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Therapeutics in Dermatology
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Keratoacanthomas

18 April 2012, by RICHARD M.-A.

1 - DIAGNOSIS

1.1 - CLINICAL DIAGNOSIS

Keratoacanthoma is a skin tumour that is believed to originate from the hair follicle. The tumour typically emerges very suddenly (even brutally), grows rapidly and then regresses spontaneously through various morphological stages and changes.

Clinically, a keratoacanthoma presents as an erythematous, scaly papule which is often overlooked in its initial stages ; then, within the space of a few days, it suddenly develops into a symmetrical, well-defined, purple, nodular tumour that sits on the surface of the skin. The nodule consists of a peripheral ring covered with smooth and telangiectasic epithelium surrounding a central crater filled with a keratin plug. The keratoacanthoma involutes spontaneously within two to 12 weeks, as the raised peripheral ring flattens out and the contents of the central crater are eliminated, leaving a sometimes unsightly puckered scar, often scattered with milia. Widespread ulceration may develop, mainly on the nose and eyelids.

Several variants of keratoacanthoma have been reported on the literature but since their biological and histological profiles are similar, keratoacanthoma is considered to be a separate histoclinical entity, irrespective of the conditions under which it arises or the number of lesions observed. 

• The single or solitary form is most common, particularly in men, most often after the fifth decade of life. The tumour is most often located on sun-exposed and hair-covered areas such as the face, leg below the knee and forearm. It generally measures less than 3 cm in diameter and regresses spontaneously within two to three months.

• The extensive and ulcerative form, or keratoacanthoma centrifugum marginatum, is rare. This is a single centrifugally-growing keratoacanthoma characterised by peripheral growth which can be very destructive since it has a very active polycyclic border despite concomitant central healing. It has been described on the lips, nose and eyelids.

• The giant forms which are probably actually the result of confluence of several keratoacanthomas ; these are found mainly on the forearms.

• Multiple keratoacanthomas may also arise. These have been reported in three different contexts :

— familial keratoacanthomas (Ferguson-Smith type) are seen in young patients with an autosomal dominant genodermatosis who will present with rapidly-regressing keratoacanthomas affecting all parts of the body throughout their lives ;

— generalised eruptive and sporadic keratoacanthomas (Grzybowski type) which are mainly seen in patients with a cell immune deficiency (lymphomas, etc.) and who develop hundreds of tiny keratoacanthomas on the skin and mucosa ;

— Muir-Torre syndrome is another genodermatosis in which multiple keratoacanthomas combine with sebaceous tumours to become cutaneous markers of primary visceral (colon) or larynx carcinomas.

1.2 - CLASSIFICATION – NATURE OF THE LESION

The clinical and histological relationship between keratoacanthoma and squamous cell carcinoma is still the subject of much debate. Many authors consider keratoacanthoma to be a specific and generally spontaneously-involuting type of squamous cell cancer [6]. According to the experts involved in drawing up the French Dermatology Society’s recommendations for the diagnosis and treatment of squamous cell carcinoma and its precursors (published in 2009), “keratoacanthoma and squamous cell cancer share epidemiological factors, affect patients of the same age and are observed on the same parts of the body. Furthermore, a small number of what were initially considered to be keratoacanthomas developed into squamous cell carcinomas although their initial appearance was "typical" ” [24]. Owing to its spontaneous involution and thus its good prognosis, others see keratoacanthomas as a harmless condition.

1.3 - PATHOLOGY

The entire structure of the lesion, rather than its cytological characteristics, is used to make the diagnosis. Biopsy sampling through of the whole lesion, encompassing the central crater and the peripheral fleshy edges (wedge biopsy) is recommended. Histological analysis shows a central crater with a keratin plug, surrounded by and resting on a keratinocyte wall which corresponds to the peripheral ring observed clinically and whose dyskeratotic features, structural changes or even atypical aspects are sometimes misleadingly squamous cell carcinoma-like. However, the generally symmetrical arrangement around a central crater filled with keratin, the abrupt transition between this worrying tumour mass (beak or spur shaped attachment) and the normal epidermis, with no dysplasia of the type seen in squamous cell carcinomas, possibly with the presence of neutrophil-rich micro-abscesses, the clear cytoplasm, large size of the keratinocytes and low mitotic index are all suggestive of keratoacanthoma. Different markers distinguishing keratoacanthoma from squamous cell carcinoma have recently been assessed in immunohistochemistry studies but the results have not yet been confirmed. These include cortactin, subunit p50 of kappa nuclear factor [9] , Bcl-x [31] and laminin 322 [5].

1.4 - DIFFERENTIAL DIAGNOSIS

Keratoacanthoma may be confused clinically with actinic keratosis, a cutaneous horn, a wart or even molloscum contagiosum or prurigo nodules when multiple lesions are present. However, the main differential diagnosis remains squamous cell carcinoma which has neither the same prognosis nor metastatic course.

1.5 - PATHOGENESIS

The pathogenesis remains incompletely elucidated but several factors, some interconnected, could play a role :

— genetic factors, hence the familial forms ;

— infectious factors, particularly some of the human papilloma viruses ;

— immune factors, since keratoacanthomas have been described in patients with malignant blood disorders or after immunosuppressant treatment ;

— environmental factors, for instance, the role played by exposure to sunlight, since keratoacanthomas arise mainly on exposed areas of skin and are often seen with actinic lesions, chemical carcinogens (tars) or trauma.

__ on the molecular level, the recent arrival of targeted therapies should shed light on the molecular pathways involved in the formation of the lesions. Thus, cases of sudden onset keratoacanthoma have been reported in patients receiving treatments targeting Ras/Raf/MAP kinase pathways, such as vemurafenib [4] and sorafenib [15, 19, 29].

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