Keratoacanthoma (KA) is a skin neoplasm that histologically and pathologically resembles a squamous cell carcinoma (SCC). It is relatively common in the general population and arises from the squamous epithelium of pilosebaceous glands.
There are three clinical stages for the lesions of keratoacanthoma – proliferative, mature and resolving. The lesions grow very rapidly initially and develop into a lesion of 1-2 cm within few weeks. This active stage is followed by a slow growth phase characterized by slow involution. This may continue for many months and, sometimes, years. If not excised, the lesions may then develop into a scar after spontaneous resolution. Lesions of keratoacanthoma are round, firm and reddish in color. The papules gradually develops into a dome-shaped structure with a shiny surface. The central part of the nodule may develop ulceration and become filled with keratin. The keratin plug in the center may hang like a horn-like structure in the center.
Lesions usually develop in areas exposed to the sun like face, neck, and the dorsal side of hands and legs. In same rare cases keratoacanthoma lesions may form intraorally. Intraoral lesions lack the central keratin plug. Other unusual variants of keratoacanthomas include:
- Agglomerated lesions
- Keratoacanthoma centrifugum – lesions that grow up to 20 cm in diameter
- Giant keratoacanthoma – lesions that are larger than 9 cm
- Subungual keratoacanthoma
- Multiple eruptive keratoacanthoma
- Multiple persistent keratoacanthoma
- Generalized eruptive keratoacanthoma – characterized by severely pruritic, eruptions of numerous small follicular papules
- Keratoacanthoma associated with xeroderma pigmentosa, nevus sebaceous of Jodassohn
Histopathological studies to observe the tissue structure is a confirmatory diagnostic test for keratoacanthoma. Shave biopsy may not be helpful as it is difficult to distinguish between SCC and keratoacanthoma through this test. Thus, for histological studies, excisional or deep incisional biopsy are more beneficial. Histology of the tissue show squamous epithelium with mild pleomorphism. Central core of the lesion is composed of keratin. The lesion do not usually grow beyond the level of sweat glands as the infiltration is pseudocarcinomatous. Peripheral zones of the lesions are formed by squamous cells that contain hyperchromatic nuclei.
Differential diagnosis include actinic keratosis, cutaneous horn, merkel cell carcinoma, metastatic carcinoma of skin, molluscum contagiosum, Muir-Torre syndrome, Prurigo nodularis, SCC, and verrucous carcinoma.
Surgical excision is the primary treatment strategy for keratoacanthomas. Medical treatment is usually used only for those cases where surgery is not possible. Surgery is generally not suitable in case of multiple keratoacanthomas lesions, and also in patients who have comorbid conditions that deter surgical intervention. In some cases, surgery may not be suitable because of the size or location of the lesion. For multiple lesions, systemic retinoids like isotretinoin are recommended. Bleomycin, intralesional methotrexate, 5-flurouracil, and steroids are also considered if surgery is not suitable . Studies show that intralesional injections of methotrexate have 92% clinical resolution rate. Topical imiquimod is also being used with some success rate.
If left untreated, keratoacanthoma lesions may result in local destruction of tissues, recurrence and, occasionally, metastasis. Early diagnosis and treatment may help to prevent substantial morbidity. Keratoacanthoma lesions increase the risk of nonmelanoma skin cancer. It may also be an indicator of malignancy, particularly in the presence of certain syndromes. Thus, education and periodic follow-up are very important.
Keratoacanthoma in general have a good prognosis, particularly with excisional surgery. In most of the cases, these lesions resolve spontaneously, leaving a scar. More aggressive form of treatment is needed for recurrent lesions. Occasionally these lesions may become aggressive in growth pattern. Although lesions are usually solitary, it may be seen as multiple tumors in some patients. The lesions may enlarge in size and metastasize in some very rare cases .
The actual cause for the formation of keratoacanthomas is not clear. A number of etiological factors are considered to be involved in the process. SCC, Bowen disease, and keratoacanthoma are presumed to have common etiological factors. Sunlight is considered to be a very important causative factor in the development of these lesions. Exposure to pitch and tar increases the incidence of keratoacanthoma . Smoking is also associated with increased incidence of this neoplasm . Some other factors that are presumed to increase the risk of keratoacanthoma include HPV infection, genetic factors, and immune deficiency . A large number of keratoacanthomas are known to have chromosomal abnormalities . Treatment of metastatic renal cell carcinoma with multi-kinase inhibitor is reported to increase the chances of these lesions .
