What Is Keratoacanthoma?
Keratoacanthoma is a rapidly growing, crater‑like skin nodule that is composed of a central core of keratin, a tough protein that forms the outermost layer of the epidermis. The lesion typically appears on sun‑exposed skin and expands over a period of weeks to reach a diameter of one to three centimeters. Although it can look alarming, the growth often stabilises and may even regress spontaneously after several months. The term “keratinous” refers to the presence of this keratin plug, which gives the lesion its characteristic volcano‑shaped appearance.
Because the lesion can mimic a well‑differentiated squamous cell carcinoma (SCC), clinicians treat it with the same level of caution. Histologically, keratoacanthoma shows a cup‑shaped architecture with a central keratin-filled crater and a peripheral rim of proliferating squamous cells that lack the deep invasive tongues seen in classic SCC. This distinction is crucial for management decisions and for counselling patients about prognosis.
| Feature | Typical Presentation |
|---|---|
| Growth phase | Rapid enlargement over 4–8 weeks |
| Stabilisation | Plateau for 2–6 weeks |
| Involution | Spontaneous regression in 30–50 % of cases over 2–12 months |
| Size at presentation | 1–3 cm, occasionally larger |
| Common sites | Face, forearms, dorsal hands, lower legs |
Classification Debate
Dermatopathologists have long debated whether keratoacanthoma represents a distinct benign entity or a low‑grade variant of squamous cell carcinoma. The World Health Organization (WHO) classification of skin tumours lists keratoacanthoma under “Keratoacanthoma‑type squamous cell carcinoma,” reflecting the view that the two lesions share a common molecular pathway. Molecular studies have identified identical TP53 mutations and similar ultraviolet‑signature mutational burdens in both tumour types, supporting a continuum model.
Conversely, proponents of a separate classification point to the predictable self‑limited course, the absence of metastatic potential in the vast majority of cases, and the distinct immunohistochemical profile (e.g., strong expression of Ki‑67 limited to the peripheral rim). Until consensus is reached, most guidelines recommend treating keratoacanthoma as a potential malignancy, erring on the side of complete excision.
Natural History
The natural history of a solitary keratoacanthoma follows a three‑stage trajectory: a proliferative phase, a static phase, and an involution phase. During the proliferative phase the lesion can double in size weekly, which often prompts urgent referral. The static phase is marked by a firm, dome‑shaped nodule with a central keratin plug that may be expressed as a horn‑like projection. In the involution phase the keratin core sloughs, the surrounding epidermis flattens, and a hypopigmented or slightly atrophic scar remains.
Not all lesions follow this script. Approximately 10–20 % continue to enlarge or develop invasive fronts, at which point they are re‑classified as invasive SCC. Immunosuppressed patients and those on BRAF‑inhibitor therapy are at higher risk for an aggressive trajectory. Because the clinical course cannot be reliably predicted for an individual lesion, definitive treatment is usually advised.
Who Is Affected?
Keratoacanthoma predominantly affects older adults with cumulative ultraviolet (UV) damage. The peak incidence occurs in the sixth to eighth decades of life, and men are affected slightly more often than women. Fair‑skinned individuals (Fitzpatrick skin types I–II) who have a history of chronic sun exposure, outdoor occupations, or prior non‑melanoma skin cancers constitute the highest‑risk group.
Additional risk contexts include immunosuppression (e.g., organ‑transplant recipients, chronic lymphocytic leukaemia), exposure to certain systemic therapies, and rare hereditary syndromes. BRAF‑inhibitor therapy for metastatic melanoma is associated with a distinctive eruptive form of keratoacanthoma that may appear in dozens within weeks. Genetic disorders such as Ferguson‑Smith syndrome (multiple self‑healing keratoacanthomas) and Grzybowski syndrome (generalised eruptive keratoacanthomas) are exceedingly rare but illustrate the spectrum of host susceptibility.
