes prove fatal if left untreated. The catch is that distinguishing the two remains one of the trickiest problems in dermatology, and experts still argue about whether they are truly separate entities or points on the same biological spectrum.
The Classification Debate
Dermatologists and pathologists have been going back and forth on keratoacanthoma’s identity for decades. One camp considers KA a benign, self-limiting growth that arises from hair follicle cells and resolves without treatment. The other camp argues that KA is better understood as a well-differentiated variant of squamous cell carcinoma that happens to regress more often than typical SCC. This is not just an academic turf war. How a clinician classifies a KA directly shapes whether they recommend watchful waiting or immediate surgery.1PubMed. Keratoacanthoma: Update on the Debate
The tension exists because KA shares many histological and molecular features with well-differentiated SCC, yet its clinical course is strikingly different. Many pathologists now hedge their reports with a combined diagnosis such as “well-differentiated squamous neoplasm, keratoacanthoma type,” essentially acknowledging that the line between the two is blurry. For the patient, the practical consequence is that most clinicians treat KA as though it could be SCC unless there are very clear signs of spontaneous regression already underway.2PubMed Central. Keratoacanthoma versus Squamous-Cell Carcinoma: Histopathological Features and Molecular Markers
How They Look and Grow Differently
The single most useful clinical clue is timing. A keratoacanthoma is famously fast. It typically balloons from nothing to a firm, dome-shaped nodule over just four to eight weeks, often reaching one to two centimeters across. The classic shape is a volcano-like mound with a central crater filled with a keratin plug, a hard, waxy-looking core. After this explosive growth phase, the tumor usually plateaus for a few weeks and then gradually shrinks over two to six months, sometimes leaving behind a depressed, slightly pitted scar.
Squamous cell carcinoma, by contrast, tends to develop more slowly and progressively over months to years. It appears as a rough, scaly plaque or an ulcerated nodule that bleeds easily and does not resolve. SCC lacks the neat symmetry and defined crater that characterize a textbook KA. While both tumors favor sun-exposed skin in fair-skinned, middle-aged, and older adults, SCC is more likely to appear on the lips, ears, and backs of hands, whereas KA has a particular affinity for the central face and arms.
A disease duration of three months or less at the time of presentation is itself a strong pointer toward KA rather than SCC, and dermoscopic features can sharpen that initial impression. KA under dermoscopy typically shows a layered pattern: a dense yellow-white central keratin mass, an ivory-white peripheral zone, and hairpin-shaped blood vessels radiating symmetrically outward from the core.3PubMed Central. Dermoscopy of Keratinizing Skin Tumors: Cutaneous Squamous Cell Carcinoma, Keratoacanthoma, and Bowen’s Disease A recent study found that a “bud-like appearance” on dermoscopy was present in about 92% of KA cases compared with only 14% of well-differentiated SCCs, and hairpin vessels showed up in roughly 45% of KAs versus under 6% of SCCs. Combining the bud-like appearance with a disease duration of six months or less yielded a specificity above 97%.4PubMed Central. The utility of a dermoscopic ‘bud-like appearance’ combined with disease duration in differentiating keratoacanthoma from well-differentiated cSCC: A preliminary study
Why Keratoacanthomas Regress on Their Own
The fact that a tumor can grow aggressively and then vanish without treatment is unusual enough to demand an explanation. Research points to at least two mechanisms working together. The first is an immune response. KA tumors contain significantly higher numbers of certain immune cells, particularly activated T cells, infiltrating the tumor compared with SCC. These immune cells express markers indicating they are actively fighting the tumor, and the tumor cells themselves express adhesion molecules that help the immune system target them. SCC tumors, by comparison, appear to evade this immune attention more successfully.5British Journal of Dermatology. Evidence that regression in Keratoacanthoma is immunologically mediated: a comparison with squamous cell carcinoma
The second mechanism involves signaling pathways borrowed from normal hair follicle biology. Keratoacanthomas appear to hijack Wnt signaling, the same molecular program that drives hair follicle regeneration, to fuel their initial burst of growth. The tumor’s eventual collapse is linked to activation of the retinoic acid pathway, which puts the brakes on Wnt signaling and tips the balance toward regression. In other words, the tumor’s own biology contains a built-in off switch that SCC apparently lacks.6PubMed Central. Spontaneous tumour regression in keratoacanthomas is driven by Wnt/retinoic acid signalling cross-talk
These two findings together help explain why KA sits in such an awkward spot between benign and malignant. The tumor grows like a cancer in its early phase, but the body has multiple mechanisms to shut it down that simply do not kick in, or do not kick in effectively, for conventional SCC.