Reports of incidence of this benign tumor is negligible due to several factors including spontaneous resolution, variability in diagnosis, and paucity of epidemiological studies. Epidemiological data of keratoacanthoma resembles that of SCC and Bowen’s disease with respect to age, sex and location. The ratio of keratoacanthoma to SCC is reported to range from 1:0.6 to 1:5 in different parts of the world . This condition is more common among people with fair skin . It is often seen as a solitary lesion, and occasionally as multiple eruptive keratoacanthomas . Incidence in children is uncommon, but incidence increases with age. Prevalence is more common in the age group of 40-70 years. Incidence increases in people above 60 years. Lesions are found to be more common among men when compared to women. The male to female ratio for the occurrence is about 2:1.
A number of etiological factors are implicated in the development of keratoacanthoma lesions. Pathogenesis of this disease is presumed to be similar to that of SCC as seen from similar epidemiological features . Lesions of keratoacanthoma is believed to originate from the infundibulum of the hair follicle. Different etiological factors like exposure to sun, genetic factors, exposure to certain chemicals, and HPV infection may trigger the papillary proliferations of keratinocytes. Lesions are usually cup-shaped with a keratin-filled central region. Keratinocytes at the base develop abundant eosinophilic cytoplasm. Squamous epithelium at the lower part of the lesion may infiltrate the dermis collagen. Neutrophil, lymphocyte and eosinophil infiltration into the dermis is noted in most of the lesions. Lesions may also contain microabscesses of neutrophils within the dermal fibers. Active fibrosis can be noted in older lesions. Focal lichenoid inflammation is also observed in some keratoacanthomas.
Keratoacanthoma (KA) is a skin neoplasm that histologically and pathologically resembles a squamous cell carcinoma (SCC). It is relatively common in the general population and arises from the squamous epithelium of pilosebaceous glands and is often categorized as a variant of SCC . Lesions form in the sun exposed areas of the skin. The crater shaped lesion of keratoacanthoma grows rapidly over few weeks or months. In most of the cases, the lesion resolves on its own within few months. They leave an atrophic scar on the skin after resolution. Occasionally the lesion may become aggressive and are referred to as invasive variant.
Keratoacanthoma is associated with the autosomal-dominant disease, Ferguson-Smith syndrome, in which it appears as multiple keratoacanthomas. Surgical removal is one of the common treatment of choice.
Keratoacanthoma is a low-grade skin tumor that develops from the hair follicles. It is similar to squamous cells carcinoma (SCC). This benign tumor grows rapidly in the first few weeks to months. Most of the keratoacanthomas lesions resolve on its own within few months or years. These lesions generally do not spread to other parts of the body or become cancerous. But in some rare cases it may become aggressive and spread to lymph nodes. Once diagnosed, it is better to surgically remove these lesions to prevent further complications.
The actual cause of these lesions are not known fully. Many factors are presumed to cause these lesions. It includes exposure to sun, exposure to chemicals like pitch and tar, trauma, certain genetic factors, and human papilloma virus. Keratoacanthoma and SCC have similar pathogenesis and epidemiology. This suggests that both the conditions are caused by exposure to sunlight. People who have prolonged exposure to sunlight have a high risk of developing these lesions. Peak incidence is usually seen after 60 years of age. Lesions are very rare in those below 20 years. Occurrence of these lesions is more common among fair-skinned people. Males are more frequently affected when compared to females.
There are three stages in the development of keratoacanthoma lesions – proliferative, growth and resolving. In the proliferative stage, the lesions grow rapidly and develop into 1-2 cm lesions within few weeks. This is followed by a slow grow phase which may continue or few months. Lesions may spontaneously resolve over 4-6 months later. Lesions usually occur in the sun exposed areas like face, and dorsal side of arms and legs. They form as round, firm and reddish papules which grows into a dome-shaped nodule with a smooth shiny surface. The central part of the nodule may be filled with keratin. If left untreated, the nodule may resolve spontaneously, leaving a scar. In some rare cases, multiple keratoacanthoma may develop as multiple lesions as a part of larger group of symptoms. In most of the cases, these lesions are painless. Biopsy of the lesion is an important diagnostic criteria for keratoacanthoma. A minor surgery is usually the first level of treatment for these lesions. If there are multiple lesions, or if the size or location of the lesions are difficult for performing surgical procedures, medications are recommended. Systemic retinoid drugs are suggested for patients with multiple lesions. Intralesional methotrexate, 5-flurouracil, bleomycin and steroids are also used in the treatment of these lesions.
Those who develop keratoacanthoma have an increased risk of recurrence. There are increased chances of non-melanoma skin cancer. Thus preventing the occurrence of these lesions is very important. Avoiding exposure to sun by wearing sunscreen and hats helps to prevent the occurrence of these lesions. One should take care to regularly examine the skin for new moles.
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