Keratoacanthomas are dome-shaped skin lesions that grow rapidly. They often have a central crater or keratin plug, resembling a small volcano. While visually similar to certain skin cancers, especially squamous cell carcinoma, their behavior and natural history differ significantly.
What are Keratoacanthomas?
Keratoacanthomas typically begin as a small, round, flesh-colored or slightly reddish bump, often appearing on sun-exposed areas like the face, forearms, or hands. This initial growth phase is rapid, with the lesion often reaching 1 to 2.5 centimeters in diameter within one to two months. During this period, a characteristic central keratin plug develops, giving the lesion its crater-like appearance.
Following this rapid proliferation, the keratoacanthoma enters a maturation phase, where it maintains its size and crateriform shape for several weeks to months. The final stage is involution, during which the lesion may spontaneously regress, often leaving a depressed, puckered scar. Although generally considered benign, keratoacanthomas are a type of squamous cell proliferation, meaning they originate from the outer layer of the skin.
Causes and Risk Factors
The precise cause of keratoacanthomas is not fully understood, but several factors are believed to contribute to their development. Prolonged exposure to ultraviolet (UV) radiation from the sun is a risk factor, as it can damage skin DNA and trigger the formation of these lesions. This is why keratoacanthomas commonly appear on sun-exposed areas and are more prevalent in fair-skinned individuals, particularly those over 60 years of age, and in men, possibly due to higher outdoor occupational exposure.
Other factors include exposure to certain chemical carcinogens, such as tar, pitch, and mineral oils, and cigarette smoking. Trauma or local injury to the skin, human papillomavirus (HPV) infection, and a weakened immune system (due to immunosuppression or certain cancer immunotherapies) are also risk factors. Genetic predispositions, such as Muir-Torre syndrome or Ferguson-Smith syndrome, can increase the likelihood of developing multiple keratoacanthomas.
How Keratoacanthomas are Diagnosed and Distinguished
Diagnosing keratoacanthomas is challenging due to their close resemblance to invasive squamous cell carcinoma (SCC), a form of skin cancer. Diagnosis begins with a clinical examination, where a doctor assesses the lesion’s appearance, including its dome shape, central crater, and rapid growth history. However, visual assessment alone is insufficient for a definitive diagnosis due to close similarities with SCC.
To confirm diagnosis and distinguish from SCC, a biopsy is necessary. This involves removing a piece of the lesion for microscopic examination by a pathologist. An excisional biopsy, which removes the entire lesion, is preferred as it allows pathologists to evaluate its full features. If full excision is not immediately feasible, an incisional or shave biopsy may be performed, though partial samples can make differentiation more difficult.
Under the microscope, pathologists look for specific features. Keratoacanthomas show symmetry and well-differentiated squamous cell proliferation with a central keratin-filled crater. In contrast, SCCs may exhibit more cellular atypia, an irregular keratin-epithelial interface, and invasion into surrounding tissues. While some pathologists classify keratoacanthomas as a variant of SCC, differentiation is important due to their differing biological behaviors: keratoacanthomas regress spontaneously, while SCCs can progress and potentially metastasize.
Treatment and Prognosis
Keratoacanthoma management involves various approaches, considering their potential for spontaneous regression and resemblance to squamous cell carcinoma. While “watchful waiting” is an option for some lesions due to their tendency to resolve, uncertainty in differentiating them from SCC often leads to proactive treatment. Surgical excision is a common and preferred treatment, allowing complete lesion removal and providing tissue for histopathological examination to rule out malignancy. This ensures complete removal and comprehensive assessment.
Other treatment options include curettage and electrodesiccation (scraping and cauterizing the base), and cryotherapy (freezing the lesion). For patients with multiple lesions or those not suitable for surgery due to size or location, topical or intralesional medications are options. These include agents like 5-fluorouracil, methotrexate, or bleomycin injected directly into the lesion. Oral retinoic acid may also be an option for patients with multiple or recurrent keratoacanthomas.
The prognosis for keratoacanthomas is good, especially after surgical excision. While spontaneous regression is common, leaving a scar, recurrence after treatment is uncommon but can occur, particularly with larger lesions or those on the lower legs. Patients with a history of keratoacanthoma, especially with sun exposure, should be monitored for new skin lesions, as they may have a higher risk of other sun-related skin cancers.