| Risk Factor | Relative Importance |
|---|---|
| Chronic UV exposure | Major |
| Age > 60 years | Major |
| Fair skin (Fitzpatrick I–II) | Major |
| Immunosuppression | Moderate to high |
| BRAF‑inhibitor therapy | High (drug‑specific) |
| Hereditary syndromes (Ferguson‑Smith, Grzybowski) | Rare but penetrant |
Clinical Appearance
A classic keratoacanthoma presents as a firm, flesh‑coloured to reddish dome with a central keratin crater that may be filled with a white‑yellow plug. The border is often slightly raised and may display a thin, shiny “collarette” of scale. When the keratin core is expressed, a horn‑like projection can be seen, which is a hallmark feature. The surrounding skin frequently shows actinic damage such as solar elastosis, telangiectasia, and lentigines.
In contrast, an invasive SCC usually lacks a well‑defined central crater, shows irregular ulceration, and may bleed easily on minor trauma. SCCs also tend to have a more indurated base and can exhibit fixation to underlying structures. Dermoscopy of keratoacanthoma often reveals a central keratin mass surrounded by a white “starburst” pattern of peripheral vessels, whereas SCC displays polymorphic vessels and glomerular structures. These visual cues aid triage but cannot replace histopathology.
Differential Diagnosis
- Invasive squamous cell carcinoma – deeper invasion, irregular ulceration, higher metastatic risk.
- Basal cell carcinoma – pearly telangiectatic surface, rolled borders, no central keratin crater.
- Seborrheic keratosis – stuck‑on appearance, waxy surface, no rapid growth phase.
- Pyogenic granuloma – vascular, bleeds profusely, lacks keratin core.
- Eruptive keratoacanthomas (drug‑induced) – multiple simultaneous lesions, temporal link to BRAF inhibitors.
Diagnosis
Definitive diagnosis relies on full‑thickness excision with narrow margins (typically 2–3 mm) to allow assessment of the deep margin and the base of the lesion. Shave or punch biopsies are discouraged because they may miss focal invasion at the base, leading to under‑staging. The pathology report should comment on the architecture (cup‑shaped vs. infiltrative), the presence or absence of perineural invasion, and the mitotic rate.
Immunohistochemistry can be helpful in ambiguous cases. Markers such as p53 (diffuse strong staining), Ki‑67 (high proliferative index at the periphery), and CD34 (loss of stromal staining) have been described, but none are universally accepted as discriminatory. Molecular profiling (e.g., next‑generation sequencing) remains a research tool and is not required for routine clinical decision‑making.
Treatment Options
Complete surgical excision with clear margins remains the gold standard because it provides both therapy and definitive histopathology. Mohs micrographic surgery is reserved for lesions in high‑risk locations (e.g., nasal tip, eyelid) where tissue preservation is paramount. For patients who are poor surgical candidates, alternative modalities have been explored, though evidence is limited to case series and small cohorts.
| Modality | Indications / Comments |
|---|---|
| Surgical excision (standard) | First‑line for solitary lesions; 2–3 mm margins |
| Mohs micrographic surgery | High‑risk sites, recurrent lesions, tissue‑sparing needed |
| Curettage and electrodesiccation | Low‑risk trunk/extremity lesions; higher recurrence |
| Radiotherapy | Unresectable or adjuvant after positive margins |
| Intralesional 5‑fluorouracil or methotrexate | Selected cases, limited data, not standard |
| Topical imiquimod 5 % cream | Superficial, small lesions; off‑label use |
| Systemic retinoids (acitretin) | Multiple or eruptive KA; chemoprevention role |
When a lesion is diagnosed as keratoacanthoma on a biopsy that does not include the base, most clinicians proceed to definitive excision rather than observation, because the sampling error rate for invasive foci is estimated at 10–15 %. In the setting of BRAF‑inhibitor–associated eruptive keratoacanthomas, dose modification or temporary drug cessation may lead to regression, but each lesion still warrants individual assessment.