The Limits of a Biopsy
You might assume that putting a tissue sample under a microscope would settle the question, but pathologists routinely struggle with it. KA and well-differentiated SCC share many overlapping features: large, glassy-looking squamous cells, a pushing margin of growth, and keratin production. The key architectural difference is that a well-oriented KA biopsy shows a symmetrical, cup-shaped silhouette with a central keratin-filled crater, while SCC tends to invade in irregular tongues and cords. But this distinction depends on getting a complete excisional or deep shave biopsy. A superficial punch biopsy that samples only part of the lesion often misses the overall architecture, leaving the pathologist with an inconclusive report.7PubMed Central. Giant keratoacanthoma of the upper extremity treated with mohs micrographic surgery: a case report and review of current treatment modalities
Researchers have tried various molecular markers to break the tie. Markers like p53 and PCNA (a protein associated with cell division) were investigated early on, but studies found no statistically significant difference in their expression between KA and KA-like SCC. The staining patterns overlapped so much that these markers proved unhelpful for individual cases.8PubMed. Keratoacanthoma versus squamous cell carcinoma. An immunohistochemical reappraisal of p53 protein and proliferating cell nuclear antigen expression in keratoacanthoma-like tumors This overlap is actually one of the strongest arguments for viewing KA and SCC as a biological spectrum rather than two neatly separated diagnoses.
How Each Is Treated
Because a misdiagnosed SCC could spread and become dangerous, most dermatologists treat a suspected KA much like they would treat an SCC: they remove it. Surgical excision with clear margins is the standard first-line approach for both. For larger or anatomically tricky lesions, Mohs micrographic surgery, where the surgeon maps and examines tissue layer by layer during the procedure, offers the highest cure rate while preserving surrounding healthy tissue.7PubMed Central. Giant keratoacanthoma of the upper extremity treated with mohs micrographic surgery: a case report and review of current treatment modalities
For patients who are poor surgical candidates or who have multiple KAs, intralesional injections offer an alternative. A comparison of methotrexate and 5-fluorouracil injected directly into KA tumors found complete clearance in 70% of the methotrexate group and 80% of the 5-fluorouracil group, with no statistically significant difference between the two. The 5-fluorouracil group did achieve clearance in fewer sessions, typically requiring about two injections compared with three for methotrexate.9PubMed Central. Intralesional methotrexate versus 5-flurouracil in the treatment of keratoacanthoma
Watchful waiting, simply observing the lesion to see if it regresses, is only realistic when the clinician is highly confident the lesion is a KA, the patient can be monitored closely, and the tumor is in a cosmetically forgiving location. Even then, many patients prefer removal because the regressing tumor can leave an unsightly scar and the uncertainty is difficult to live with.
Does Perineural Invasion Change the Picture?
One of the feared complications with squamous cell carcinoma is perineural invasion, where tumor cells grow along nerves. This finding on a pathology report usually prompts aggressive follow-up because it is associated with higher recurrence and metastasis rates in SCC. In keratoacanthoma, however, perineural invasion appears to carry a very different meaning. A review of 40 KA cases with perineural invasion found no metastases and no deaths attributable to the nerve involvement. Local recurrence occurred in only one case, and the authors did not consider the perineural invasion the cause.10PubMed. Keratoacanthoma with perineural invasion: a report of 40 cases
This finding underscores just how differently these tumors behave biologically. A feature that would be alarming in an SCC report may be essentially benign in the context of a true KA. The problem, of course, circles back to the diagnostic challenge: the perineural invasion data only reassures you if you can be confident the original diagnosis was actually KA and not a misclassified SCC.
