Keratoacanthoma (molluscum sebaceum)

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This is a lesion that occurs in elderly patients, most commonly men, in sun – exposed areas such as the face and nose (75%), although it may occur on any skin surface. It appears as a rapidly growing nodule, which may reach 3 cm or more in diameter in a few weeks, with a characteristic central crater filled by a keratin plug. It closely resembles a squamous carcinoma or rodent ulcer in appearance, and it is only the history of very rapid growth that helps differentiate it from the latter.

Histologically, it consists of a central crater filled with keratin surrounded by hypertrophied squamous stratified epithelium. There is no invasion of the surrounding tissues.

If left untreated, the lesion disappears over a period of 4 – 5 months, leaving a faint white scar. It appears to be of hair follicle origin, and may be associated with a minor injury.


Multiple etiologies have been suggested including ultraviolet (UV) radiation, exposure to chemical carcinogens, immunosuppression, use of BRAF inhibitors, genetic predisposition including mutations of p53 or H-Ras, viral exposure including human papillomavirus (HPV), and recent trauma or surgery to the location. Keratoacanthoma can be commonly associated with syndromes such as autosomal dominant Muir-Torre syndrome, autosomal dominant Ferguson-Smith syndrome, autosomal recessive xeroderma pigmentosum, X-linked dominant incontinentia pigmenti, autosomal dominant Witten-Zak. The rare sporadic pruritic generalized eruption of keratoacanthoma is known as generalized eruptive keratoacanthoma of Gryzbowski


Keratoacanthomas are reported in all age groups although they rarely appear before the age of 20. The peak incidence of solitary keratoacanthoma is between 50 and 69 years of age. They occur more frequently in men with a male to female ratio of 2:1. The majority have been described in fair skin individuals with the highest rates found in those with Fitzpatrick I-III classification. Additionally, they occur more frequently in immunosuppressed individuals and are more locally invasive. Keratoacanthomas and squamous cell carcinoma share many epidemiological features.


Originating in the pilosebaceous unit, keratoacanthomas are derived from an abnormality leading to hyperkeratosis of the infundibulum. Although associated with hair-bearing areas and sunlight, these tumors can develop in other areas including within the mouth, lip, gingiva, hard palate, and other mucosal surfaces. There are 3 stages of keratoacanthomas. These stages include proliferation, maturation, and involution. In the proliferative phase, rapid growth occurs up to approximately 6 to 8 weeks. The maturation phase lasts several weeks to months where the keratoacanthoma maintains its crateriform appearance. Involution is the final stage where the keratoacanthoma regresses into an atrophic scar. Stages are variable in length of time. Ultraviolet light, trauma, human papillomavirus (HPV), genetic factors, immune status, use of hedgehog pathway inhibitors for basal cell carcinoma (BCC), and use of BRAF inhibitors in patients with melanoma have been implicated as risk factors.


Differential Diagnosis

Differential diagnosis of a lesion consistent with a keratoacanthoma includes squamous cell carcinoma, amelanotic melanoma, molluscum contagiosum, prurigo nodularis, metastatic lesion to the skin, Merkel cell carcinoma, nodular basal cell carcinoma, ulcerative basal cell carcinoma, nodular Kaposi sarcoma, hypertrophic lichen planus, deep fungal infection, atypical mycobacterial infection, foreign body reaction, and verruca vulgaris.


Keratoacanthoma has an excellent prognosis following surgical excision. Generally, patients with keratoacanthoma will have a history of sun exposure and will need to be followed for the development of new primary skin cancers. Metastases are rare, but there are reports of this in addition to perineural spread in literature.

Treatment / Management

While keratoacanthoma is recognized as benign, treatment is recommended due to the relation to squamous cell carcinoma. Treatment of choice consists of an excisional procedure with 4 mm margins. Lesions less than 2 cm on the extremities may be treated with electrodessication and curettage.

Aggressive large tumors that are greater than 2 cm in cosmetically sensitive areas require tissue sparing and should consider Mohs micrographic surgery. For those with perineural invasion, Mohs micrographic surgery is the treatment of choice.

Nonsurgical interventions have been limited to case reports and retrospective reviews consisting of topical 5% imiquimod cream, topical 5% 5-fluorouracil (5-FU) cream, intralesional methotrexate injections, intralesional bleomycin, intralesional 5-FU, and oral isotretinoin. Of the intralesional therapy, 5-FU and methotrexate have the most data.

Intralesional 5-FU have been administered at a dose of 40 to 75 mg weekly for 3 to 8 weeks. Some data supports a 98% response rate. A case series for intralesional methotrexate have been in smaller numbers of patients but noted regression in 83% to 100% of patients.

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