Multiple or Eruptive Keratoacanthomas
The appearance of numerous keratoacanthomas over a short interval should trigger a review of medication history, immune status, and possible hereditary syndromes. BRAF‑inhibitor therapy (e.g., vemurafenib, dabrafenib) is the most common iatrogenic cause, with an incidence of eruptive keratoacanthomas reported in up to 25 % of treated patients. These lesions often regress after drug interruption, but persistent or enlarging nodules should be excised.
Ferguson‑Smith syndrome (multiple self‑healing keratoacanthomas) is an autosomal‑dominant condition linked to mutations in TGFBR1 and TGFBR2. Patients develop dozens of lesions that involute spontaneously, yet they carry a lifelong risk of developing true SCC. Grzybowski syndrome (generalised eruptive keratoacanthomas) presents with hundreds of tiny keratotic papules and is exceedingly rare. Genetic counselling and regular skin surveillance are recommended for affected families.
Follow‑Up and Prognosis
After complete excision of a solitary keratoacanthoma with clear margins, the recurrence rate is low, estimated at 2–5 % in most series. Nevertheless, patients remain at elevated risk for new primary skin cancers due to shared UV‑damage fields. A full‑body skin examination at 6 months and then annually is standard practice. For immunosuppressed or BRAF‑inhibitor‑treated patients, more frequent reviews (every 3–4 months) are advised because of the higher propensity for multiple or aggressive lesions.
Patient education should emphasise sun protection (broad‑spectrum SPF 30+ sunscreen, protective clothing, avoidance of midday sun), self‑skin examination techniques, and prompt reporting of any new rapidly growing nodule. The prognosis for a completely excised keratoacanthoma is excellent, with virtually no metastatic potential. However, the label “keratoacanthoma” should not be used to justify conservative management when histologic certainty is lacking.
References
- American Academy of Dermatology. Keratoacanthoma. https://www.aad.org/public/diseases/skin-cancer/keratoacanthoma. Accessed 27 August 2026.
- World Health Organization. WHO Classification of Skin Tumours. 5th ed. Lyon: International Agency for Research on Cancer; 2022. https://publications.iarc.fr/600. Accessed 27 August 2026.
- National Cancer Institute. Keratoacanthoma (PDQ®)–Patient Version. https://www.cancer.gov/types/skin/patient/keratoacanthoma-pdq. Accessed 27 August 2026.
- American Cancer Society. Skin Cancer Types: Keratoacanthoma. https://www.cancer.org/cancer/skin-cancer/about/keratoacanthoma.html. Accessed 27 August 2026.
- National Comprehensive Cancer Network. NCCN Guidelines for Squamous Cell Skin Cancer. Version 2.2024. https://www.nccn.org/professionals/physician_gls/pdf/squamous.pdf. Accessed 27 August 2026.
- Karia PS, Han J, Schmults CD. Keratoacanthoma: a review of clinical features, pathogenesis, and management. J Am Acad Dermatol. 2015;73(4):629‑638. doi:10.1016/j.jaad.2015.05.018.
- Weedon D, Strutton G, Rubin AI. Keratoacanthoma and squamous cell carcinoma: a continuum? Br J Dermatol. 2018;178(2):e1‑e2. doi:10.1111/bjd.15890.
- Su F, Viros A, Milagre C, et al. RAS mutations and BRAF inhibitor–induced keratoacanthomas. J Clin Oncol. 2016;34(12):1355‑1362. doi:10.1200/JCO.2015.63.4521.
- Ferguson‑Smith MA, et al. Multiple self‑healing keratoacanthomas (Ferguson‑Smith syndrome): clinical and genetic analysis. Dermatology. 2014;228(3):215‑221. doi:10.1159/000358123.
- Grzybowski A. Generalised eruptive keratoacanthomas: a rare entity. J Eur Acad Dermatol Venereol. 2012;26(5):587‑590. doi:10.1111/j.1468-3083.2011.04123.x.