When Medications Trigger These Tumors
An unexpected chapter in the KA-versus-SCC story emerged with the rise of BRAF inhibitors, a class of targeted therapy used to treat melanoma. Patients taking drugs like vemurafenib began developing keratoacanthomas and cutaneous SCCs at surprisingly high rates, sometimes within weeks of starting treatment. The mechanism involves a paradox: while BRAF inhibitors block signaling in melanoma cells that carry a specific BRAF mutation, they can actually accelerate signaling through the same pathway in skin cells that harbor pre-existing RAS mutations. This paradoxical activation promotes uncontrolled cell growth in those cells.11PubMed Central. RAS mutations in cutaneous squamous-cell carcinomas in patients treated with BRAF inhibitors
Preclinical experiments confirmed this: treating cell lines that carry a mutant HRAS gene with a BRAF inhibitor led to increased colony formation and activated a gene signature associated with the growth-promoting signaling pathway.12PubMed Central. Paradoxical oncogenesis and the long term consequences of BRAF inhibition in melanoma The clinical solution has been to combine BRAF inhibitors with MEK inhibitors, which blocks the paradoxical activation and greatly reduces the occurrence of these secondary skin tumors. For dermatologists, BRAF-inhibitor-associated KAs and SCCs are a reminder that the relationship between the two diagnoses is not just academic; it has real consequences for patients on certain cancer therapies.
Immunosuppression and Transplant Recipients
Organ transplant recipients sit at a unique crossroads of KA and SCC risk. The immunosuppressive medications they take to prevent organ rejection are well known to increase the incidence of skin cancers, particularly squamous cell carcinoma, which in transplant recipients tends to be more aggressive, recur more often, and carry a higher mortality rate than in the general population. Reports also suggest that these patients develop keratoacanthomas at elevated rates.13PubMed Central. Keratoacanthoma Accompanied by Multiple Lung Squamous Cell Carcinomas Developing in a Renal Transplant Recipient
The immunological data discussed earlier helps explain this pattern. If KA regression depends heavily on an active immune response involving T cells and adhesion molecules, then dampening that immune response with medications would logically make it harder for the body to shut down a KA. In transplant recipients, the line between KA and SCC may be even blurrier than in the general population, because the immune “off switch” that normally drives KA regression could be partially or fully disabled. Clinicians generally treat any suspicious keratinizing tumor in a transplant patient aggressively rather than waiting to see if it resolves.
Trauma as a Trigger
A less common but well-documented trigger for keratoacanthoma is physical injury to the skin. Case reports describe KAs arising at surgical scars, burn sites, and even puncture wounds. In one instance, a construction worker developed a rapidly growing KA on the back of his hand after a penetrating injury.14PubMed Central. Post-traumatic keratoacanthoma of the hand: A case report A review of the literature on post-traumatic KA found that while the phenomenon is rare, there does appear to be a real predilection for these tumors to arise at trauma sites, suggesting that the wound-healing process itself can create conditions favorable for KA development.15Journal of the American Academy of Dermatology. Keratoacanthoma developing in sites of previous trauma: A report of two cases and review of the literature
This is notable because it provides circumstantial support for the idea that KA originates from hair follicle stem cells or other regenerative cell populations that become activated during wound repair. When those cells proliferate to close a wound, something occasionally goes wrong and a KA results. SCC, while it can also arise in chronic wounds or scars (the classic Marjolin ulcer), typically takes much longer to develop in that context and does not share the same rapid, self-limiting trajectory.
A Genetic Syndrome That Produces Self-Healing Tumors
Perhaps the strongest natural evidence that KA is biologically distinct from SCC comes from a rare inherited condition called Ferguson-Smith disease, also known as multiple self-healing squamous epithelioma. People with this syndrome develop KA-like tumors repeatedly throughout their lives, and the tumors consistently regress on their own. Genetic research traced the condition to loss-of-function mutations in a gene called TGFBR1, which acts as a tumor suppressor. Investigation further revealed that the disease is actually digenic, meaning a second linked genetic locus is also necessary for the tumors to develop.16PubMed. Digenic/multilocus aetiology of multiple self-healing squamous epithelioma (Ferguson-Smith disease): TGFBR1 and a second linked locus
Ferguson-Smith disease is instructive because it effectively replays the KA lifecycle over and over in the same patient, yet these patients do not develop metastatic disease. If KA were truly just well-differentiated SCC, you would expect at least some of these repeatedly occurring tumors to progress. The fact that they reliably self-resolve, generation after generation, suggests a fundamentally different biology at work, one where the tumor’s genetic programming includes an expiration date that conventional SCC lacks.